Class practical

Observations of the stars, planets and the Moon for students to make.

Apparatus and Materials

• Camera with B (open shutter) setting

Health & Safety and Technical Notes

Caution students about where and when (and with whom) they make their observations of the night sky, so that they do not put themselves at risk. If appropriate, inform parents/guardians.

Read our standard health & safety guidance

Procedure

1. Ask students to observe the sky at least twice in one evening, with an interval of about two hours between observations. (It will help if pictures of a few easy-to-identify constellations are available before the observing time, so that students will recognize them and can direct their observations towards them.)
2. Ask students to watch the Moon and to note its position relative to the stars. Then, one or two hours later, look again and note the new position of the Moon relative to the stars. The Moon appears to travel across the star pattern.
3. Extend the previous experiment to a month. Note the position of the Moon at the same hour on each possible night for a month. The observations should relate to the stars, and also to the position in the sky relative to the horizon. Ask students to draw the phases of the Moon throughout the monthly cycle. (There will be times when the Moon is invisible during the night and will only be seen during the day. The rising and setting of the Moon can often be found from diaries or the newspapers.)
4. Show students the brightest planets - Venus, Jupiter and, possibly, Saturn.

Teaching Notes

• Students will need to be prepared for this observation in anticipation that a clear starry night appears. Normally the best times are during winter when the skies are predicted to be clear and a frost is forecast. Viewing the sky away from the city lights is recommended. These observations will probably have to be done at home for many students.
• A compass is helpful so that students know in which direction they are looking.
• A record of observations should be made.
• For step 1 it will help if pictures of a few easy-to-identify constellations are available before the observing time, so that students will recognize them and can direct their observations towards them.
• In step 2 the Moon appears to travel across the star pattern.
• For step 3. there will be times when the Moon is invisible during the night and will only be seen during the day. The rising and setting of the Moon can often be found from diaries or the newspapers.

This experiment was safety-tested in April 2007

Demonstration

Using a time exposure photograph to illustrate the apparent motion of the stars.

Apparatus and Materials

• Tripod, or other means of holding camera still during exposure
• Camera with B (open shutter) setting

Health & Safety and Technical Notes

Use a torch when setting up the camera and tripod. If students do this at home, they should make arrangements with parents or guardians to do it in a safe place.

Read our standard health & safety guidance

It is a good idea to cool down the camera by leaving it outside for some time before the exposure is set up, so that no condensation forms inside the camera.

The photograph will be more impressive if the picture includes the silhouette of the school building or of well-known trees near by. Avoid doing this at a time of month near a full Moon.

To get a B (Bulb) setting (open shutter) on a digital camera, you need to have your camera on manual setting and then decrease shutter speed. You will also need a cable release that you can lock. Otherwise the shutter only stays open as long as you keep your finger down on the button!

Have the lens aperture as wide open as possible so that you photograph more than just the brightest stars.

A digital camera or colour film will show the different colours of the stars.

Use a torch when setting up the camera and tripod. If students do this at home, they should make arrangements with parents or guardians to do it in a safe place.

Procedure

1. Take a photograph of the night sky by exposing a film in a rigidly fixed camera for an hour or more, and make it available for discussion.
2. To take such a photograph, attach a simple camera with an ordinary lens (not telephoto) to a firm stand or tripod. Point it towards the Pole Star, open the shutter on a setting that keeps it open indefinitely (though the aperture will usually have to be found by trial), and leave undisturbed for the period chosen (at least 2 hours, preferably 4 to 8 hours).
3. Encourage students who are interested to make a photograph themselves.

Teaching Notes

• The photograph will show arcs of a circle as the stars in the northern hemisphere appear to revolve around the Pole Star. The length of the arc, as a fraction of the circumference of the circle of which it forms a part indicates the time for the exposure as a fraction of 24 hours.
• For the southern hemisphere, there is no bright star close to the celestial pole. The southern pole star, Sigma Octantis, is only of the 5th magnitude, so the direction to point the camera will have to be judged from other stars.

This experiment was safety-tested in April 2007

Class practical

Tracking the motion of the Sun through the sky for a day, month or year.

Apparatus and Materials

• Either: Card with hole in it
• Or: Bowl (transparent, plastic, hemispherical), Card (large piece), Card (smaller, with hole in it)

Health & Safety and Technical Notes

It is essential to remind students that they must never look at the Sun directly, not even through a pinhole camera. This could blind them.

Read our standard health & safety guidance

One way to observe the Sun's changing position is to put a card with a hole in it on a window which faces the Sun. A bright spot will appear on an opposite wall. A short time later, move the card so that the same spot on the wall is illuminated, and mark the position of the hole in the card on the window (with a yellow sticker?). Record the path of the Sun until it disappears from that window (but there may be other convenient windows). This demonstration can be continued at daily/weekly intervals.

