How far is the Earth from the Sun? And the other planets? Here is a simple laboratory to propose in the classroom. Thread and clothespins are enough.
Astronomy fascinates and intrigues students of all ages, but the magnitudes involved escape everyday experience: how far is the Earth from the Sun? And Saturn? The billions of kilometers do not help understanding or imagination.

In this astronomy laboratory we propose a very simple method to restore the distances between the planets of the Solar System and the Sun to everyday gestures. We will use thread and clothespins (and images of the planets or socks, if you prefer) to make a “scaled” solar system without even having to do a calculation.

We will help you with three clues that stimulate different skills.


  • Required material (for each group):
  • 10 clothespins
  • A rope of a few meters
  • Images of the planets and the sun
  • Pull the thread between two chairs or simply lay it on the floor. Fix the Sun at one of its ends with a clothespin.

Fix Pluto at the opposite end with a second clothespin. Where are the other planets? Using the pegs and images of the planets, try to identify the position of all remaining planets by hanging each image in the relative position.

If you do not have the images of the planets, just write on each clip the planet it represents.


Divide the boys into groups of three or four. Give each group the material to create the Solar System.

The students will have to hang the clips at the wire at distances such as to correctly represent the real distances, shown in the table and expressed in astronomical units (ua).

Remind students of the definition of an astronomical unit, motivating it: 1 ua corresponds to the average distance between Earth and the Sun, which is about 149,000,000 km. Why are astronomical units used instead of kilometers? To simplify your life: even the distances of the planets are more understandable. Saturn, for example, is far from the Sun 9.6 ua, that is almost 10 times more than the Earth.

The clue 1 is the following: hang the Sun at one end of the wire and Pluto at the other.


The second clue must be given together with the previous one: observe the numerical sequence in the table very carefully.

You can solve the problem with good precision and without making calculations. Let your students rack their brains and discuss for about ten minutes. Each group will choose one of these two ways: a pure “eye” strategy or a “hand” or “foot” strategy. In the second case, students try to use hands or feet or fingers as a unit of measurement on which to set approximate proportions.

Over the years, the most creative group I worked with tried to use the nose, attempting the word play with NASA, evidently with terrifying results.


Give the groups the clue 3: Find a graphic or an image (ex: the one present on the number 2 of Focus School) where the distance between the planets is proportional to the real one. Look at it carefully and try to find the regularities.

In my experience, at least one group arrives at the solution within a few minutes.


Notice that Pluto is about 40 ua from the Sun. Divide by two and find 20 ua. It is about the distance of Uranus from the Sun. Operationally this means that if you fold the thread in half, Uranus is at the midpoint.

Now take the thread folded in half and fold it again (model sheet). The middle point of the first half corresponds to about 10 ua, while that of the second half to about 30 ua.

Take the folded thread twice already and fold it in half again: the outer part of the thread is now empty, there are no intermediate planets, but between Saturn and the Sun there are several. With this move you found Jupiter (at 5 ua). Again bend the line between Jupiter and the Sun and at 3 ua (about half of 5, and this is the largest approximation) find the asteroid belt, with the dwarf planet Ceres. If you continue to bend, you will find Mars (1.5 ua), Venus (0.7 ua) and Mercury (0.4 ua) one after the other. The only exception is the Earth.

But we know that the Earth is exceptional: we are on it. Let each group explain its strategy and discuss it with others.

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