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Did Herschel discover the rings around Uranus?

1797 vs 1977

A British scientist wants to re-write history and attribute the discovery of the ring around Uranus (no sniggering at the back) to Sir William Herschel.

Officially, Uranus' rings were identified in 1977. Herschel, who discovered the planet in 1781, was long dead by then, having shuffled off this mortal coil in 1822.

But Dr Stuart Eves, who works for Surrey Satellite Technology Limited, says that Herschel's writings indicate that he saw a possible ring orbiting Uranus in 1797, some 180 years earlier.

Speaking at the annual meeting of the Royal Astronomical Society (RAS) at the University of Lancashire, Eves told reporters that in his notes detailing his observations of Uranus, Herschel had written "February 22, 1789: A ring was suspected".

Eves was tipped off, the BBC reports, by a framed page from an 1815 edition of an encyclopaedia. This showed Uranus, in the correct spin plane, with six smaller objects orbiting it. Eves traced this back to Herschel's earlier writings.

Herschel's "discovery" was initially dismissed as an error, and critics of Eves idea say that it would not have been possible for the astronomer to have made the observation using 18th century telescopes.

But an extra detail caught Eves' eye: Herschel had noted that the ring was "a little inclined to the red", something that has been confirmed since by observers using the Keck telescope.

He told the BBC: "I was thinking, 'could he have got all of that right'? [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."

Eves notes that Herschel identified a single ring, of roughly the right size and positioning.

He told the Beeb: "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, but there are reasons for thinking that the ring plane moves about a bit...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."

There are several reasons Eves thinks the rings might have been visible to Herschel, even though they have not been "viewable" from Earth in between then and 1977.

The skies would have been much clearer back then, Eves argues, since the industrial revolution was only just beginning. In addition, the skies could still have been cleared of moisture, thanks to the so-called Maunder Minimum. This "cold snap" lasted from 1645 to 1715, and saw global temperatures five degrees lower on average than we see today.

And finally, Eves notes that Saturn's rings are darkening. This same phenomenon could be occurring around Uranus, and contributing to the rings' apparently temporary invisibility. ®

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