School Science Lessons
Astronomy experiments
2009-01-19
Please send comments to: J.Elfick@uq.edu.au
History
Table of contents
36.1.0 Instruments for astronomy
36.4.0 Sundials
36.14.0 Stars and planets
36.28.0 Celestial phenomena, the Sun and the Moon
36.36.0 Effects of the Earth's motion
36.44.0 Models and demonstrations
36.51.0 Space science
35.3.0 Elements in the Earth's crust
Laboratory safety
36.1.0 Instruments for astronomy
36.1 Simple refracting telescope
36.2 Simple reflecting telescope
36.3 Simple theodolite or astrolabe, sextant
36.4.0 Sundials
36.4 Demonstration sundials
36.5 Flowerpot sundial
36.6 Find north by the length of a shadow during the day
36.7 Sunrise and sunset
36.8 Make a sundial
36.9 Measure the altitude of the Moon and the Sun
36.10 Make a range finder
36.11 Make a model earth
36.12 Time zones and sidereal time, ships watches
36.12.1 Great circles
36.13 Find due north
36.68 Demonstration sundials (Southern hemisphere)
36.69 Sundial for your home
36.70 Universal globe sundial
36.70B Parallel rays of the sun
36.70.5 Building sundials
36.70.6 Make a pocket sundial
4.42 The sun and sundials (primary)
36.14.0 Stars and planets
36.14 Find the main constellations
36.14.1 List of constellations
36.15 Magnitude
36.16 Albedo
36.17 Azimuth and altitude, the horizontal system of co-ordinates
36.18 Find constellations from north of the equator, Northern hemisphere
36.19 Find constellations from south of the equator, Southern hemisphere
36.20 The equatorial system of co-ordinates, latitude and longitude, declination and right ascension, zenith, star chart for the tropics
36.21 Apparent daily rotation of the sky, axis of rotation of the Earth
36.22 Ecliptic, apparent yearly path of the Sun against the background of stars
36.23 Obliquity of the ecliptic
36.24 Is Pluto a planet?
36.25 Model of the solar system
36.26 The "Morning Star" and the "Evening Star"
36.26a "Falling stars" and "shooting stars"
36.27 Movements of planets
3.3.3 Astronomical unit, AU
3.8.0 Ellipse
36.76 Make a constellarium
36.76B Umbrella constellarium
36.77 Seasonal shift of the sky
36.78 Tell the time and the date by the stars
36.78A Star calendar
36.78B Star clock
6.20 The Southern Cross constellation (primary)
36.28.0 Celestial phenomena, the Sun and the Moon
36.28 The phases of the Moon and its apparent position in the sky
36.29 Observe the Moon for four weeks
36.30 Observe the positions of the Moon
36.31 Observe "the man in the Moon"
36.32 Observe the rising and setting moon
36.33 Observe a solar eclipse
36.34 Observe a lunar eclipse
36.35 Rotation period of the Sun
6.19 The moon and tides (primary)
36.36.0 Effects of the Earth's motion
36.36 Foucault pendulum
2.239.3 Foucault pendulum
36.37 Miniature Foucault pendulum
36.38 Seasonal change of position of the Sun, solstice
36.39 Photograph star trails
36.40 Astrology and the zodiac
36.41 Circumference of the Earth, the method of Eratosthenes, 250 B.C.
36.42 Equinox, celestial co-ordinate system, latitude and longitude, right ascension, precession of the equinoxes, knots, logbook
36.43 Find the north-south line from the Sun
36.44.0 Models and demonstrations
36.44 Phases of the Moon and lunar eclipses
36.45 Simulated solar eclipse
36.46 Why an eclipse does not occur at every new and full moon
36.47 The cause of the seasons
36.48 Differences in the length of day and night
36.49 Effects of the angle of the Sun's rays on the Earth
36.50 Calendars, the Star of Bethlehem and birth of Jesus
36.101 Make a spectroscope for materials analysis
15.3.0.2 Rotational and translational kinetic energy of the earth
36.1 Simple refracting telescope
See diagram 36.1: Refracting telescope
Use two cardboard tubes, one fitting inside the other. Fix a lens of focal length 2 cm as an eyepiece, mounted in a cork with a hole in it. Fix a lens of focal length 25 cm in the wider cardboard tube. Adjust both lenses to the same optical axis. Focus by sliding the tube. You can probably observe Jupiter's moons, but not Saturn's rings.
36.2 Simple reflecting telescope
See diagram 36.2.1: Reflecting telescope | See diagram 36.2.2: Ray diagram
Make a simple reflecting telescope with a concave mirror, e.g. a shaving mirror. Mount the mirror in a wooden box to tilt it at different angles, Attach a wooden upright to the box to vary its angle of inclination. Fix two short focus lenses in corks then put them in a short length of a mailing tube as an eyepiece. Attach this eyepiece to the wooden upright and make the necessary adjustments.
36.3 Simple theodolite or astrolabe, sextant
See diagram 36.3: Astrolabe
The astrolabe is perhaps the most iconic of all scientific instruments. Used by astronomers, astrologers, mariners and those with pretensions, it flourished for the first half of the second millennium as a calculation tool and a thing of beauty. They are widely represented in art and many survive in museums.
Use an astrolabe to show the appearance of the celestial sphere then and estimate the altitude of celestial bodies. Astrolabes were used to give approximate measurements of time, terrestrial measurement of heights and angles, and for navigation. Make a simple theodolite or astrolabe by fixing a drinking straw to the base line of a protractor with adhesive. Hang a plumb line hung from the head of a fixing screw to show if the support is upright. Also, use it to measure the elevation of a star seen through the drinking straw. To make an improved model for finding the altitude and the bearing of a star, fix the upright to a baseboard with a screw and two washers, leaving it free to rotate. Fix a piece of tin to the upright as a pointer to show the angle on a horizontal scale.
The phrase " to shoot the sun" means to use a sextant to measure the meridional altitude of the sun, usually at mid day.
36.4 Demonstration sundials
See diagram 36.4: Shadow stick sundial, Circular metal plate sundial
1. Make a shadow stick sundial. Demonstrate a simple sundial by placing an upright stick in the ground so that it is not in the shade. At hourly intervals, mark the position of the shadow from the top of the stick on the ground.
2. Make a simple dial from a circular metal or plastic plate divided into 24 equal arcs. Push a steel knitting needle through the centre of the plate so that the plane of the plate is at right angles to the needle. Fix the plate so that the gnomon, i.e. the needle, points towards the celestial pole. If the noon position of the shadow of the gnomon falls on the XII marking, the shadow will then fall on the other markings close to correct time. Mark the plate on both sides because the shadow of the gnomon will move from one side to the other as the Sun's declination changes.
36.5 Flowerpot sundial
Use a stick fixed through the hole in a flowerpot. Mark the position of the shadow on the flowerpot rim each hour.
36.6 Find north by the length of a shadow during the day:
Use a large sheet of paper and a 50 cm stick fixed vertically on the paper. Select an open space exposed to the Sun. Mark the position of the base of the stick. Every 15 minutes mark the position of the end of the stick's shadow and write the time of observation next to the mark. Use a soft pencil to draw a curve linking the positions of the ends of the shadows. Mark where the shadow was at minimum length. Record the date. Draw the position of true north.
36.7 Sunrise and sunset
Sunrise is the time when the upper part of the Sun appears above the horizon, i.e. when the zenith distance of the Sun is 90o50' and decreasing. Twilight is the period when the illumination of the sky increases after sunrise and decreases after sunset caused by the air molecules and dust scattering sunlight. Twilight lasts longer at higher latitudes because it depends on the steepness of the apparent path of the Sun.
