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The Human Orrery

Mark Bailey, Apostolos Christou and David Asher have developed ideas for a new outdoor exhibit in the Observatory grounds, called `The Human Orrery'. This will feature an accurate scale model of the positions and orbits of the six naked-eye planets, two comets, and an asteroid. The objective is to engage visitors to Armagh in a thought-provoking and inspiring open-air exhibit, and to use accompanying leaflets and activity sheets to introduce fundamental ideas about the Earth's position in space. The Orrery, which is the first major addition to the Armagh Observatory Grounds and Astropark for more than a decade, will be located in an area roughly 24 metres across located south of the existing Library between the Robinson Dome and the planned new Library, Archive and Historic Scientific Instruments building (see Figures 13 and 14). It will help educators to communicate and explain key ideas in solar system astronomy, particularly orbits and time-dependent phenomena, and will eventually provide pointers to galactic and extragalactic objects.

Figure 13: Location of the Human Orrery, between the Robinson Dome and the new Calver Dome. The position of the proposed new Library, Archive and Historic Scientific Instruments building is shown in outline. Drawing courtesy of Leighton Johnston Associates.

Figure 14: The orbits of the six naked-eye planets, two comets (1P/Halley and 2P/Encke) and the main-belt asteroid (1)Ceres. Tiles are indicated at 16-day intervals for the terrestrial planets (Mercury, Venus, Earth and Mars), at 80-day intervals for Ceres and the two comets, and 160-day intervals for Jupiter and Saturn. The indicated scale is in metres.

The most dynamic feature of the Human Orrery is its capacity to explain Kepler's laws of planetary motion in a fun and interesting way. A leader has simply to clap out time intervals of 16 days, and his or her assistants move around the Orrery tracing out the orbit and speed of each object. For example, little Mercury whizzes round, followed by Venus, the Earth and a more sedate Mars, while Jupiter and Saturn creep forward, making just one revolution for every approximately 12 and 29 of the Earth respectively. This is a great way to appreciate the decrease of Keplerian velocity with heliocentric distance, and for the Educator to demonstrate concepts such as parallax and the occasional retrograde motion of Mars.

Even a cursory inspection of the model can be used to illustrate the division of the inner planetary system into two principal parts, while the heliocentric ecliptic coordinate system provides an introduction to astronomical frames of reference. The model 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 will allow investigations of the law of equal areas (Kepler's Second Law). Similarly, measurements can be used to illustrate the fact that the squares of the orbital periods of revolution of the planets are proportional to the cubes of their mean distances from the Sun (Kepler's Third Law). In this way, the Human Orrery can be used by educators as a rich, innovative and dynamic outdoor exhibit.

It is intended that these and other concepts will be developed through a series of Human Orrery leaflets and activity sheets. For example, each numbered tile will be labelled with the planet's heliocentric distance, true anomaly and ecliptic longitude, so that a look-up table can be used to locate the positions of the planets `tonight', or when you were born, or at any other potentially interesting date. Visitors to the Observatory grounds will be able to identify planetary alignments, transits of Mercury and Venus, the morning and evening visibility of these and other planets, and indulge in a host of other engaging, interactive pursuits.

Although the Human Orrery is primarily a tool to explain astronomy, the questions of distance-scales, time-scales, and the dynamics and spatial interrelationships of solar system bodies lead naturally to issues such as the calendar, seasons, the equation of time, and precession of the equinoxes. This leads to a possible extension of the basic model so as to incorporate a circular band, indicated in the Figure beyond Saturn's ellipse as simply a red circular path, which will display ecliptic longitude, the First point of Aries, , and the boundaries of the 13 zodiacal constellations through which the Sun passes in the course of a year, as well as the directions to various Galactic and extragalactic objects. The Human Orrery and its possible extension provides a further important opportunity for the Observatory to reach out to new audiences, and to explain why our modern world-view is so very different from that considered, for example, by the ancient Greeks or during much of recorded history.

It is planned that the Human Orrery will be constructed during 2004 towards the end the main work associated with the HLF Telescopes and Telescope Domes restoration project.