A second way to observe the Sun's changing position over the course of a day is to invert a transparent plastic hemispherical bowl on a large piece of card on which the central point is marked. Place a smaller card with a hole in it on the surface of the bowl so that the Sun shines down onto the central spot and the position of the hole in the card can be marked on the hemisphere. Repeat this throughout the day and for as many days throughout the year as possible.

Procedure

1. Ask students to watch the Sun and its daily movements, if possible from month to month, so that the changes in the height of the Sun's daily arc can be observed.
2. Over a period of time, note the star pattern in the vicinity of the Sun immediately after sunset and before sunrise.

Teaching Notes

Students should note the Sun's position at noon from month to month (take care if summer time is introduced).They should note the height of the Sun at noon at different times of the year. The Sun's path changes with the seasons: high in the sky in mid-summer and low in mid-winter. It rises precisely in the east to set in the west on those days which we call the spring and autumn equinox, (at about 21 March, 22 September), when day and night are equal in length.

This experiment was safety-tested in April 2007

User suggestion

Geoff Sargent: Mon 2 February 2009

Use software eg Winstars2 or Photodesk's Orrery. Either will model the sky from any point on the surface of the Earth and produce animations which show precisely what happens as declination changes.

Photodesk's Orrery permits the position of the observer to be dragged in real time about the Earth in real time as the animation runs. Interesting discussions.

Demonstration:

Students collect data and gain experience in using the inverse-square law for intensity of radiation. They use simple but ingenious apparatus to deduce a value that cannot be measured directly.

Apparatus and Materials

• Lamp, 240 V 150 W or 100 W
• Mains extension cable fed through earth-leakage circuit-breaker (ELCB)
• A4 paper, plain white, one sheet
• Optical pin or similar pointed object (e.g. drawing compass)
• Cooking oil, a few ml, in a small beaker or cup
• Tape measure or metre ruler

Health & Safety and Technical Notes

Make sure that participants do not look directly at the Sun. Ensure that the extension cable is safely positioned so as not to trip up passers-by, and that connections to the lamp and power supply are protected from moisture. Check that the ELCB is operating (by using its Test button) before use.

Read our standard health & safety guidance

The lamp should ideally be clear glass and held in a standard batten holder.

Procedure

This activity needs to be performed outdoors on a clear sunny day.

1. Use the pin to place a very small drop of oil on the paper – it should spread to form a translucent patch no more than 5 mm diameter and ideally smaller.
2. Hold the paper so that it is illuminated by the Sun on one side and by the lamp on the other, as shown in the diagram.

3. Viewing the paper from the ‘Sun side’, adjust its distance from the lamp so that the oil spot appears to merge with the surrounding paper.
4. Record the lamp distance d.
5. Assume that the light from the lamp and from the Sun varies in intensity according to an inverse-square law, and using the Earth-Sun distance of 1.50 x 10¹¹m, obtain a value for the Sun’s luminosity, L_sun.
6. Discuss factors that might affect the result.

Teaching Notes

• This activity can be carried out in a few minutes as a quick demonstration, followed by calculation and discussion.
• Alternatively, the demonstration could be followed by setting a challenge to students: how can they design their own experimental set-up so as to reduce uncertainties in measurement?
• When the spot appears to merge into the surrounding paper, the intensity of illumination due to the lamp (seen through the translucent spot) is the same as that due to the Sun on the surrounding paper.
• With D the Earth-Sun distance, d the lamp distance and L _lamp the luminosity (power) of the lamp (150 W or 100 W):
• L_sun /(4π D ²⁾ = L_lamp /(4π d² ) which can be rearranged to obtain a value for L_sun.
• The calculated value generally lies within an order of magnitude of the accepted value for the Sun’s luminosity: L_sun = 3.9 x 10²⁶W.
• In addition to uncertainties in judging and measuring the correct position of the paper, the result is affected by two sources of systematic error.
• Solar radiation that reaches the Earth’s surface is absorbed by the atmosphere. The amount of absorption depends on the elevation of the Sun above the horizon, and atmospheric conditions. Near midday in the UK in summer, on a clear day, about 30% of the radiation may be absorbed.
• The judgement of the correct position for the paper depends on sampling only the fraction of radiation to which human eyes are sensitive. As the Sun and the lamp have very different temperatures, they do not emit the same fraction of visible radiation.

This experiment comes from University of York Science Education Group:

Salters Horners Advanced Physics

Diagrams are reproduced by permission of the copyright holders, Heinemann.

