A Projecting Planetarium

In April 2005 the Whipple Museum acquired a planetarium for use with a candle or lamp, which projects an image of the planets onto a wall. We don't know who made it and no contemporary catalogue or text describing it has been found. By examining the planetarium closely, we can suggest what astronomical phenomena it was meant to demonstrate.

The astrolabe was a key astronomical instrument during the Middle Ages. It can be used to tell the time, measure the heights of stars and buildings, and for many other calculations and observations. It works because it is a 2-dimensional representation of the 3-dimensional Universe as understood by the medieval astronomer - with the Earth at the centre of the apparent motion of the Sun.

A planetarium for projecting images

A planetarium demonstrates the motions of the planets. On this particular instrument (Image 1), the rings are rotated by turning a crank, moving a system of cogs. Each ring carries a planet or one of its satellites. The signs of the Zodiac appear on the edge in reverse order. This planetarium was intended to be used with a projecting device, such as a magic lantern.

The holes through which Mercury, Venus and the Moon are displayed are sometimes partially covered by part of the mechanism, enabling the phases of the Moon and planets to be shown (Image 2).

The three outer planets, Jupiter, Saturn and Uranus, are fixed in place. The holes through which their satellites can also be covered by a cog. The projected image then shows the satellites passing into the shadow of the planet, and out of sight of observers on Earth.

The planet Mars is absent, probably because it doesn't demonstrate phases and wasn't known to have satellites at the time the instrument was made.

How do we date this object?

When there is no date of manufacture on an instrument, a date can often be determined by the appearance of the instrument and by comparing it to others that are similar. In the case of this planetarium, other information offers clues about the date. For example, in 1848 an eighth satellite of Saturn was discovered. Our planetarium has only seven. It is very likely that this planetarium was made before 1848.

Another piece of astronomical information is also helpful in dating the planetarium. William Herschel discovered the planet Uranus in 1781; he named it Georgium Sidus (George's Star) in honour of King George III, the reigning monarch. Herschel discovered two of Uranus' satellites in 1787, and a year later suggested that there were probably four or more in total. Then in 1797 he announced a total of six satellites.(1) Our planetarium has Uranus with six satellites (Image 3), so it was probably made after 1797.

How many satellites does Uranus have, according to different astronomers?

Following Herschel's discovery, astronomical textbooks normally stated that Uranus had six satellites. But some astronomers in the first half of the 19th century questioned this. Thomas Keith (1759-1824) lists six satellites in his A New Treatise on the Use of Globes, but when R. A. Le Mesurier revised the text  in 1848, he suggested that Herschel had "imagined" six, but that only two had been observed by others.(2) Herschel's own son, John Herschel (1792-1871) had already questioned the number when he wrote in 1833 that there were "probably" five or six.(3)