Draw an outline diagram of the eastern horizon as seen from a convenient location. Name the main features of the outline, e.g. big tree, a house, a hill. Observe the eastern horizon just before sunrise on three occasions, one week apart. Record the date, time, place and direction on the horizon of the Sunrise on the three occasions. Mark the position of the Sun as it first appears over the horizon. On the first morning continue to plot the path of the Sun each hour until 10.00 a.m. Note any differences in the position of the Sunrise from day to day. Note whether the Sun rises due east. Use a compass to observe the direction of sunrise from the observation point.
1. Seasonal sunrise and sunset
Record the path of the Sun from sunrise to sunset on December 22, March 30, June 22 and September 23.
2. Record the altitude of the Sun at different times and dates using the following formula:
tan altitude angle = length of shadow stick / length of shadow, e.g. 1 January 20o, 1 April 47o, 1 June 65o, 1 September 52o
3. At noon on October 5 a vertical stake casts a shadow. Sketch where the tip of the shadow will be on the following dates:
3.1 On 1 January,
3.2 On 1 April,
3.3 On 1 June,
3.4 On 1 September.
36.8 Make a sundial
See diagram 36.8: Sundial for the Northern hemisphere
Make the base with a flat rectangular piece of wood. The gnomon ABC is a thin triangular piece of metal. Angle ABC = latitude and angle ACB = 90o. Use a spirit level to test that the base is horizontal. The central line must lie along the north-south line, i.e. the meridian. Fix the gnomon vertically so that the hypotenuse points towards the pole star, (north star, lodestar), in the Northern hemisphere and the celestial south pole in the Southern hemisphere. For approximate results, make the hour markings by noting the position of the shadow of the gnomon at hourly intervals, using a watch set to local mean time. Get more accurate results by making the markings 15 April, 15 June, 1 September or 24 December, when there is no difference between watch time and dial time. The markings are symmetrical about the central line XY so do not calculate other angles. If the base of the dial is made vertical, then the angle between the gnomon and the base must equal 90o minus the latitude.
The difference between time on a perfect clock and the apparent time on a sundial is called the equation of time. The difference is greatest early in November when the sun is more than 16 minutes slow. However, there are days in December, April, June and September when the clock and the sundial agree.
36.9 Measure the altitude of the Moon and the Sun
See diagram 36.9: Simple astrolabe, sextant
1. Cut out a rectangular piece of cardboard slightly larger than a protractor. Trace the shape of the protractor on the cardboard and mark the main points of a scale at 10 degree intervals. Start with zero degrees at the bottom of the scale. Punch a small hole through the cardboard at the point corresponding to the position of the cross hairs of the protractor. Attach a drinking straw to the edge of the cardboard closest to the hole. Attach a washer as a plumb bob to one end of a piece of string. Thread the other end of the string through the hole in the cardboard and tie a knot at the end. The plumb bob should swing freely from the cross hairs. Sight through the drinking straw at any object, e.g. top of a tree, and measure the angle showing the altitude of the object above the ground. At night, use the simple astrolabe to measure the altitude of the Moon.
2. Measure the altitude of the Sun during the day. Cut out a 4 cm X 4 cm piece of cardboard. Punch a hole in the middle to form a tight fit over the drinking straw. Attach the cardboard to one end of the drinking straw. With the back to the Sun, adjust the alignment of the astrolabe so that the shade forms a shadow on a screen. When you can observe a point of light in the middle of the shade patch, you can read the altitude of the Sun.
36.10 Make a range finder
See diagram 36.10: Range finder
1. Cut a slit in a square piece of cardboard and attach the square to a metre rule. Place the end of the rule to the eye and move the card on the rule until a distant object just fits into the slit height.
Measure the following:
1.1 the slit height,
1.2 the distance along rule from eye to slit,
1.3 the estimated size of the distant object,
1.4 the estimated distance to the distant object.
2. Repeat the procedure for the full moon. The diameter of the Moon is 3 476 km. The only unknown is the Earth's distance from the Moon. Calculate the distance from the Earth to the Moon at different times of the year. The average distance is 384 000 km, depending on its position in its elliptical path and the method of calculating an average. For the full moon, draw the slit height and rule length to scale. Use a protractor to measure the angle shown and find the angular size of the full moon.
36.11 Make a model earth
See diagram 36.11: Model earth
Inflate a balloon to 20 cm in diameter. Tie a knot at the entrance and use a marker pen to mark the knot with "N" to represent the north pole. Mark the point opposite with "S".
Draw lines on the model earth to represent the following:
1. the Greenwich meridian,
2. the international dateline,
3. the equator,
4. The closest longitude to the school,
5. the standard meridian, e.g. Brisbane 150o east.
The 15o longitude is equivalent to one hour and 1.0 degrees every four minutes (4 X 15 = 60).
36.12 Time zones and sidereal time, ships watches
Zone time is the local mean time of the standard meridian for the zone. The standard meridian for Brisbane, Rockhampton, Mackay, Townsville and Cairns is longitude 150o E. So all these locations have the same zone time. However, the local mean time varies with the observer's longitude. For example, Brisbane, Rockhampton, Mackay, Townsville and Cairns have different local mean times because they are situated at slightly different longitudes. Local apparent time, as kept by a sundial, differs from local mean time because the Earth's orbit is an ellipse and its linear velocity varies during a year. The equation of time (EOT) is the difference between local apparent time (LAT) and local mean time (LMT), i.e. the difference between mean solar time from a clock and apparent solar time from a sundial. The difference is caused by the eccentric orbit of the Earth and the obliquity of the ecliptic, now about 23o26' but regularly changing over a period of 40 000 years. There is no difference between LAT and LMT on 15 April, 14 June, 1 September and 25 December but he difference may be as much as 16 minutes. Usually, each geographic time zone within a country differs by 15o of longitude, unless determined by a political decision, as in Queensland, Australia. Sidereal time is time related to the movement of the Earth with respect to the stars, not the Sun. A sidereal day is 24h 56m 4s of mean solar time. A sidereal month is 27.32 mean solar days. A sidereal year (astral year) is 365.25636 mean solar days (365 days 6 hours 9 minutes and 9.6 seconds). Sidereal time is the right ascension (RA, alpha) of an object on the meridian of the observer, i.e. the angular distance from the vernal equinox (spring equinox) (First Point of Aries) to where the great circle passing through both celestial poles and an object meets the celestial equator, expressed as time or angle. One hour of right ascension = 15o.
The astronomical, equinoctial, natural, solar, tropical year is the time taken by the Sun to return to the same equinox and has mean length of 365 days 5 hours 48 minutes and 46 seconds.
Ship's watches
12 00 to 16 00 hours, the afternoon watch
16 00 to 18 00 hours, the first dog watch
18 00 to 20 00 hours, the second dog watch
20 00 to 24 00 hours, the middle watch
04 00 to 08 00 hours, the morning watch
08 00 to 12 00 hours, the forenoon watch
36.12.1 Great circles
A great circle is a line on the surface of a sphere which lies on a plane through its centre, or lies on any circle that divided the sphere into two equal parts. So the shortest distance between two points on the earth's surface is on a great circle. The equator and all lines of longitude are great circles.
36.13 Find due north
See diagram 36.13: North-south meridian
1. Find due north to align the gnomon of the sundial along the north-south meridian. Draw a circle on a cardboard base. Attach a shadow stick to the base at the centre of the circle and put the apparatus in a sunny location. Use a plumb bob to check that the shadow stick is vertical. Mark Point M where the shadow just touches the circle in the morning. Mark Point A where the shadow just touches the circle in the afternoon. The line drawn from the shadow stick to the midpoint of MA represents due north-south.