Demonstration

To show the apparent movement of the stars through the sky.

Apparatus and Materials

• Umbrella, plain black

Health & Safety and Technical Notes

Make sure the umbrella is in good condition, with no exposed ribs at its edges.

Read our standard health & safety guidance

A simple model can be made from an ordinary umbrella. The star pattern drawn will depend upon your terrestrial location. You may be able to find a suitable star chart from the internet. For those in the northern hemisphere, when it is opened, the ferrule can represent the Pole Star, and constellations such as the Plough and Cassiopeia can be represented by paper discs stuck in the appropriate places to the underside of the umbrella, as shown in the diagram.

Chalk marks are not satisfactory: they tend to rub off when the umbrella is closed again. The eight ribs provide a useful guide in the marking up. Pairs of ribs enclose 45° (or three hours of time). The surface can include the circumpolar stars visible from your latitude.

An alternative would be to use a model of the celestial sphere.

Procedure

1. Spin the umbrella to show the apparent movement of the stars.

Teaching Notes

It is better to avoid the spinning Earth interpretation for beginners.

This experiment was safety-tested in April 2007

Demonstration

Making a model that shows the apparent motion of the Sun and stars around the Earth.

Apparatus and Materials

• Flask, large (e.g. 2 litre wide neck)
• Bung to fit flask
• Knitting needle, long
• Polystyrene ball ( 2.5 cm or suitable diameter to fit through neck)
• Retort stands, bosses and clamps, 2
• Sellotape, coloured
• Tripod with round top

Health & Safety and Technical Notes

Take normal care with glassware. Keep the model out of sunlight since flask and water can act as a convex lens and produce localized heating.

Read our standard health & safety guidance

Procedure

1. Mark the celestial equator and place the flask, with its neck slanting downward, in a chemistry tripod with a round (instead of triangular) top; or in a horizontal ring on a retort stand. Or support it in a clamp (it may be necessary to use a second boss to prevent the clamp turning under the weight of the model) so that the polar axis is inclined at about 50° to the horizontal.
2. The level of the water provides a horizon whilst the small sphere represents the Earth at the centre of the celestial sphere.

3. Add a band of coloured tape to represent the Zodiac, within which the ecliptic path will be included. This band should be at 23 1/2° to the equatorial plane of the model.
4. Rotate the model about its axis to provide a model of the repetitive, daily motion of the stars.

Teaching Notes

• The model: The Earth support rod represents the axis round which the Earth spins.
• The plane of the celestial equator is at right angles to the ‘Earth’s support’ rod through to the Pole Star. It is the ring where the plane of the Earth’s equator meets the celestial sphere. The celestial sphere revolves around an axis through the Pole Star.
• In a single night and day each star makes a complete circle around the Pole Star. Some stars are always above the horizon. Others dip down below the horizon for part of their journey. These stars rise and set.
• The model does not provide a description of the lagging and wandering motion of the Sun, the Moon and the planets. These are best explained by ignoring the Earth's diurnal rotation.
• You can stick twelve yellow stickers on the ecliptic band to represent the position of the Sun at monthly intervals. To early astronomers the ecliptic was the path followed by the Sun; today we know it represents the Earth’s yearly orbit round the Sun.
• With the daily motion frozen we can see the Sun crawling backwards from west to east along the ecliptic, which is inclined to the celestial equator at 23.5°. The Sun’s speed round the ecliptic varies; in the northern winter the Sun travels a little faster (the Earth is closer to the Sun) and so the four seasons are not exactly equal in length.
• At the equinoxes, the daily path of the Sun makes just half a circle slanting above the horizon plane between sunrise, due east, and sunset, due west. The Sun, which is always on the ecliptic, is also on the celestial equator. People living on the Earth’s equator will see the Sun directly overhead at noon.

This experiment was safety-tested in April 2007

Demonstration

The apparent motion of the Sun and planets around the Earth, including epicyclic motion.

Apparatus and Materials

• Links to animations, planetariums and orreries that could be used in this experiment are given in the guidance note:

  Teaching aids: star charts and model planetariums

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Draw a large sketch of the path of a planet as seen from the Earth. Planets that are currently observable in the night sky can be found in The Yearbook of Astronomy (Macmillan).

2. Show students an oblique view of an epicycloid. This can be drawn freely with a felt-tip pen by moving the pen round in a small circle (diameter about 10 cm) whilst sweeping the hand round a larger circle of diameter about 50–100 cm.
3. Tear out a patch of paper from the resulting pattern and hold it obliquely so that students can see the pattern almost edge on.

