Museum Notebook: Device explains the puzzle of motion

Orrery, a mechanical model of the solar system. Whanganui Regional Museum Collection ref: 1802.2605

Here's a quiz question for today: what is an orrery? Is it a) a group of owls, b) a musical instrument, or c) a mechanical solar system model?

The correct answer is c) and there are two of them in the Whanganui Regional Museum collection. They are small, not very impressive to look at, but have the very useful function of modelling the motion of planets orbiting the sun.

The apparent movement of planets and stars across the sky has fascinated humans in every culture and every time period. The daily rotation of Earth on its axis gives us sunrise and sunset but also causes the night-time sky-show of stars rising and setting. The Earth also maintains an annual orbit around the sun, producing predictable appearances and disappearances of constellations throughout the year. For thousands of years this has helped us to organise our lives and inspired seasonal festivals including the celebration of Puanga-Matariki here in Aotearoa. Ancient stone structures such as Stonehenge and the pyramids of Egypt are believed to be aligned with the celestial events such as the solstice and the rising of particular constellations.

Until the development of modern astronomy, people in many cultures around the world believed that it was the sun and stars that moved, while humans and the Earth remained fixed. It was shocking, to the point of blasphemy, for early European astronomers to assert that it was the sun which remained fixed, while Earth and all the other planets revolved around it.

One of the clues to this puzzle of motion was the difference between the stars and the planets. Like the sun, stars rise and set in very predictable places. The planets, once referred to as "wandering stars" have their own specific orbits around the sun and this can make them appear to change direction over a period of nights. However, planetary movements are still predictable, and a mechanical orrery makes this much easier to see and understand.

Thanks to centuries of clockwork inventing, people already knew how to get hour, minute and second hands doing their own things at the right relative speeds. For an orrery the cog wheel gear sizes are adjusted to move at the correct relative speeds for each of the planets. The earliest clockwork orreries were made in the 1700s. The much more recent examples in the Whanganui Museum collection were made in the 20th century as educational tools from modern materials such as perspex.

Over time, people have invented a variety of ways to model the movement of stars and planets to help us understand and learn about the night sky. Some cities around New Zealand have purpose-built planetariums that project the movement of stars and planets onto a spherical ceiling.

During the coming week, Whanganui Regional Museum has the loan of an inflatable planetarium in which the night sky is simulated using tiny points of light projected onto the ceiling. Families are warmly invited to join us to learn all about the special stars Puanga and Matariki that mark Aotearoa New Year when they return to the morning sky. Sessions times are Wednesday, May 26, from 4pm-5pm and Thursday, May 27, from 7pm-8pm. The cost is $2 per person. Call the museum on 06 349 1110 to book your space.

• Margie Beautrais is the educator at the Whanganui Regional Museum