2. Another way of finding due north-south is to use the shadow stick to find the shortest shadow of the day. The direction of the shortest shadow is due north-south.
3. Set up the sundial so that the shadow is aligned with local apparent time of 10 h 15 m at exactly 10 h 30 m zone time, so that the gnomon is pointed due north-south. Use a shadow stick to find the direction of the Earth's daily rotation.
36.14 Find the main constellations
See diagram 36.14: 35 mm slide of a constellation
1. Find the constellations during new moon when there is no moonlight. Prepare a piece of brown paper with pinholes pricked through as constellations. Hold the brown paper up to a light so the pinholes become visible and rotate the brown paper to recognize a similar star pattern. The stars appear to make one full revolution every 24 hours and one full revolution each year. So the constellations cannot be seen in the same position at different times of the night and at different times of the year. The north celestial pole and the south celestial pole are points in the sky that do not move and around which the stars appear to rotate.
2. Perforate underexposed and discarded 35 mm film slides with a pinpoint as constellations then project them on a screen or view them at the end of a cardboard tube held up to the light.
36.15 Magnitude
The magnitude measures the brightness of stars. About 150 B.C. the Greek astronomer Hipparchus classified stars by their brightness with the brightest star at magnitude 1 and the faintest star that could just be seen at magnitude 6. One hundred stars together of magnitude 6 are as bright as a single star of magnitude 1. For each change in level of magnitude the light energy or brightness decreases by about 2.5 (more exactly, the fifth root of 100 = 2.512). The faintest visible star from the Earth is about magnitude 30. Sirius, Venus and the Sun are so bright that they have negative magnitudes. Apparent magnitude is as seen from the Earth. Absolute magnitude is the brightness adjusted for the distance from the Earth. Ancient astronomers named some stars, e.g. Sirius, Rigel. Other star names show the constellation to which a star belongs and the order of brightness of the star in the constellation using the order of letters of the Greek alphabet. For example, the brightest star in the constellation Crux (Southern Cross) is Alpha Crucis. The pointers are the brightest stars in the constellation Centaurus, Alpha Centauri and Beta Centauri. All known stars are listed in catalogues by a code number. For example, Sirius has code number AE41.
36.16 Albedo
The albedo is a measure of reflectivity or brightness, 1 for perfectly reflecting white body and 0 for perfectly absorbing black body. It is also used to express the fraction of the Sun reflected by bodies in the solar system.
36.17 Azimuth and altitude, the horizontal system of co-ordinates
See diagram 36.17: Altitude and azimuth
Zenith is th point immediately over the head of the observer. The opposite point is the nadir.
The altitude of a celestial object is its angular elevation from the horizon from 0o on the horizon to 90o at its zenith. The azimuth is its angle measured eastwards from north in a horizontal plane, i.e. the horizontal angular distance of an arc passing through the celestial object. Note that altitude and azimuth defines the position of a point in the sky only at a certain time. Point your extended arms north-south, with your extended right arm pointing due south. Start from your extended left arm pointing due north to observe the azimuth of a celestial body, e.g. a star has an azimuth of one hand span clockwise from north and its elevation is two hand spans above the horizon. Show the position of this star on a sky diagram. Measure the positions of the Sun during the day and record them on the sky diagram. Make tables of positions from the sky diagram. For example: 5 p.m. 11 March 2006, Sirius azimuth 10o, elevation 70o, Aldebaran azimuth 320o, elevation 30o, Rigel azimuth 330o, elevation 60o, Betelgeuse azimuth 340o, elevation 40o.
36.18 Find constellations from north of the equator, Northern hemisphere
See diagram 36.18: Northern hemisphere constellations
For the Northern hemisphere, the pole star, Polaris (north star, lodestar), will be very close to the north celestial pole. So in the Northern hemisphere, the stars appear to revolve around it.
1. To find constellations in the October sky, turn the diagram through 90o so that the Big Dipper is lowest. Hold the diagram as a map above your head with its face down.
2. Find the most obvious constellation, Ursa Major, known as Big Dipper or Plough.
3. Extend a straight line through the two stars that form the front edge of the dipper cup to find the pole star, Polaris.
4. The two dippers, two bears, are the Big Dipper, Great Bear, Ursa Major, and the Little Dipper, Little Bear, Ursa Minor. The pole star is the last star in the handle of the Little Dipper. The Little Dipper appears to pour into the Big Dipper.
5. The four stars of Pegasus, the mythological winged horse, form a box. The north-east star belongs to the constellation Andromeda. Find Pegasus by continuing the straight line from the two stars that form the outer edge of the Big Dipper cup through and beyond the pole star, Polaris.
6. Find the Cassiopeia constellation opposite the Big Dipper beyond the pole star. It forms the letter w and is known as "Cassiopeia's Chair".
7. The constellation Orion, the "great hunter" contains three bright stars in a line, the "Orion's Belt". Below the "belt" are three fainter stars, the "sword".
8. Observe Venus, known as the "morning star", "day star" and "evening star", and record when it rises or sets in respect to sunrise or sunset.
36.19 Find constellations from south of the equator, Southern hemisphere
See diagram 36.19.1: Southern hemisphere constellations | See diagram 36.19.2: Southern Cross constellation
1. To find constellations in the December sky, hold the diagram as a map above your head with its face down. For the Southern hemisphere, start with the Southern Cross constellation to find the south celestial pole. Extend the longer axis X 3.5, then drop vertically to the horizon. South of the equator the stars appear to revolve about a point in the sky, the south celestial pole. There is no star at the south celestial pole.
2. Find the south celestial pole from the Southern Cross constellation and the two pointers. Imagine a perpendicular bisector of the pointers. Where this line crosses an extension of the largest diagonal of the Southern Cross constellation is the south celestial pole. A point on the horizon exactly below the south celestial pole is due south from you.
3. The Southern Cross constellation, Crux, is kite-shaped, almost surrounded by Centaurus. Crux is the smallest constellation. Its stars are as follows: Alpha (Acrux), the brightest in the constellation, magnitude 0.77, about 320 light-years away, Beta (Mimosa, Becrux) magnitude 1.2, Gamma (Gacrux) magnitude 1.6, Delta magnitude 2.8, Epsilon magnitude 3.6.
4. At the beginning of December see the constellation Crux, the Southern Cross, low down on the southern horizon at midnight. Two magnitude 1 bright stars, Alpha Centauri and Beta Centauri, known as the pointers, are almost in line with Gamma of the Southern Cross towards the south-west. Alpha centauri, also known as Rigel Kentaurius, is the pointer farthest away from the Southern Cross and is the brightest star system in the constellation of Centaurus. It is a "star system" because it was known to be a double star, but lately a third star has been found. It is famous because it is our nearest "star" at 7.39 light-years. The pointers to the Southern Cross constellation cannot be seen from the Northern hemisphere.
5. Follow the milky way to the north of the Southern Cross to find Canis Major constellation, the great dog. This constellation contains Sirius, the dog star. Sirius is the brightest star in the sky, with magnitude -1.44, distance 8.6 light-years away and luminosity 22 X luminosity of the Sun. A few stars are nearer to the Earth than Sirius. North of Canis Major find the constellation Orion. It can also be seen from north of the equator.
36.20 The equatorial system of co-ordinates, latitude and longitude, declination and right ascension, zenith, star chart for the tropics
1. For the identification of stars, imagine them to be on the inside of a sphere, the celestial sphere, that is concentric with the Earth. The pole star is about at the north pole of the celestial sphere and is almost directly above the north pole of the Earth. The celestial equator circles the celestial sphere directly over the equator of the Earth.