Teaching Notes

• A few stars show entirely different behaviour from the others. They were the ones singled out by some of the earliest observers to be watched with great care and awe. We call them planets, using the Greek name which means wanderer.
• Like the Sun, the planets sweep round the star pattern in daily motion. Freezing out that daily motion, we find that each planet slips slowly backwards from west to east through the star pattern in the course of years, along a path in the Zodiac belt. Unlike the steady motion of the Sun round the ecliptic, the outer planets have an irregular motion through the star pattern. They slide backward for a time, come to a stop and then move forward again. The backward motion from west to east predominates, carrying the planet, Jupiter, for example, all the way round the Zodiac in 12 years. The short forward motion in which the planet makes a loop (seen almost sideways on) occurs almost once a year.

• The path that we see the planet taking through the star pattern seems to be an epicycloid; that is, a compound of motions round a small circle and a big one.
• Optional: display the Information about the planets (see below).

This experiment was safety-tested in March 2007.

Demonstration

A demonstration of epicyclic motion.

Apparatus and Materials

• Turntable
• Small DC motor on a base plate
• Polystyrene ball
• Knitting needle
• Battery, 1.5 V

Health & Safety and Technical Notes

Read our standard health & safety guidance

Make sure that the polystyrene ball is firmly fixed and the sharp point is protected.

Procedure

1. Attach the motor base plate to the turntable. When the battery is connected it drives the motor, which rotates a polystyrene sphere in a small circle as shown. At the same time rotate the turntable slowly by hand.
2. The epicyclic motion of the small sphere can be observed by the class looking edgewise at the model.

Teaching Notes

This experiment was safety-tested in April 2007.

Demonstration

A home-made planetarium.

Apparatus and Materials

• Round-bottomed flask, 2 litre
• Lamp, 12 V 36 W
• Aquadag (colloidal graphite in water)
• Clamp stands, 2
• Suitable support for flask (see diagram)

Health & Safety and Technical Notes

Care is needed to make sure the flask is not dropped. The flask surface will get very hot after a while.

Read our standard health & safety guidance

Improvise a planetarium using a two litre round-bottomed flask (preferably the type with a wide neck).

Support a 12 V, 36 W lamp at the centre of the flask. Coat the surface of the flask with Aquadag (colloidal graphite in water). Scratch holes in this surface to represent the pattern of some of the major constellations.

Procedure

1. Switch the lamp on in a darkened room. Rotate the flask and the spots of light on the ceiling will rotate and display the simple daily motion.

Teaching Notes

• It is recommended that a model of the Solar System (orrery) should not be shown at this stage unless the heliocentric world of the solar system (the work of Copernicus) has been discussed.
• Where a visit to a planetarium can be arranged this can be a valuable part of the teaching, particularly if a special programme is arranged. There are many simple planetarium models which can be purchased from Science Museums and toy shops.

This experiment was safety-tested in April 2007

Demonstration

Drawing diagrams of eclipses of the Sun and the Moon.

Apparatus and Materials

None required.

Health & Safety and Technical Notes

Remind students that they must never look directly at the Sun (except through approved solar eclipse viewers). This could blind them (eclipse viewers too must be carefully checked for damage before each use – hold each one in front of lamp – check no cracks or pinpoints of light.)

Read our standard health & safety guidance

Procedure

1. Sketch an eclipse of the Sun for the class as shown (not drawn to scale); the Sun is much too near and Moon is too big and too near.

2. Sketch the same situation, but to scale, showing the shadow cones of the Moon and the Earth.

3. Sketch another diagram to scale, but reducing the scale so that the Sun, Moon and Earth are in the picture. The small circle is the Moon’s orbit. The Earth, at the centre of that circle, is too small to show. On this scale the Earth is a dot 1/800 centimetre in diameter. The Moon is much too small to show. (The diameter of the Moon's orbit is half the diameter of the Sun. The Earth's diameter is 109 times smaller than the Sun's diameter.)

Teaching Notes

• Eclipses have always excited interest and sometimes fear. Early astronomers concluded from eclipses that the Moon shines only by reflected sunlight and that the Earth is round.
• The Moon produces a cone-shaped shadow, with an angle of half a degree to its axis at the apex. In an eclipse of the Sun, the tip of the shadow cone only just reaches the Earth.
• In an eclipse of the Moon, the shadow of the Earth, which itself will have narrowed by one Moon diameter out at the distance of the Moon, just covers 2.5 Moon diameters as the Moon passes through it.
• From that, and the half degree angle subtended by the Moon, you can show that the Moon must be about 60 Earth radii away, a distance now known precisely to the nearest metre.

This experiment was safety-tested in April 2007.

