Ever wanted to play the role of a planet, asteroid or comet? Now you can, thanks to the new Human Orrery at Armagh Observatory, writes John Moore.
How many planets are there in our solar system and where is Earth situated among them? Many people will know the answer to this question: nine; and third planet from the sun, respectively. Probably fewer can describe Earth's position in space in relation to Mars or Jupiter, or when Venus is a morning or evening star.
The new Human Orrery at Armagh Observatory - launched last week - can help answer these more difficult questions. Constructed on the ground as a series of stainless steel tiles - each representing the positions and orbits of six naked-eye planets, an asteroid and two comets - the Human Orrery explains how our heliocentric solar system (a system centred on the sun) works.
"The heliocentric system of celestial bodies orbiting around the sun isn't very well understood," says Prof Mark Bailey, director of the observatory.
"The Human Orrery lets people play the role of a planet, an asteroid or comet on any particular date and time, immediately giving them a feel both visually and physically for where each is, how fast they are moving, their distances to each other or the sun."
Each tile is positioned on the ground to scale accuracy better than 1 centimetre, explains Prof Bailey. The terrestrial planets (Mercury, Venus, Earth and Mars) are placed at 16-day intervals, the asteroid and comets placed at 80-day intervals and Jupiter and Saturn at 160-day intervals.
Several bits of information are written on each tile about the celestial bodies - name, positional date, distance from the sun, ecliptic longitude and more - and this allows the user to work out which objects are visible at any given time, relative to each other or to the sun.
"You can say to people: 'Go and discover planetary alignments, conjunctions and transits of past and future events, or work out the orbital mechanics of Kepler's three laws'," says Prof Bailey.
"Suppose that you wanted to explain the orbit of the Cassini spacecraft and how did it get to Saturn? You could lay out a rope from an Earth tile and with sufficient information, show all the complicated fly-bys that got it there."
Or suppose a totally new object like a comet is discovered. The Orrery would allow you to see when it's going to be visible. There are so many ways you can use it, Prof Bailey enthuses.
"When you play long enough on it you get a more grounded view of each object's place in the solar system," he says. "You can easily write formulae to work out where each is located at any particular time, but it really doesn't sink in until you actually see it on the ground before you.
"It's really a way of turning what used to be a rather dull, dry subject of celestial mechanics, co-ordinates systems, geometry of the orbits, into something much more exciting and interesting."
Orreries were popular during the 18th century as educational models that used a mechanical, clock-driven system of gears to show the relative movements of the planets around the sun. Named after Charles Boyle (1676-1731), Fourth Earl of Cork and Orrery - who happened to commission a fine example - they ranged from "grand orreries" a metre in diameter to small portable devices featuring just the Earth, moon and sun.
The Human Orrery project is the first major addition to the observatory's grounds for more than a decade. It is free to all visitors every day during daylight hours. Activity sheets and personalised introductions are available, explaining how the Orrery works, and visits by schools and other parties associated with and interested in astronomy are encouraged.
* For more information, visit star.arm.ac.uk/orrery.html