Bringing the planets closer home

Veena Prasad shows you how to elevate lessons about the solar system from being just another boring activity to an awe-inspiring experience.

We all remember a picture of the solar system that we drew in our school books — The sun in the centre, surrounded by somewhat elliptical orbits, each punctuated with a small circle representing a planet. Those of us who paid attention made sure Jupiter was the biggest circle of all, but smaller than the sun. But how many of us wondered exactly how much smaller? Or how far the planets were from each other?

Did we get it right on the paper? Did some of us have a suspicion that perhaps a sheet of notepaper was too small to represent the immense distances of space? Looking up at the night sky, and seeing glowing objects a little bigger than pin heads certainly seemed to confirm this suspicion. They were so far away — how did anyone even measure this distance? Were there some of us who wanted answers to these questions, and did we try and find them in the text books or in the classroom? Did we find them there?

We found some information, but not perhaps in a way that inspired the awe that this immensity deserves. We learnt about ‘light years’, about masses of planets, and in a vague way understood that they are enormous. We learnt well, no doubt, and our teachers were pleased with our answers, but there was no drive to find out more, pose questions, and work hard till the answers were found — simply for the sake of finding answers.

A little exercise while trekking through the solar system may provide just the spark. This will require the students to be familiar with three ideas. First of course, they need to know the general layout of the solar system. Secondly, they need to know the units of measurement for distances in space, such as Astronomical Units and light years. And finally, they must have a good understanding of “drawing to scale”.

Following these initial requirements, we need to list down the nine planets (yes, nine — who has the heart to leave out Pluto?), their sizes and distances from the sun. A planet’s distance from the sun is an average one, because its orbit is elliptical and this distance is not constant at all times.

Given in the table is a chart listing the planets, their distance from the Sun, and their sizes relative to the Earth. While drawing a model to scale, it is useful to have a relative measure rather than an absolute one, as we shall see in the coming paragraphs.

The distances in the solar system are usually measured in Astronomical Units. As it turns out, 1 AU is defined as the distance from the Earth to the sun, making each planet’s distance from the sun also relative to the Earth’s.

Armed with this, the class can jump into an adventure. But first, weneed to have a small discussion.

Teacher: “Look at the sizes of the planets. Do you think you have drawn them to scale?” A lively debate should follow, after which the class comes to a conclusion. Most Jupiters drawn in the classroom are perhaps not 11 times Earth, and the size of Sun in the last row should definitely be an eye-opener. The debate is settled.
Teacher: “Perhaps your notebook is not big enough. Shall we try fitting it on the blackboard?”

Again this question is thrown open to discussion. Perceptive students might notice that the Sun, being 109 times larger than the Earth, might not fit into a blackboard. If you drew a 1 cm wide Earth, you would have to draw the Sun 109 cm (or three and a half feet) wide! Should we try moving to the playground?

Since we have chosen such a big canvas, we might find it easier to place objects to represent the planets and Sun instead of drawing them. A hula-hoop or a large beach ball would make a good Sun. Proportional to this you need a tiny earth. A shirt button would be ideal. (To decide on the right size of shirt button, imagine 109 shirt buttons fitting across the hula-hoop.) Then we choose other circular objects to represent the rest of the planets  — For correct sizes, refer to the table we just drew up, comparing everything to the button that represents our earth.

Mercury would have to be a really tiny button, and the Venus button can be the same size as Earth, but a different colour. Mars is half the size, but Jupiter is big. A tennis ball might be just the size. Saturn is a little smaller, so a golf ball should do. Uranus and Neptune can be shown as marbles. But what of Pluto? It is so tiny that you only have to make a little dot.

Now that we know what objects to use, let us look at where to place them, and how far apart. All the distances in our chart are in Astronomical Units. We need to know how long an Astronomical Unit is, compared to the size of the earth. This is because we are thinking in terms of relative measures which will help us in making this model to scale. A little reference will tell us that 1 AU is approximately equal to 10,000 times the diameter of the Earth! So if our “Earth button” were 1 cm across, you would need to place the Sun and Earth 10,000 centimetres, or 100 metres apart.

Assuming we have a playground with a 100 metre racetrack (or enough space and a measuring tape), we can put the hula-hoop (sun) on one end and the button (Earth) on another. Between these, we place Mercury and Venus, as per our chart — Mercury about 40 metres away from the Sun, and Venus 70 metres. Beyond Earth lies Mars. In our model, this would be a small button 50 metres away from Earth. When we come to Jupiter, we realise that our troubles are just about to begin.

Jupiter is about five times as far away from the Sun as Earth is. In our scale model, that would make it 500 metres, or half a kilometre away. At this point a familiar landmark such as a traffic signal half a kilometre away from the school can be suggested to the children to get an idea of how far this planet is. Saturn is almost one kilometre away. Another familiar landmark can be thought of. But what of Pluto? At almost 40 kilometres away from the Sun, you may have to step into the neighbouring city. Or at least, imagine doing so.

Phew. Is all this really necessary for —what is it — five marks? Perhaps not. But for sheer awe-inspiring sense of the scale of our solar system, nothing can beat this exercise. As a bonus, you may have even created an astronomer or two!