2. Identify the position of a point on the surface of the Earth by its latitude and longitude. The latitude of a point is the angular distance north or south of the equator, e.g. latitude 45o S. The longitude, the meridian, is the line joining the north and south poles and passing through the point. The 00 longitude, the Greenwich meridian, passes through the north pole, Greenwich in England, and the south pole.
3. Identify the position of a star on the celestial sphere by its declination and right ascension. The declination corresponds to latitude and is measured north and south of the celestial equator. The right ascension corresponds to longitude.
4. The zenith is a point on the celestial sphere immediately overhead an observer, 90o from the horizon. The pole star would be at the zenith of an observer at the north pole of the Earth. At about midday on 15 May the Sun would be at the zenith of an observer in a place of latitude 200 N.
5. A star chart for the tropics represents that part of the celestial sphere that an observer on the Earth's equator would see. It extends from 35o N to 30o S. Orion's belt, when visible, gives an approximate east-west direction and the line joining the midpoints of the shorter sides of the Orion quadrilateral gives a guide to the north-south direction. The distances are measured in angular degrees and the equator is divided roughly into months. Each date sets the chart at midnight for an observer on the equator, i.e. whose zenith is on the equator.
36.21 Apparent daily rotation of the sky, axis of rotation of the Earth
1. Choose a place where you have a clear view of the sky, including parts close to the horizon. Find your north or south celestial pole. Fix a plumb line so that it appears to go through the celestial pole. Note where the lower end of the plumb line appears against the stars. Draw a line on the star chart to represent this position of the plumb line, and note the time to the nearest minute. Make the same type of observation two hours later. Mark a second line on the star chart and note the time to the nearest minute. Record the calendar date. Note whether the sky appears to turn clockwise or anticlockwise. Measure the angle in degrees between the two lines with a protractor. Calculate the change in degrees per hour. Calculate the time required for one complete rotation, 360o. You can also do this with photographs of star trails.
2. Identify a prominent constellation and sketch its position relative to a prominent landmark, e.g. a big tree. Note the time. Make the same observation and sketch two hours later. Calculate the change in degrees per hour. Calculate the time required for one complete rotation, 360o.
3. Repeat the above observations one month later.
4. Observe the diurnal aberration of a star. An observer at the equator can observe a movement of any star to the east at a rate of 0.32 seconds of arc per day because of the rotation of the Earth on its axis. However, that observed movement reduces to zero as the observer approaches the poles. Diurnal aberration of a star is the direct evidence that the Earth is not fixed in space.
36.22 Ecliptic
The ecliptic is the apparent yearly path of the Sun against the background of stars. It is an imaginary line based on the earth's motion about the sun. The ecliptic is in the middle of the Zodiac.
On consecutive days, note the position of the Sun against the stars just before the Sun rises and just after the Sun sets. Each day the position of the Sun moves East. The ecliptic is a line but in practice it is thought of a narrow band each side of the ecliptic. So the ecliptic is a circle on the celestial sphere where the celestial sphere is cut by the orbit of the Earth. The ecliptic intersects the celestial equator at the two equinoxes.
36.23 Obliquity of the ecliptic
The obliquity of the ecliptic is the angle between the plane of the ecliptic and the celestial equator, or the angle between the axis of rotation of the Earth and the pole of its orbit. It is responsible for the seasons. The obliquity of the ecliptic of the Earth is now about 23o26' (year 2000 23o26'34"). It varies from 21o55' to 28o18'. It is caused by precession and nutation. The precession is caused mainly by the gravitational pull of the Sun and the Moon on the equatorial bulge of the Earth, 43 km diameter than pole to pole. Other planets have a small effect but in the opposite direction so the total effect is called the general precession, with a decrease of about 50 arc seconds per year, about 1o every 72 years. These gravitational pulls constitute a torque so that the axis of the Earth traces a circle in the sky like a wobbling spinning top. The axis completes a circle in 25,800 years. Nutation is a periodic oscillation of the axis of the Earth caused by the relative changing positions of the Sun, moon and Earth.
36.24 Is Pluto a planet?
As at 24 August 2006, the International Astronomical Union, IAU, demoted Pluto as a planet. The IAU voted to redefine Pluto as a "dwarf planet" along with the "body, UB313" outside Pluto (and bigger than Pluto), Pluto's moon Charon, and Ceres (the biggest asteroid between Mars and Jupiter). The IAU stated that planets must be large enough to "clear the neighbourhood" around their orbits, must be in orbit around a star while not being a star and must be large enough in mass for their own gravity to pull them into a nearly spherical shape. So in 7.79 Model of the Solar System, you may delete Pluto as a planet and / or insert the dwarf planets, Pluto, Ceres, and Eris.
36.25 Model of the solar system
Make models of the solar system to understand the relative size and distance of the planets from the Sun. Make two separate models: 1. showing the relative size of the planets 2. showing their relative distances of the planets from the Sun. Make paper circles or balls to represent the Sun and planets using the table below. The figures in parentheses give a scale for distances, taking the Earth's average distance from the Sun and the Earth's diameter as units. The Sun is about 1 400.000 km in diameter (110). Attach the models to the wall of the classroom. An astronomical unit, AU, is the mean distance between the Earth and the Sun, about 149 598 000 km (92 956 000 miles). It is used as a convenient way to measure distance in the solar system.
The planets (Greek: planes, wanderer) revolve around the sun in approximately circular orbits. The planets listed below are called the primary planets. Secondary planets are satellites or moons. The asteroids between the orbits of Mars and Jupiter are called the minor planets.
Planet Distance Diameter
Mercury 58 (0.4) 4 800 (0.4)
Venus 108 (0.7) 12 000 (1.0)
Earth 150 (1.0) 13 000 (1.0)
Mars 228 (1.5) 6 800 (0.5)
Jupiter 778 (5.2) 140 000 (11.2)
Saturn 1 420 (9.5) 120 000 (9.5)
Uranus 2 870 (19.2) 50 000 (3.7)
Neptune 4 490 (30.1) 53 000 (7.1)
Pluto 5 900 (39.5 2 700 (0.2)
36.26 The "Morning Star" and the "Evening Star"
Observe the planet Venus and note when it rises or sets in respect to sunrise and sunset.
36.26a "Falling stars" and "shooting stars"
Note the position, time and date of "falling stars" or "shooting stars", i.e. meteors. A small rock in space is called an asteroid. If it enters the earth's atmosphere and starts to burn it is called a meteor. The unburned remains of a meteor, if found on the ground, is called a meteorite. Most meteorites contain iron-nicket minerals but they may also be composed of carbon, iron carbides and sulfides, oxides, phosphides and silicates.
36.27 Movements of planets
Use a tall, narrow jar, some water, S.A.E. 30 grade motor oil, 90% alcohol, and a pencil. Half fill the jar with water. Slowly pour alcohol on top of the water, do not agitate the two liquids or you will disturb the interface. Dip a pencil into the motor oil, and let several drops of the oil fall into the liquid filled jar. Gently rotate the jar to cause the oil drop "planets" to revolve. Alcohol has a lower density than water, so it floats on the water. Oil sinks in alcohol, yet floats on water. In such a "free" state, the oil forms spheres and stays at the interface between alcohol and water.
