Drum model orrery, 1813-1822.
This planetary model was designed by the astronomer William Pearson (1767-1847) and made in London by John Fidler. Called an orrery or planetarium, it is a demonstration model to show the motions of the Earth, Moon and planets around the Sun. It has a brass drum base with a pedestal stand and seven planets (Mercury-Uranus) on long rod-arms. Such devices became popular during the 17th century, especially after Sir Isaac Newton (1642-1727) published his universal theory of gravity in 1687. To reproduce the motions of the solar system this model employs a hand crank to drive the gear wheels. This elaborate orrery is the only surviving example of several planetary models devised by William Pearson.
Credit: Science Museum
Uranus rings 'were seen in 1700s'
In a paper presented at the National Astronomy Meeting in Preston from 16 - 20 April, Dr Stuart Eves of Surrey Satellite Technology Limited will challenge the orthodox view that the rings around the planet Uranus were first detected during an occultation experiment in 1977. Remarkably, a paper presented to the Royal Society in December 1797 by the then King's Astronomer, Sir William Herschel, (who had discovered Uranus in 1781), includes a description of a possible ring around the planet.
In a paper presented at the National Astronomy Meeting in Preston from 16 - 20 April, Dr Stuart Eves of Surrey Satellite Technology Limited will challenge the orthodox view that the rings around the planet Uranus were first detected during an occultation experiment in 1977. Remarkably, a paper presented to the Royal Society in December 1797 by the then King's Astronomer, Sir William Herschel, (who had discovered Uranus in 1781), includes a description of a possible ring around the planet. Dr Eves believes this is the first observation of the rings that were not seen again for almost two hundred years.
Even Herschel was unable to confirm his possible sightings, and they were not repeated by several generations of astronomers who came after him. (Prior to 1977, when astronomers thought that Uranus lacked rings, Herschel's claims were dismissed as "clearly erroneous". And even after 1977, when the existence of the rings was finally established, it was suggested that the rings were far too dim to have been detected by Herschel's telescopes, and so his claim to priority was ignored).
However, a recent re-evaluation of Herschel's 1797 paper by Dr Stuart Eves of Surrey Satellite Technology Limited, suggests that Herschel's claim to have seen one of the rings may well have been correct.
"Herschel got a lot of things right", notes Dr Eves, "He has a ring of roughly the correct size relative to the planet, and he also has the orientation of this ring in the right direction. In addition, he accurately describes the way the appearance of the ring changes as Uranus moves around the Sun, and he even gets its colour right. Uranus's Epsilon ring is somewhat red in colour, a fact only recently confirmed by the Keck telescope, and Herschel mentions this in his paper."
But if Herschel could see the Epsilon ring in the late 1700s, why did no-one else follow up his observations in subsequent years as the telescopes astronomers used improved? "There are several mechanisms that could account for this", suggests Dr Eves. "The current Cassini satellite mission to Saturn is telling us that its rings are becoming darker and also expanding, (becoming more diffuse), over time. If these same mechanisms are also operating at Uranus, then the appearance of its rings could have changed quite markedly over 200 years, making them much harder to detect." Herschel's observations could thus be proof that planetary ring systems in our solar system are far more dynamic than has previously been supposed.
Dr Stuart Eves
Surrey Satellite Technology Limited
Tel: +44 (0)1483 803 803
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From 16 to 20 April, Dr Eves can be contacted via the NAM press office
Monday, 16 April 2007
'A ring suspected'
When Dr Eves tracked down Herschel's notes detailing his observations of Uranus, he found the following passage: "February 22, 1789: A ring was suspected".
Herschel even drew a small diagram of the ring and noted that it was "a little inclined to the red". The Keck Telescope in Hawaii has since confirmed this to be the case. Herschel's notes were published in a Royal Society journal in 1797.
Dr Eves told BBC News: "I was thinking, 'could he have got all of that right'? He has one ring, rather than multiple rings as there are at Saturn; it is relatively close to the planet and it's about the right size.
"The opening angle is about right. Astronomical software indicates that it may have been slightly more open as viewed from Earth on the dates Herschel was observing," he added.
"But there are reasons for thinking that the ring plane moves about a bit, and he has the major axis of the ring plane in the right direction. I started to add up all the statistics and I said: I reckon he had a point.
"[Herschel] is not just superimposing a saturnian-style ring system on Uranus. I think it is compelling from a psychological point of view, because he really didn't have much to compare it with at the time."
Other astronomers have dismissed the possibility that Herschel discovered rings around Uranus. They claim that it would have been far too faint for him to have seen from the ground, using contemporary telescopes.
Clear skies
The ring was officially discovered in 1977 during an occultation experiment. One question some will be asking is why no one saw the ring in the intervening years.
Stuart Eves thinks there may be a few reasons for this. Firstly, there are only a few windows of opportunity during which the rings present themselves to Earth.
The Cassini-Huygens mission has also observed darkening of the rings of Saturn. This may be due to dust accumulating on the icy material in the rings.
If this process is happening on Saturn, Dr Eves argues, it could be happening on Uranus. The seventh planet's rings may have been brighter in 1787, allowing Herschel to spot them from Earth. Several other effects could also cause variability in the rings, including loss of material from them.
Another factor may lie with the Earth's atmosphere. As the industrial revolution proceeded apace, light pollution and smog may have prevented subsequent observers from seeing the planet's rings.
More speculatively, a cold snap called the Maunder Minimum, which lasted from 1645 to 1715 and saw temperatures that were on average five degrees lower than today, might have removed water vapour from the atmosphere, locking it up as ice.
If the climate was still relatively cold by the time Herschel made his observations, less water vapour may have made skies clearer and therefore more suitable for astronomy.
