This is pretty interesting– the rings of Uranus were supposedly discovered in the 1970s, but there is some indication that William Herschel — a brilliant observational astronomer in the 18th century who discovered Uranus in the first place — may have observed them 200 years before!
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| This image of Uranus and its rings was taken using the monster 10 meter Keck telescope in Hawaii. It’s a near-infrared image. The bright ring is the epsilon ring which Herschel may have seen — it’s bright in the IR, which means it’s red… and that’s how Herschel described it in his 1797 paper. |
Herschel wrote a paper in 1797 where he describes something very much like a ring around Uranus, but his findings have been dismissed because the rings are so faint that no one could believe he saw them. However, Dr. Stuart Eves of Surrey Satellite Technology Limited has re-evaluated Herschel’s claims. He says Herschel got many things right– the size, color, and orientation of the ring in the paper match what Uranus’s epsilon ring would have been like back then. Very interestingly, we know that Saturn’s rings are dynamic, changing on short timescales. It’s possible (though I have no idea how likely) that over 200 years that Uranus’s rings too have gotten fainter making them harder to detect now than in the 1790s.
I personally have no idea if this idea holds water or not, but it’s certainly cool to go over old papers by those brilliant, accomplished, and extremely well-seasoned observational astronomers of yore and see how what they did then compares to what we can do now. It’s best not to dismiss such old observations out of hand… but you also need to keep a jaundiced eye open. Not every observations turns out to be correct, but there may be a few cosmic needles in our celestial haystack still waiting for us to uncover.
April 23rd, 2007 at 2:21 pm
Here is a BBC story on the subject, which reveals more about Dr. Eves’ theory:
http://news.bbc.co.uk/2/hi/science/nature/6569849.stm
The six loops around Uranus in the orrery are almost certainly orbits of the six moons Herschel thought to have discovered. Until mid-nineteenth century the existence of these six moons was accepted by almost everybody.
As for the Herschel’s observations of a ring, it seems like he did not take them too seriously himself, otherwise they would have probably been accepted just like his moon claims (he had the best telescopes in the world at the time). Dr. Eves also does not appear to be an expert on planetary rings; it’s safe to say the timescales for significant evolution of rings tend to be longer then 200 years.
There is a brief mention of the “six moons” controversy on the Nine planets’ “hypothetical planets” page:
http://seds.lpl.arizona.edu/nineplanets/nineplanets/hypo.html#ura
And here is description of another orrery from ca. 1800 which features six moons of Uranus:
http://www.etesseract.com/Astronomy/Astronomy.html
April 23rd, 2007 at 3:45 pm
I guess my question is, do we have an exact wavelength on those rings? Are they close enough to the “visible spectrum” to speculate that Herschel might have had just slightly more sensitive eyesight than average, and could see them where most people could not?
And of course, other questions, like whether his viewing location and/or meteorological conditions may have helped? How about the glass in his telescope? I know from doing infra-red photography that you have to focus shorter than you would for the visible spectrum, so was there something in his scope that assisted the observation?
And has anyone looked for the Martian Canals in infra-red?
April 23rd, 2007 at 4:16 pm
The rings aren’t red because they’re bright in the IR. The rings are bright in the IR because of thermal emission, which contributes virtually nothing to the visible red flux.
The rings in visible light would all be due to backscattered sunlight, which should have the same colors as the solar spectrum, but the rings are slightly reddish because of their composition, I think.
April 23rd, 2007 at 5:25 pm
Stephen James O’Meara spotted the dark spokes in Saturn’s B ring before the Voyager 1 imaged them. Maybe William Herschel’s eyes were as good.
O’Meara was also the first to detect the rotation period of planet Uranus.
April 23rd, 2007 at 7:42 pm
I could almost believe the story. Tim Ferris in “Coming of Age in the Milky Way” has already described how good Herschel’s powers of observation were.
April 23rd, 2007 at 7:43 pm
Interestingly, Starry Nights shows that in the year of 1797, the ring inclination toward us was about half of the Keck image I found of 2005, thus it would have twice the surface brightness due to reflection, I think. Perhaps three times better than this more recent Uranus image.
[I wish y'all would rename it to the original name he gave it.
]
April 23rd, 2007 at 8:11 pm
This image seems to suggest that the rings are more exothermic than the planet itself?
Is this, in fact, the case?
April 23rd, 2007 at 8:52 pm
Thomas,
I think it just means that the epsilon ring is brighter than the planet. This could indicate any of a number of things…
1) The particle size in that ring is such that IR back-scatter is stronger than for other rings, and for the planet.
2) That ring is darker (thus absorbing more visible light, and warming it more) than the other rings, or the planet’s upper atmosphere.
3) The upper layers of Uranus’ atmosphere are relatively cool.
4) The upper layers of Uranus’ atmosphere are opaque to IR wavelengths.
I don’t know enough about Uranus to know which (if any) of the above items is true, just wanted to point out that any number of things could be responsible for the relatively bright epsilon ring.
Sam
(who is really glad to see no Klingon jokes in these comments)
April 23rd, 2007 at 9:13 pm
‘Exothermic’ isn’t really the correct word for it, Thomas. ‘Exothermic’ would imply a release of internal energy as a result of some process. I think that you’re thinking that the rings looks like they are hotter than the planet because they are brighter.
However, here, the rings are actually very bright compared to the planet because the image was purposefully taken in a narrow band filter around 2.2 microns (10^-6 meters) in the middle of a big methane absorption feature. The planet emits little light here and the rings shine brightly in contrast.
April 23rd, 2007 at 9:15 pm
Right, Sam. Phil’s kindly linked the image to a Keck press release for your enjoyment.
April 23rd, 2007 at 9:25 pm
If Herschel could just detect rings with naked-eye observations, wouldn’t a long time exposure with IR film easily confirm it? Of course that would have had to wait another century or so for the technology to be developed.
April 24th, 2007 at 2:41 am
I have worries about the accuracy of data in standard planetarium software to accurately predict the inclination of the rings back in the 18th century. Does anyone know how good the input data for Uranus’s rings, in software such as Starry Night, is?
April 24th, 2007 at 6:22 am
I can say that when I test Starry Nights Pro Plus on known solar eclipses as far back as 2167 BC in China, that it got it right. But, I do not know if this accuracy extends to planetary inclinations.
April 24th, 2007 at 10:56 am
In the visible part of the spectrum the rings are much darker than the planet. The composition of the rings includes lots of hydrocarbons which makes the reflectivity very dark. Ground-based observations need to look at particular wavelengths in the near-IR where the planet is dark and the rings are relatively bright in order to detect the rings. The orientation of the rings is easy to predict back in time to 1797 or any other time as long as one assumes that the angular momentum of the rings has not changed significantly. That is, the ring plane is fixed in inertial space, so you just have to track Uranus back in its orbit to get the orientation relative to the Earth. The rings are close to the planet’s equatorial plane, and there is no mechanism known that would change that.
All that said, I think it is very unlikely that Herschel was able to see the rings.
April 25th, 2007 at 12:00 am
Maybe seeing rings was an artifact of the optics of his telescope. Chromatic or spherical aberration for example. Maybe it is possible to infer that what he was seeing is a known optical effect. Sometimes people do get lucky; For example it was believed Mars had 2 moons way before they were discovered, though it had to do more with numerology: Earth=1, Mars=2, Ceres=3 (not yet discovered and of course is moonless), Jupiter=4, Saturn=5. Sometimes it is possible to stumble on the correct answer even though the logic or the observation was completely wrong.