36.28 The phases of the Moon and its apparent position in the sky
See diagram 36.28: Phases of the moon
1. The phases of the Moon are visible because different portions of the illuminated and non-illuminated parts of the Moon are facing towards earth at different times. The Moon shines because it reflects light from the Sun. At any particular time, half the Moon is illuminated by the Sun. the Moon takes 27 days 7 hours and 43 minutes to travel around the Earth. As it orbits the Earth, it takes the same length of time to rotate once on its axis so the same side of the Moon is always facing the Earth. On 2006-09-22 the Moon was farthest from Earth, apogee, at 406 498 km. On 2006-09-08 the Moon was closest to earth, perigee, at 357 174 km. Phase refers to the illuminated part of a celestial body. The different relative positions of the Moon and Sun cause the phases of the Moon (new, crescent, half, gibbous, full moon). When the Moon and Sun are on opposite sides of the Earth, you can see the sunlight reflected from all of the face of the Moon, a full moon. When the Sun is on the same side of the Earth as the Sun, little light is reflected back towards the Earth, a new moon. When the angle made by the Sun and the Moon at the Earth is between 0o and 180o you see the light from only a part of the Moon, a crescent moon. From just after the new moon, the crescent shape changes into a quarter moon then a gibbous moon and finally into a full moon. Then the changes reverse.
2. A "blue moon" means a second full moon in the same calendar month that occurs about seven times in each nineteen years, i.e. "once in a blue moon". The Moon has no atmosphere so you see a clear separation between the lit and unlit portions of its surface, the terminator. It is an arc of an ellipse. A lune or crescent is the area enclosed by the terminator and the nearer edge of the Moon.
3. A "harvest moon" is the full moon nearest to the autumnal equinox (autumn equinox) during 22 September, 2008, in the Northern hemisphere and during 20 March, 2008. in the Southern hemisphere.
4. From the Southern hemisphere, the Moon appears to move around the Earth in a clockwise direction, while from the Northern hemisphere, the Moon appears to move around the Earth in an anticlockwise direction. The Moon rises about 50 minutes later each day. For a few days after the new moon to a few days before the full moon, the Moon appears to move clockwise from west to east and can be seen in the morning during school time. The best time to observe the Moon is 7.00 p.m. The waxing crescent moon is visible low in the western sky, the first quarter is visible high in the Northern sky and the full moon is visible low in the eastern sky.
36.29 Observe the Moon for four weeks
At the same time each evening, e.g. 8.00 p.m. record the date, time, apparent shape (full, gibbous, half, crescent, new, crescent, half, gibbous, full), azimuth and altitude. Draw a moon each night so that the lune remains white and the rest of the Moon is shaded black. When the Moon is a gibbous moon, use circles to represent the Moon and show the orientation of the terminator of the gibbous moon through the night, i.e. when the Moon is in the east, north and west. Record the dates of the phases. Make these observations during four weeks. Always observe from the same place. Consult an almanac so you can begin the observation on the date when the crescent moon is just visible in the evening, two or three days after a new phase. The horns of the crescent moon are turned away from the sun. A lunar month is from new moon to new moon, about 29.5 days, i.e. the time taken for the moon to revolve around the Earth., however, most people think of the lunar month as being a period of 28 days.
new moon waxing crescent first quarter waxing gibbous full moon waning gibbous last quarter waning crescent
March 14 .
March 22 . March 29 . April 5 .
36.30 Observe the positions of the Moon
1. On the first night, draw the position of the Moon relative to prominent landmarks, e.g. above a tower or church steeple. Measure its height above the horizon in degrees, using your fist or your fingers extended, e.g. a fist at arm's length = 100, a span of a thumb and little finger = 200. Record these measurements and the time on a sketch. Also, record the direction of the horns of the Moon, and the shape of the crescent. Two hours later, repeat the observations and note the time.
2. Make repeated observations in the same way every night for two weeks.
Record the following observations:
2.1 how the shape of the Moon's illumination changes from night to night,
2.2 how its apparent location changes,
2.3 how its horns, or cut-off edge, are oriented relative to the position of the Sun below the western horizon
2.4. how the Moon changes position during one night.
A drawing of an "impossible moon" shows horns pointing down!
36.31 Observe "the man in the Moon"
1. Observe the craters and flatter areas, "seas" (Mare) and oceans (Oceanus). The space craft Apollo 12 was launched 14 November 1969 and landed on the Oceanus Procellarum on 19 November 1969. Then the astronauts walked to the remains of previous lunar probe Surveyor 3 and retrieved some pieces of it. The arrangement of craters, sea oceans and other features allow different people and cultures to see figures in the Moon. Although "the man in the Moon" in the Northern hemisphere looks like an old man walking away carrying sticks or leaning on a fork, some people can see different faces and figures, even a frog. In China and Japan they see a large rabbit stretched across the Moon with the ears pointing down from the upper right and the legs crossed at the lower left. The rabbit is making something in a box. Most figures can be seen only at or near a full moon. Some of these figures appear differently in the Northern and Southern hemisphere. Stare at the Moon at different phases until you can see figures. Record the figures on a moon diagram and note the time and date of the observations.
2. Measure the diameter of the moon, 31 minutes 4 seconds. So 347 full Moons side by side would fill a circle across the sky from horizon to horizon.
36.32 Observe the rising and setting moon
During the last quarter phase of the Moon, make the above observations during the morning and compare them with the same observations during the evening.
Phase Rising time Time in eastern sky Time highest in sky Time in western sky Setting time
New moon Sunrise Morning Noon Afternoon Sunset
Waxing crescent Just after sunrise Morning Just after noon Afternoon Just after sunset
First quarter Noon Afternoon Sunset Evening Midnight
Waxing gibbous Afternoon Sunset Night, before midnight Midnight Night, after midnight
Full moon Sunset Night, before midnight Midnight Night, after midnight Sunrise
Waning gibbous Night, before midnight Midnight Night, after midnight Sunrise Morning
Third quarter Midnight Night, after midnight Sunrise Morning Noon
Waning crescent Just before sunrise Morning Just before noon Afternoon Just before sunset
36.33 Observe a solar eclipse
See diagram 36.33: Solar eclipse
1. By observing eclipses you can learn about the shape, size, and motions of the Sun, moon, and earth. The coming dates of eclipses are in newspapers and almanacs so you can plan to be outdoors when an eclipse occurs in your area.
Be careful! Do not allow students to look directly at the eclipse with the naked eye or through smoked glass or exposed photographic film.
2. One safe method of observing an eclipse is to view it indirectly. Punch a hole through a piece of cardboard. Turn your back to the Sun and hold the cardboard over one shoulder to permit the Sun's image to shine through the hole on to a second piece of cardboard held in front of you. Be careful! Do not look at the Sun through the hole in the cardboard.
36.34 Observe a lunar eclipse
See diagram 36.34: Lunar eclipse
Direct observation of a lunar eclipse is safe. Observe the shape of the Earth's shadow as its edge crosses the Moon as evidence that the Earth is spherical. However, the effect could be caused by a disc-shaped earth.
36.35 Rotation period of the Sun
See diagram 36.35: Observe position of sunspots with binoculars
1. Find the rotation period of the Sun and the direction of its axis by observing the position changes of sunspots. Use a small telescope or binoculars, a large box, a clipboard, paper and pencil.
Be careful! Do not look directly at the sun through this instrument.
2. Mount binoculars in the front end of a box. Make a sunshade for a telescope. Leave one long side of the box open for viewing. Elevate the box that the front end is perpendicular to the direction of the Sun's rays. Put the clipboard with attached paper inside the box at the back end so that the solar image can be projected on it. Make observations each day at noon. Draw a circle and mark in the position of any sunspots. Show their relative sizes and approximate shapes. From day to day, the spots will appear to change position as the Sun rotates. Measure the differences between several daily sketches to estimate the rate of motion. After some weeks a spot group may return or new spot groups may appear.
