Build a tellurion.
Construct a home-built working model of the Sun, the Moon and Earth
This month’s project is a tellurion – a working model of Earth, the Moon and the Sun. As well as being educational, this model looks great when displayed in your living room or study. We designed our tellurion so that it’s simple to build, utilising parts that are readily purchased online. By making some careful calculations we were able to choose gear ratios to produce accurate movements of the Moon and Earth as the main arm is rotated by hand around the Sun.
The design is built around a central ‘year disc’. This has a section of GT2 belt glued around its circumference, forming a gear with 267 teeth. The model Moon is rotated by a 20-tooth pulley meshing with this gear, which gives a ratio of 20/267 (1/13.35). This means it will complete 13.35 orbits of the Earth per year or one orbit every 27.34 days – very close to the real Moon’s sidereal month of 27.32 days. Like the real thing, model Earth’s axis is inclined on its mounting by 23.4° from vertical and a pair of 20-tooth pulleys and a toothed belt make sure it maintains this inclination as the system orbits the Sun.
One full orbit around the Sun represents a calendar year. By adding a printed disc (free to turn so it can be synchronised with the positions on a known date) you can indicate the current month. During the northern hemisphere’s winter, Earth’s south pole is tilted in the general direction of the Sun, which appears low in the sky for us in the UK. In the northern hemisphere’s summer, the north pole is tilted in the Sun’s direction with a correspondingly high Sun for us. This demonstrates our seasons very nicely.
The tropics are observed as the most northerly and southerly points where the Sun is directly overhead.
It is possible to turn the Earth sphere by hand to demonstrate the passage of days and how the Sun appears to rise and set. By imagining the ‘fixed’ star field on the walls of your room and varying the position of Earth around the Sun it is possible to explain how the stars appear to move across the sky during the night, and why much of the night sky is only visible for part of the year.
Phases of the Moon
Because the Moon is fixed on its axis arm the same face always looks towards Earth (as it does in reality). We incorporated a small bulb in our model so the Sun can be illuminated, but a torch can also be used
to simulate the Sun’s rays striking Earth and the Moon. It should therefore be possible to demonstrate how the phases of the Moon are formed during a month. Note that in the model there is a solar and a lunar eclipse every month instead of the usual New Moon and Full Moon. The reason we get far fewer eclipses than this in reality is because the real Moon’s orbit of Earth is inclined to Earth’s orbit of the Sun by 1.54°, which means that most of the time the Moon is slightly above or below the plane of Earth’s orbit as it passes behind us or in front of the Sun – hence no eclipses. It is beyond the scope of our model to recreate this, but nevertheless it is easy to imagine the New and Full Moon.
The crucial elements of the model are the supporting arms. We found that model shops sell packs of 4mm, 5mm and 6mm brass tube that fit inside each other – perfect for the job. Earth and the Sun are table tennis balls; the Moon is a wooden ball from a craft shop. To build the rest of our model we used 6mm plywood, which is flat and quite easy to cut. We were going to stick a few layers together for the base but sourced a nice offcut of hardwood instead. It got us thinking that there are many ways to customise the tellurion. Whether you keep yours simple or get fancy, we are sure that you will have a lot of fun building and demonstrating it.
A tellurion shows the movements of Earth and the Moon around the Sun
An illuminated Sun means the tellurion can demonstrate Moon phases