Props

Demonstration 1

The first demonstration concerns the relative sizes of the planets and the Sun. You will need:

The Sun

I normally use a scale in which the Sun is 1 m across. Cut a circle of reasonably stiff yellow card 1 m in diameter. Since even A0 paper is less than 1 m wide, you'll probably need to make two half circles and tape them together. This is a good idea in any case, since it means you can fold the thing in half, which makes it more portable!

The planets

On a scale of Sun = 1, the diameters of the planets are as follows:

Mercury	Venus	Earth	Mars
0.0035	0.0087	0.0092	0.0049
Jupiter	Saturn	Uranus	Neptune	Pluto
0.1025	0.0867	0.0367	0.0349	0.0017

A page with images of the planets to the correct scale for a solar diameter of 1 m can be downloaded from the Downloads page.

Mount the images of the large outer planets on cardboard. The smaller inner planets and Pluto should be mounted between pieces of Sellotape or transparent plastic film (Like the stuff you can buy to cover books) to make them large enough to handle: a piece of Sellotape about an inch (2.5 cm) square is enough.

Notes

In a 45 minute talk, you do not want to display all the planets: it will take too long. I normally use the Sun, Jupiter (largest planet), Earth and Pluto (smallest planet). But it's useful to have the other images in case there is a particular reason to look at a different one (e.g. images of Saturn on the news). Get the kids to hold the planet images in fron of the Sun (held by two children) so that the audience can see the relative sizes clearly. Small children may need to be manoeuvred into position, as they often don't seem to have a clear idea of the audience's viewpoint.

Demonstration 2

The second demonstration concerns the relative sizes of planetary orbits. You will need pieces of string or ribbon to represent the orbital radii (strictly, the orbital semi-major axes). I use a scale in which the Sun's radius is 1 mm: this is handy because you can say that you have shrunk the Sun down to about the size of Pluto in the last demonstration (the scale image of Pluto is actually 1.7 mm across rather than 2 mm, but it's near enough to give them an idea).

The orbital sizes, on a scale of Sun's radius = 1, are as follows:

Mercury	Venus	Earth	Mars
83.2	155.5	214.9	327.5
Jupiter	Saturn	Uranus	Neptune	Pluto
1118	2050	4125	6461	8497

You can use either string or narrow Christmas parcel ribbon (don't use wool because it stretches). It is advisable to use different colours for the different planets, rather than trying to stick labels on: small sweaty fingers will quickly pull the labels off! Use mnemonic colours: white for Venus, light blue for Earth (the "pale blue dot"), red for Mars, purple for Jupiter the king of the gods, yellow for Saturn, green for blue-green Uranus, blue for blue Neptune, for example (Mercury and Pluto don't lend themselves to obvious mnemonics: I use purple for Mercury and red for Pluto, since they can't possibly be confused with Jupiter and Mars, but it's up to you: silver ribbon for Mercury, from the colour of the metal, and black for Pluto the king of the underworld would work well).

You can do the inner planets and Jupiter yourself quite quickly; get volunteers to hold one end of the string for the outer planets (I usually just do Saturn, Uranus and Pluto). If you're in a classroom, you may find yourself out in the corridor for Pluto - don't worry about it, it always gets a laugh!

Notes

On this scale:

the nearest star is 59 km away (pick a town at an appropriate distance, e.g. Leeds from Sheffield);
the centre of our Galaxy is 355,000 km away (approximately the distance of the Moon);
the nearest large galaxy (M31 in Andromeda) is 31 million km away (about one-fifth of the distance to the Sun).

(Equivalent distances in light years: 8 light minutes from here to the Sun, 4 light years to the nearest star, 25000 ly to the centre of the Galaxy, 2 million ly to Andromeda.)

Other Useful Props

It is useful to have a ball available (any size from tennis ball to football). A bright torch can be useful in case you are asked questions relating to the curriculum material on Earth, Moon and Sun, which includes discussion of the Moon's phases.