Effects of the Earth's motion
36.36 Foucault pendulum
See diagram 36.36: G-clamp support
1. Use a G-clamp with a ball bearing soldered to the inside of the jaw to makes a good support for the pendulum. Hang the pendulum indoors with the ball bearing resting on a razor blade or another hard surface. Use nylon fishing line to suspend the bob. It can be a solid rubber ball. For a pointer, use a short knitting needle pushed into the bob and continuous with the suspending fishing line. The pointer should just touch a reference line drawn in fine sand in a tray on the floor. The length of the pendulum can be from 3 m to 30 m.
2. To set the pendulum in motion, attach a long cotton thread to a drawing pin pushed into the bob. Align the thread along the direction of the reference line, then burn the thread near the drawing pin. After the pendulum is set in motion note that the plane of the swing has changed after a few hours compared with the reference line. If the ceiling-mounted pendulum swings freely, note the change in the path of the pendulum after one hour. Then note the plane of swing at try six X ten minute intervals. A pendulum releasing ink can mark a clear pattern. Getting good quantitative results without many refinements is not easy, but observing the effect is not difficult.
3. Note the variation of rotation of the Foucault pendulum with latitude. The Earth rotating beneath the bob causes the change. The precession period for an ideal pendulum is 23.93 hours / sine of the latitude. For example, at Sydney, Australia, at latitude 34o S, the period is about 43 hours, i.e. about one degree every seven minutes. At the south pole the pendulum precesses through 360o in a day. At the equator the pendulum does not precess.
36.37 Miniature Foucault pendulum
Mount a small Foucault pendulum from a stand set upon a turntable or office chair that can be rotated. Have the students observe the behaviour of the pendulum when the turntable is rotated slowly.
36.38 Seasonal change of position of the Sun, solstice
See diagram 36.38: Morning and afternoon shadows
1. From a fixed location with a good view, note accurately the point where the Sun disappears behind landmarks as it sets. Repeat the observations at intervals of a week for four weeks at least, and find the rate of change in degrees per day. To measure degrees, a clenched fist at arm's length equals about 100.
2. Mark a line on the floor or the wall where the Sun shines in your room and makes a shadow's edge. Note the exact month, day and hour. At the end of each week make another line at the same hour. Repeat this throughout the year to obtain an interesting set of observations. The variation in position of the line from week to week and from month to month is caused by the movement of the Earth around the Sun.
3. In an open space, drive a 150 cm vertical thin rod, the gnomon, into the ground. Mark a north-south line on the ground from the base of the gnomon. Record the length of the shadow of the gnomon at different times of the day and at different seasons of the year. Note whether the noon shadow is north or south of the north-south line. Mark the position of the end of the shadow at noon each day. By the end of a year, join the positions to form a figure eight, an analemma. The highest position is at the summer solstice and the lowest position is at the winter solstice, caused by the axial inclination of the Earth. The variation across the short axis is because of the eccentricity of the orbit of the Earth. A summer solstice is when the Sun reaches the farthest point north of the equator, at 21 or 22 December, the longest day in the Southern hemisphere. A winter solstice is when the Sun reaches the farthest point south of the equator, at 21 or 22 June, the longest day in the Northern hemisphere. The sun reaches its extreme Northern and southern points on the ecliptic and appears to stand still before it reverses its apparent course. These two points of the ecliptic are midway between the equinoxes. The hours of light and darkness become the same a few days before the spring equinox and a few days after the autumn equinox.
36.39 Photograph star trails
See diagram 36.39: Star trails around the north celestial pole
1. Photograph star trails as the Earth revolves. Wait for a clear moonless night where you can see the horizon. Avoid a place with extraneous light, e.g. motor car headlights. Face the camera on a tripod at a celestial pole, i.e. pole star or south celestial pole. Record the time. Focus for infinity, open the diaphragm to full aperture, set the shutter for time exposure and start the exposure. Leave the camera with the diaphragm open for two hours. Close the shutter for two minute without moving the camera then open the shutter again for one minute and finally close it. The last short exposure identifies the end of the exposure. Record the time.
2. The developed film show star trails as concentric arcs with centres at the celestial pole. Measure the longer arcs to show how many degrees of rotation occurred and use this to calculate the period of full rotation. Each star near the pole traces a tight circle in its movement, and as the distance from the pole increases, the radius of curvature increases until the stars above the equator appear to travel in straight lines.
3. Record the apparent path of the Moon by taking two seconds exposures every fifteen minutes until the Moon moves out of the field of the camera.
4. Record the apparent path of the Sun during the day with the lens stopped down. Be careful! Do not look at the sun through the viewfinder.
36.40 Astrology and the zodiac
1. The ecliptic is divided into 12 equal sections of 30o, each containing a constellation, a sign of the zodiac. On or near 21 March each year the Sun moves into 0o of Aries, first point of Aries, which defines the start of the tropical year of 365.242 194 mean solar days. The timetable for the Sun passing through the 12 signs of the zodiac as follows, may vary plus or minus 1 day depending on leap years: Aries (Ram) 21 March to 20 April, Taurus (Bull) 21 April to 20 May, Gemini (Twins) 21 May to 21 June, Cancer (Crab) 22 June to 23 July, Leo (Lion) 24 July to 23 August, Virgo (Virgin) 24 August to 23 September, Libra (Scales) 24 September to 23 October, Scorpius (Scorpion) 24 October to 22 November, Sagittarius (Archer) 23 November to 22 December, Capricornus or Capricorn (Goat) 23 December to 20 January, Aquarius (Water carrier) 21 January to 19 February, Pisces (Fish) 20 February to 20 March.
2. The zodiac is the circular band of stars seen along the same path as the Earth's orbit around the Sun. It is a belt on the celestial sphere 8o on either side of the ecliptic, forming a background to the motion of the Sun, moon and planets. In twelve groups, these stars make up the twelve signs of the zodiac, each 30o long. They are named after the constellations identified during the time of the ancient Greek astronomers. Astrologers believe that the positions of heavenly bodies when you were born influence what you are so they match zodiac signs with human characteristics.
3. Some traits associated with signs of the zodiac
Aries: aggressive, courageous, self-motivating, impulsive, dynamic, selfish, irascible
Aries was the first constellation of the zodiac but the vernal equinox, the point at which the Sun crosses the celestial equator from south to north, also called the spring equinox and the first point of Aries, is now moved into the area of Pisces because of precession causing the movement westwards by one seventh of a second of arc daily.
Taurus: determined, practical, unemotional, inflexible
Gemini: versatile, restless, talkative, superficial
Cancer: persistent, possessive, moody, cautious
Leo: leadership ability, generous, egotistical, patronising
Virgo: modest, diligent, reliable, fussy
Libra: fair minded, diplomatic, urbane, indecisive
Scorpio: subtle, determined, possessive, compulsive
Sagittarius: friendly, optimistic, enthusiastic, restless
Capricorn: resent interference, patient, careful, fatalistic
Aquarius: erratic, detached, honest
(The "age of Aquarius" is a time of freedom, including sexual freedom, and general brotherhood.)
Pisces: creative, changeable, emotional, intuitive
3. List which of the traits in the list describe yourself and a friend. Then ask the friend to make a similar list. How many traits in the list were according to the astrological prediction?
36.41 Circumference of the Earth, the method of Eratosthenes, 250 B.C.
See diagram 36.41: Looking down the well
36.41.2 A Alexandria S Syrene, E centre of the Earth, 1. To zenith at A, 2. To sun at noon, 3. To sun at Syrene
1. At noon on the day of the summer solstice the Sun is directly ahead in Syrene and there is no shadow but at Alexandria there is a shadow. He looked down a deep well at Syrene (now Aswan) and observed that a circle of light was reflected from the surface of the water in the well. The Sun was vertical and cast no shadow. At the same time in Alexandria, using a shadow stick, the angle between the vertical and the Sun was measured at 7.2o. The Sun is far from the Earth so the rays of the Sun falling on Syrene and Alexandria are parallel. The angular difference between the two places 800 km apart = 7.2 / 360 = 0.02 = 1 /50. So the circumference of the Earth = 50 X 8000 = 40 000 km.