Demonstration

Using the model celestial sphere to show the precession of the equinoxes.

Apparatus and Materials

Model of the celestial sphere from the experiment

Model of the celestial sphere

Health & Safety and Technical Notes

Take care with glassware. Keep the model out of sunlight as flask and water can act as a convex lens and produce localized heating.

Mark a new axis on the sphere, perpendicular to the ecliptic. This can be marked on the flask with small circles of coloured sticky paper indicating where the axis meets the surface. (An ideal arrangement is to fix two small suction caps at these points.)

Read our standard health & safety guidance

Procedure

1. Hold the model so that it can revolve very, very slowly about that axis, whilst the whole model is imagined to be spinning very rapidly (10 million times faster) round the Pole Star axis.

Teaching Notes

• The precession of the equinoxes, as described by early astronomers (from a geocentric - Earth-centred - point of view), was a very obscure creeping motion of the whole system of stars around a special axis (the axis of the ecliptic). It was as though the whole Zodiac belt slipped very slowly round the celestial sphere, carrying all the stars with it and leaving the celestial equator attached to a fixed Earth. In that model it is difficult to describe, but Copernicus made it much simpler.
• The precession of the equinoxes is a slow rotation of the whole pattern of stars around the ecliptic axis, one revolution taking 26 000 years. This motion was discussed by Hipparchus (~190 BC to ~120 BC).

This experiment was safety-tested in April 2007

Related experiments

Model of the celestial sphere

Demonstration

Teaching about comets? Bring them to life in class with this memorable demonstration!

Apparatus and Materials

• Large polythene sheet to protect floor
• Bin-liner bag, to line bowl and draw comet together
• Mallet, to crush some dry ice to powder
• Substantial plastic bag, in which to crush dry ice
• Gardening gloves (heavy duty type)
• Gardening gloves (heavy duty type)
• Balance, to find mass of comet (and hence calculate a hypothetical kinetic energy)

For two comets...

• Dry ice pellets, 10 kg
• Garden sand, 1 kg
• Water, 2 litres
• Soil, 1 handful (organic constituent)
• Worcestershire sauce (organic constituent)
• Smelling salts (organic constituent)

Health & Safety and Technical Notes

Dry ice sublimes at -78°C and will cause serious skin burns on contact, but momentary contact is unlikely to be a problem.

Do not confine in a sealed container as it will explode.

10 kg of dry ice will produce 5 m³ of gas, raising the level of CO₂ from 0.035% (natural) to safe-limit (USA) of 0.5% in a room 3 m high by 19 m on a side.

Make sure there is adequate ventilation, although if the dry ice is transported in a substantial expanded polystyrene box, little will sublime.

CO₂ is heavier than air therefore pools at ground level.

In theory trapped gas could fracture the comet or cause it to split, but this has never been recorded.

Read our standard health & safety guidance

The essential ingredients are dry ice, sand and water. The other items represent the organic molecules thought to be present in a comet. If it feels as if the comet will not bind into a snowball, it is because you have not used enough water. There is a natural tendency not to want to use too much water for fear of evaporating all the dry ice.

Do this in a well-ventilated area. Wear safety spectacles and gardening gloves.

Procedure

1. Line mixing bowl with bin liner.
2. Pour in half a litre of water and several handfuls of sand.
3. Stir and add crushed dry ice.
4. Stir and add Worcestershire sauce, soil and smelling salts.
5. Add more water. Make sure that there is a fairly violent release of CO₂, which indicates that you are cooling the mixture.
6. Draw the mixture together with the bin liner and squeeze between your gloved hands. You will feel the comet is binding into a solid mass. If it feels loose you require more water and may require more crushed dry ice. Uncrushed pellets on their own will not cool the water fast enough to form a solid mass.

Teaching Notes

• We gave a talk about comets before making our comet. We then found its mass and from this calculated the kinetic energy it would have if it struck the Earth at speeds of order 30 kilometres per second (the Earth orbits the Sun at 30 km s^-1) and compared this with the kinetic energy of an aeroplane.
• At the start of the class, we put a banana skin in with the dry ice, taking it out just before we made the comet. When the cold banana skin is dropped on the bench it shatters like china. This is a fun demonstration and also provides a vivid warning of the danger of dry ice.
• Comets, long thought to be fearful omens of trouble and doom, are popularly described as "dirty snowballs".
• Recent comet space missions reveal dry dusty or rocky surfaces, suggesting that ices are hidden beneath their crusts. It is now thought that short-period comets originate in the Kuiper Belt (beyond Neptune's orbit) whereas long-period comets originate much further from the Sun, in the Oort cloud.