An ancient-modern orrery


or how to navigate in the night sky by using ancient technology

An orrery or a planetarium is a construction which depicts the motion of the planets in the sky. Typical orreries place the sun at the centre, with the planets moving around it. These can be simpler to contruct, because the motions are more or less circular around the sun. However, if we want to navigate in the night sky, the heliocentric model (sun at centre) is not the right one to use for us on earth. For this purpose, one instead needs to place the earth at the centre (geocentric model). In this case, however, building a planetarium is more complex due to the epicyclic motions of the planets, from the earth's reference frame. This is exactly what the above construction does: by using truly circular gears it models the retrograde motions of the planets. This is done in a very economic way, by utilizing technology inspired by the Antikythera Mechanism.

  • Quiz
  • By looking closely at the inner (blue) circle of the above planetarium, you will see the zodiac, with corresponding constellation names. What is the zodiac and its significance? Why all planets lie on it and not at other parts of the sky?

  • Take a look at the pointers as they go around in the video. Identify the sun & planets, marked with the following symbols:
    Sun: ☉
    Mercury: ☿
    Venus: ♀
    Mars: ♂

  • Can you spot the retrograde and prograde motions of the planets on the zodiac? Why is that?

  • Inferior planets are the ones closer to the sun, as compared to the earth, i.e. Mercury and Venus. Observe them closely. Can you spot how they go around the sun? Why do they always appear to be on the vicinity of the sun (within a limited angular range)?

  • Superior planets are the ones further away from the sun, as compared to the earth. Take the example of Mars. Can you see that in this case, the angular range around the sun is not restricted? Why is that?

  • The following table shows the naked-eye planet periods for orbiting around the sun. Based on the above points, can you see why these period relations are not so easy to see straightforwardly in a geocentric model?

  • Planet Orbital Period (y)
    Mercury 0.241
    Venus 0.615
    Earth 1
    Mars 1.881
    Jupiter 11.86
    Saturn 29.46
  • In fact, the periodicities modelled in this planetarium are much longer than the orbital periods around the sun. This is due to the earth rotating as well, and not just the other planets. The above planetarium uses Babylonian "Goal-Years" to keep everything in track.

  • Did you know that your astrological star-sign is actually one constellation out of step? You can learn why here.

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