2. Select two schools on the north-south axis, i.e. same longitude, 500 km apart. Both schools have a vertical flag pole five metres high. At about noon at the time of the summer solstice, note when the flag pole at the first school has no shadow, or almost no shadow. Immediately telephone a teacher at the second school and ask for the length of the shadow of their flag pole. Draw a right angle triangle ABC such that angle ABC is a right angle, AB is the length of the flag pole, BC is the length of the shadow and AC is the hypotenuse. Angle CAB is the angle of the Sun's rays. If the rays of the Sun through the two schools are parallel, angle a / 500 = 360o / circumference of the Earth. Circumference of the Earth = 360 X 500 / angle CAB.
36.42 Equinox, celestial co-ordinate system, latitude and longitude, right ascension, precession of the equinoxes, knots. logbook
See diagram 36.42.1: Parallels of latitude | See diagram 36.42.2: Longitude
1. The equinoxes are the two points on the celestial sphere where the ecliptic intersects the celestial equator, i.e. where the Sun crosses the equator. The equinoxes are named for the convenience of the Northern hemisphere. The vernal equinox (start of autumn) is when the Sun crosses from south to north, about 21 March. The autumnal equinox is when the Sun crosses from north to south, about 23 September (2.03 p.m. on 23 September, 2006). The vernal equinox is the base point of the celestial co-ordinate system. On this day, the Sun rises due east and sets due west.
2. Precession of the equinoxes
The Earth bulges at the equator such that the equatorial diameter is about 43 km longer than the north-south diameter. Also, the north-south diameter or axis of rotation is about 23.5o to the perpendicular to its orbit. Gravitational pull from the Sun and moon tend to pull the Earth back to the perpendicular, so the Earth wobbles like a spinning top. The circular path of the wobble takes 25 800 years and accounts for the precession of the equinoxes, the western or backwards movement of the equinoxes of 50.27' per year. As the vernal point moves through constellations, this period of time can be called the "age" of that constellation. From about 4 000 B.C. to 2 000 B.C. the vernal point was in the constellation of Taurus, the age of Taurus. From about 2 000 B.C. to 1 B.C. was the age of Aries, the lamb. From about 1 AD to AD 2 600 is the age of Pisces, the fish. The next age will be the "age of Aquarius", a constellation of the zodiac.
The celestial co-ordinate system is based on regarding the sky as an imaginary sphere with the Earth at the centre with North celestial pole, South celestial pole and celestial equator, you can extend latitude and longitude to the sphere for identifying the location of points on the sphere. The baseline or zero point in not based on north but the 0o Aries point on the ecliptic of the tropical zodiac, i.e. the vernal equinox.
Latitude is represented by the vertical angle above or below the celestial equator and is called the declination. Longitude is represented by the angular distance measured eastwards along the celestial equator from the vernal equinox to the semicircle of the declination and is called the Right Ascension, measured in hours, minutes and seconds. 1 hour of right ascension = 15o. (24 hours of right ascension = 360o.) Star catalogues specify locations in terms of right ascension and declination.
3. The latitude of a point P is the angular distance north or south of the equator, e.g. latitude 45o S. All points with the same latitude are on the same circle called a parallel. Two points with difference in latitude of 1o are about 110 km apart. The variation is because of the shape of the Earth that is flatter at the poles, oblate. Two points with difference in latitude of 1 minute, one sixtieth of a degree of latitude or one nautical mile, are about 110 / 60 = 1.83' km apart. The international nautical mile used by ships and aircraft is 1 852 m. If a nautical miles is one minute of arc on the meridian, then using the International Terrestrial Geoid based on the different polar and equatorial radii, a nautical mile is 1 852.276 metres. The UK nautical mile is 1 853.18 m (6 080 ft), its value in latitude 48o. A speed of one nautical mile per hour is called one knot. You cannot say "knots per hour". In a ship the speed in knots was calculated from using a log, a flat piece of wood radius six inches that floated upright fastened to a 100 fathom log-line with knots at intervals. The records of knots was entered into a logbook along with meteorological records.
4. The longitude of a point P is the angular separation between an imaginary circle called a meridian that passes through the point P and north and south poles, and the prime meridian that passes through Greenwich, England, north and south poles, e.g. longitude 30o East (of Greenwich). Another point could have longitude 25o West. Differences between degrees of longitude are greater approaching the equator and lesser approaching the poles. Longitude of a point on the surface of the Earth is sometimes called terrestrial longitude.
5. Greenwich Time is the mean time for the meridian of Greenwich, system of time in which noon occurs at the moment of passage of the mean sun over the meridian of Greenwich. This was standard time in the British Isles until 18 February, 1968 when clocks were advanced one hour and Summer Time became the standard as British Standard Time.
36.43 Find the north-south line from the Sun
1. Set a watch to the local mean solar time. If north of the equator, point the hour hand towards the Sun. The north-south line is given by the bisector of the angle between the hour hand and 12 o'clock. If south of the equator, point 12 o'clock towards the Sun. The north-south line is given by the bisector of the angle between the hour hand and 12 o'clock.
2. Watch compass (clock compass)
Hold the watch horizontal and point the 12 towards the Sun. Hold a small stick, e.g. a matchstick, vertically next to the 12, then turn till the shadow of the stick passes through the centre of the watch. Imagine a line bisecting the angle between the line through the centre of the watch face and the hour hand. This line is the north-south line.
3. If you have no watch, you can use the shadow of a stick instead. Drive a stick vertically into the ground. As the Sun crosses the sky during the day, the shadow of the stick will turn. It will also grow shorter in the morning and longer again in the afternoon. When the shadow is shortest, close to noon its far end will point north or south, depending on whether you are north or south of the equator
36.44 Phases of the Moon and lunar eclipses
See diagram 36.44: The Moon in the sky
1. Fix an electric torch to shine full on a white ball as a moon. Hold an earth ball in position to view the white ball moon from different directions and see crescent quarter phases, gibbous, and full moons. Rotate the Earth globe to show how the times of rising and setting of the Moon are closely related to the phase. For example, the first quarter moon rises about noon, is highest in the sky at sunset, and sets about midnight. By sighting across the position on the globe corresponding to your own geographic locality, simulate the relationship of the Moon to the horizon for moonrise and moon set positions
2. Place the white ball moon in the shadow cast by the Earth globe to simulate a partial or total lunar eclipse. Place the Moon between the electric torch and the globe so that its shadow is cast on the Earth. Show that an eclipse of the Sun is not visible over as great an area of the Earth as an eclipse of the Moon, which is seen from the entire half of the Earth that is towards the Moon.
36.45 Simulated solar eclipse
See diagram 36.45: Simulated solar eclipse
Represent the Sun with an opal electric bulb shining through a circular hole 5 cm in diameter in a piece of blackened cardboard. Draw the corona in red crayon around this hole. The Moon is a wooden ball, 2.5 cm diameter, mounted on a knitting needle. View the eclipse through any of several large pin holes in a screen on the front of the apparatus. The corona becomes visible only at the position of total eclipse. Adjust the Moon's position with a wire bicycle spoke attached to the front of the apparatus.
36.46 Why an eclipse does not occur at every new and full moon
See diagram 36.46: Eclipse of the sun and the moon
A eclipse of the sun B eclipse of the moon C no eclipse
The Moon's orbit is inclined enough to cause the Moon usually to pass above or below the Earth's shadow or the region between the Earth and the Sun.
