Navigating The Human Orrery



***Photo of Orrery***

The Human Orrery can be navigated on many different levels. The most simple, yet effective way of demonstrating its functionality is known as "Walking the Orrery". In this instance different people trace out the orbits of the planets and other solar system objects (Ceres, Halley and Encke).

***Walking Orrery photo***
Walking the Orrery (photo by Miruna Popescu)

Someone can be elected to clap out time regularly so that Mercury, Venus, Earth and Mars move one tile (from 1 to 2 to 3...) every clap; Jupiter and Saturn move one tile every ten claps; and Ceres, Halley, and Encke move one tile every five claps.

This demonstrates the most dynamic feature of the Human Orrery - its capacity to explain Kepler's laws of planetary motion in a fun and interesting way. The people playing the parts of the planets closer to the Sun move a lot faster than the more distant planets. (Kepler's Third Law states that the squares of the orbital periods of revolution of the planets are proportional to the cubes of their mean distances from the Sun.)

Below are two versions of an animation which show footprints moving along the orbits of Mercury, Venus and Earth. These animations loop - just press the "Return to Previous Page" button when you are finished viewing them.

The Human Orrery also shows that the planetary orbits (especially those of Mercury and Mars) are distinctly elliptical with the Sun not at the centre but at a focus (Kepler's First Law), whilst detailed measurements of the separation and radial distances of successive tiles allow investigations of the law of equal areas (Kepler's Second Law).

As well as showing planetary motions, the Human Orrery is adept at displaying alignments/conjunctions such as the "Grand Alignment" of 2040 September 8th involving Mercury, Venus, Earth, Mars, Jupiter and Saturn. The bright star Spica, which is on the Orrery's outer ring, is also aligned (the "best" date for Spica to be aligned with the planets is 2040 September 18th, when the alignment of the planets themselves is only slightly worse than on the 8th). More about this closer to the time. There are other reasonable alignments to look for in the nearer future. Other ideas that visitors to the Observatory grounds can use the Orrery for include: identifying transits of Mercury and Venus; determining the morning and evening visibility of planets "tonight" (or on any date of interest); and exploring concepts such as parallax.



Diagram of tiles for inner planets

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Created by Brendan Owens

Last Revised: 2009 November 6th