Mark Bailey, Armagh Observatory
"The Armagh Observatory Human Orrery"

The Armagh Human Orrery is a dynamic model of the solar system in which people play the role of the moving planets. The users' interactions with the model lead to greater awareness of their place in space and understanding of our planet's changing position with time. It is an innovative concept, the first in the world to show with precision the elliptical orbits and changing positions of the main bodies in the solar system. It is simple to use, and draws people into mathematics and science in a fun and entertaining way, introducing key concepts in astronomy and space science.

The model is constructed to a scale of 1 metre to 1 astronomical unit. It shows the orbits (and positions versus time) of the six classical planets, a main-belt asteroid (Ceres) and two comets (Halley and Encke). It contains more than 200 individually inscribed discs showing the positions of objects at intervals of 16 days or multiples thereof. The region beyond Saturn shows the thirteen ecliptic constellations and directions to more distant objects in the Universe. The concept is as simple to use and as versatile as a sundial. Many possible variations can be constructed, for example using chalk or paint on a playground or string and tent pegs on a playing field. Temporary objects, such as bright comets or spacecraft trajectories through the solar system can also be included. Activities include 'Walking the Orrery'; identifying which planets are visible tonight (or at any other time); and discovering phenomena such as planetary alignments, conjunctions and transits. Younger users can 'Run the Orrery' (and how fast can you go!); measure the distance between planets or a planet's speed in different parts of its orbit; or use the open space to create an innovative 'dance of the planets'. Advanced users can discover Kepler's laws by direct measurement and investigate subjects such as modular arithmetic, the properties of conic sections, and calendrical concepts such as leap years and the need for the Gregorian calendar reform.

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