36.47 The cause of the seasons
See diagram 36.47: The four seasons
The Sun travels about eight days longer in the Northern Hemisphere than in the Southern Hemisphere. Use a hollow rubber ball to represent the Earth. Push a 15 cm length of wire or a knitting needle through the ball to represent the Earth's axis. Draw a circle about 40 cm in diameter on a piece of cardboard to represent the Earth's orbit. Hang an electric lamp about 15 cm above the centre of the cardboard to represent the Sun. Place the ball representing the Earth successively at the four positions shown in the diagram with the axis slanted about 23.5o. Observe how much of the ball that is always illuminated. Observe where the direct rays of the Sun strike. Observe which hemisphere receives the slanting rays of the Sun. Repeat the experiment with the needle perpendicular to the table top in each of the four positions and observe what would happen if the axis of the Earth were not inclined.
36.48 Differences in the length of day and night
See diagram 36.48: Day and night
Days and nights are of equal length only at the equator. Draw a large circle to represent the Earth's orbit. Draw two lines perpendicular to each other through the centre. Where they cut the circle, label the intersections in counter clockwise order: 20 March, 21 June, 23 September, 21 December. These are positions of the Earth in relation to the Sun on these dates. Draw a small circle for the Earth at the 21 June position. The north pole will be off centre about radius of the circle, towards the Sun. For any other date or orbital position, which can be located by using a protractor, the Earth circle and pole will have the same orientation. The Arctic circle, tropic of Cancer, and equator can be drawn in. Then a line through the centre of the Earth circle and perpendicular to the Earth-Sun line will be the boundary between daylight and darkness. From such a diagram, estimate the duration of sunlight at different latitudes for any date, e.g. on 1 August at the Arctic circle the Sun would be estimated as up for about 18 hours, but up only 6 hours on 1 November.
36.49 Effects of the angle of the Sun's rays on the Earth
Show the effect of the angle of the Sun's rays on the amount of heat and light received by the Earth. Bend a piece of cardboard and make a square tube 2 cm X 2 cm X 32 cm. Use a piece of very stiff cardboard and cut from this a strip 23 cm long and 2 cm wide. Paste this to one side of the tube with 15 cm extending. Rest the end of the stiff cardboard on the table and incline the tube at an angle of about 25o. Hold a flashlight or lighted candle at the upper end of the tube and mark off the area on the table covered by the light through the tube. Repeat the experiment with the tube at an angle of about 15o. Repeat again with the tube vertical. Compare the size of the three spots and find the area of each. Show the analogy between this investigation and the way in which the Sun's rays impinge on the Earth's surface. Note whether the amount of heat and light received per unit area from the Sun is greater when the rays are slanting or direct.
36.50 Calendars, the Star of Bethlehem and birth of Jesus
1. An almanac is a yearly prediction of the position of celestial bodies. The ancient Egyptians used a calendar based on the solar year. The ancient Babylonians, Hebrews and Muslims used a calendar based on a lunar year of 12 months, 11 days shorter than the solar year so an extra month was added every third year. The Roman calendar had 10 months until in 46 B.C. Julius Caesar ordered a revised calendar, Julian calendar, of 12 months with an extra day, leap day, added every fourth year of 365 days. The number of days in the months became the same as now. In AD 321, Emperor Constantine ordered the seven day week with Sunday as the first day. In AD 1582, Pope Gregory XIII, ordered the change from the Old Style or Julian Calendar with a solar year of 365.25 days, longer than the tropical year by about 11 minutes, to a New Style or Gregorian Calendar with a solar year of 365.242 546 days. This produces an error of 3 days every 400 years, so 3 out of 4 centennial years are not leap years. The leap day is not inserted in century years not divisible by 400, i.e. 1700, 1800 and 1900, but year 2000 was a leap year. The Gregorian calendar was not adopted in Great Britain until January, 1752. The Jewish Calendar dates from the Creation fixed at 3761 B.C. The Mohammedan Calendar dates from 16 July 622, the date of the Hegira. The 29th February is called an intercalary day.
In England, the new style quarter days are Lady Day 25 march, Midsummer day 24 June, Michaelmas Day 11 october, "Old Christmas Day 6 January. Midsummer is the weekly period around the summer solstice 21 June. The term millennium (1 000 years) comes from St. John's gospel of the Bible and refers to the period of a thousand years when Christ will return to earth and live with His saints and finally take them to heaven.
2. In AD 325 the Council of Nicaea determined that Xmas Day be celebrated always on the 25th December, an immovable feast, but Easter day remained a movable feast. Easter Day is now determined in the United Kingdom by the Calendar (New Style) Act of 1750, as the first Sunday after the full moon that happens upon or next after the twenty first day of March, and if the full moon happens upon a Sunday, Easter Day is the Sunday after. So Easter Sunday can be from 22 March to 25 April. Most countries use the Gregorian Calendar and the date of Easter used in the United Kingdom. However Eastern Orthodox Churches may still use the Old Style Calendar and have a different date for Easter Sunday. Their Xmas Day is 7 January.
3. The 21st century started in 2001 because the first century started in AD 1. The year before it was 1 B.C., so there was no "year 0". B.C. stands for "before Christ" so the years are numbered backwards. AD stands for the Italian "anno domini", in the year of the Lord. In AD 525 Dionysius Exiguus decided on the start of the present calendar so that Jesus Christ was born on December AD 1. Jesus Christ may have been born as early as 4 B.C.
4. However, if Jesus was born Sunday, 1 March, 7 B.C., this was the year of the triple conjunction of the same two planets when in 27 May, 5 October and 1 December, Jupiter moved close to Saturn in the constellation Pisces. The conjunctions were first calculated by the astronomer Johannes Kepler in 1603. The first conjunction may have started the magi on their journey to Israel. The second conjunction may have guided them. The third conjunction in December may have pointed to the birth of Jesus. However, there was also a conjunction of Venus and Jupiter in Leo in June 2 B.C. In AD 314 Emperor Constantine the Great changed the date of the birth of Jesus from 1 March to 25 December to be the same date as a pagan Sun festival. The star seen in the east to guide the wise men is only mentioned in the Gospel according to St. Matthew.
History of experiments in this document
Astronomy and space science experiments (this document) is a revision, updating and expansion of the New UNESCO source book for science teaching, UNESCO, Paris, Third impression 1979, ISBN 92-3-101058-1 by Dr John Elfick, School of Education, University of Queensland, Australia assisted by Mr R. Smith, Central Queensland University, Australia, working under UNESCO contract 8347201.
The first stage in the editing process was done in China and was published in Chinese as "GUOWAI ZHONGXUE SHIYAN DILI (Overseas Middle School Experiments, Geography) J. Elfick editor Authors: Lin Peiying and Zeng Hongying, Capital Normal University Press, Beijing, December 1996 ISBN 7-81039-805-9/G.662 Price Yuan 7.50. The difficult work of co-ordination and interpretation was done by UNESCO Assistant Programme Officers Mr Howard Jiang and Ms Ye Mai. The publication was used for inservice training and was thoroughly reviewed by geography teachers in China. This book was on the Ministry of Education, People's Republic of China "All China Approved Book List for primary and Secondary Schools" and is on sale to the public in China. This book was designed to give a wider choice of experiments to teachers of geography in Chinese middle schools. The amount of descriptive detail in the experiments is designed to be the minimum needed for doing the experiment by a trained geography teacher. Each experiment is thought to be one of the simplest and least expensive ways of displaying the concept. However, a teacher should check the experimental details in a geography text recommended for use in that school system.