Maths Year 2000 Museum
    Orrery Maths Year 2000
© Islington Artefact Library
Click Here for Interactive Exhibit An orrery is a mechanical model of the solar system used to demonstrate the motions of the planets about the sun. The first orrery was probably invented by George Graham (d. 1751) under the patronage of Charles Boyle, 4th Earl of Orrery. The orrery presents the planets as viewed from outside the solar system in an accurate scale model of the periods of the revolution of planets about the sun (with the Earth completing a year's rotation in about 10 minutes). The planets' sizes and distances, however, are necessarily inaccurate.

Our calendar year is roughly the time it takes for the earth to go around (orbit) the sun once. At the same time as it orbits the sun the earth is spinning on its axis, which is tilted slightly. As the earth spins throughout the day, when the part of earth we are on faces the sun we receive light and call it daytime. The length of time we call a day is roughly the time it takes for the earth to rotate once. Different cultures had different ideas about when the day began. The Ancient Egyptians' day began at dawn. For the Babylonians, Jews and Muslims, day began at dusk. The ancient Romans decided that day began at midnight and this is still the convention we use today. Our calendar month is roughly based upon the time it takes for the moon to orbit the earth - a period of 28 days. In the time that the earth takes to orbit the sun once, the earth spins on its axis just over 365 times. To accommodate the fact that the length of a year is really 365 1/4 days we have an extra day every fourth year, which is called a leap year.

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