Hubble takes first image of solar eclipse on Uranus

New Scientist ^ | 01 September 2006 | Maggie McKee

[Virginia-American]

A tiny moon has been caught floating in front of Uranus for the first time, the Hubble Space Telescope reveals. The moon's shadow can also be seen on the planet's cloud tops, creating a solar eclipse on Uranus itself.

Hubble imaged the event unexpectedly in July 2006, during a set of observations meant to study the planet's clouds. "When we first got this image back, we looked at it and said, 'What's that bright spot and that dark spot?'" says team member Heidi Hammel of the Space Science Institute in Boulder, Colorado, US. "We thought, it must be a problem with the detector."

Then, recognising that it resembled "transit" events seen fairly frequently around other giant planets, such as when Jupiter's moon Io passes in front of that planet's disc, the researchers realised they had seen the first ever transit on Uranus. It featured the 1130 kilometre-wide (700 mile-wide) Ariel, one of the planet's 27 moons.

The events are so rare because Uranus, unlike the solar system's other planets, is tilted almost completely on its side, with its rotational axis lying nearly in its orbital plane.

Its moons, however, orbit the icy giant above its equator. So when either of Uranus's poles is facing the Sun during the planet's 84-year orbit, the moons do not cross its disc at all as seen from Earth. Instead, they circle it on orbits that resemble the rings around a bull's eye.

Rare chance

Uranus, its moons and the Sun are only in the right alignment to observe the effect every 42 years. Now, Uranus is approaching such an ideal viewing period – it will reach its equinox, in which the Sun will shine directly over the planet's equator, in 2007. "This is a once-in-a-career event for most of us," Hammel told New Scientist.

It is also the first time in the history of astronomy that such observations are even possible. When the planet's equinox last occurred, in 1965, existing telescopes were simply not powerful enough to resolve the transits. Even today, only two or three telescopes can image them, including Hubble.

"This planet and its system of rings and moons and magnetic fields are so far away, we need the biggest and best telescopes on Earth to do a lot of the observations," Hammel explains.

In fact, the planet is so far away that even the best telescopes do not have vision sharp enough to determine the moons' sizes simply by imaging them. "They're so far away, they're just points of light," Hammel continues.

But during equinoxes, astronomers can gauge the moons' sizes by observing the moons passing in front of each other or into each other's shadows. "By looking at how long it takes for the secondary satellite to go into and out of eclipse, we can figure out the size of the first moon, which is casting the shadow," she says. Knowing the moons' physical sizes reveals their mass and clues about their history, she says.

"It's very exciting," Hammel told New Scientist. "We're really hoping we get enough telescope time to do what we need to do."

[PistolPaknMama]

Now that is cool!

[bwteim]

Thanks for the info, Patrick.

[Drew68]

A piece of Uranus where the Sun don't shine...

LOL! The IAU should really consider renaming this poor planet!

[TimesDomain]

The difference between simulation and reality?

Let's see - you can have Olive Oil (Popeye's girlfriend) as your prefered simulations ....

.... and I'll have Raquel or Gina Lollobrigida.

THAT is the difference.

[misanthrope]

"The icy moon Ariel (white) casts a shadow on the cloud tops of Uranus - the moon is named for a mischievous spirit in Shakespeare's The Tempest"

A mischevous spirit around uranus.

Snicker.

[yankeedame]

FYI: How Uranus got its name.

Sir William Herschel, [who] formally discovered the planet on March 13, 1781, named it Georgium Sidus (George's Star) in honour of King George III of Great Britain.

When it was pointed out that sidus means star and not planet, he rebaptised it the Georgian Planet.

This name was not acceptable outside of Britain.

-- [French astronomer, Pierre] Lalande proposed in 1784 to name it Herschel, at the same time that he created the planet's symbol ("a globe surmounted by your initial"); his proposal was readily adopted by French astronomers.

-- [Swedish astronomer Erik] Prosperin, of Uppsala, proposed the names Astraea, Cybele, and Neptune (now borne by two asteroids and a planet).

-- [Finnish-Swedish-born Russian astronomer Anders Johann] Lexell, of St. Petersburg, compromised with Great Britain's Neptune.

-- [Swiss mathematician Daniel] Bernoulli, from Berlin, suggested the names Hypercronius and Transaturnis.

-- [German physicist Georg Christoph] Lichtenberg, from from Göttingen, chimed in with Austräa,, a goddess mentioned by Ovid.

The name Minerva was also proposed.

Finally,[German astronomer Johann Elert] Bode, as editor of the Berliner Astronomisches Jahrbuch, opted for Uranus, after Latinized version of the Greek god of the sky, Ouranos;

[Hungerian astromoter] Maximilian Hell followed suit by using it in the first ephemeris*, published in Vienna.

Examination of earliest issues of Monthly Notices of the Royal Astronomical Society from 1827 shows that the name Uranus was already the most common name used even by British astronomers by then, and probably earlier.

The name Georgium Sidus or "the Georgian" were still used infrequently (by the British alone) thereafter.

The final holdout was HM Nautical Almanac Office, which did not switch to Uranus until 1850.

In the Chinese, Japanese, Korean, and Vietnamese languages, the planet's name is literally translated as the sky king star (天王星).

=================================

* Ephemeris: A table giving the coordinates of a celestial body at a number of specific times during a given period.

[Petronski]

Uranus is blue!

Only if I spend too much time sitting.

[verity]

Bwahahaha!

[cyborg]

This is what I have to deal with on a daily basis.

[cyborg]

I mean dealing with his sense of humor, not his blue anus ;-)

[Petronski]

Juvenile-jokes-about-Uranus thread.

[2nd Amendment]

A thinly veiled attempt by the gay coalition to detract attention from the eviction of pluto!

[Lonesome in Massachussets]

I'm sure the orbital elements of Uranus' major moons are known with far more than the necessary accuracy to have predicted this. (They recently accurately predicted the occlusion of a star by one of Neptune's moons, iirc, a much, much harder feat.)

I think that the guys studying Neptune's atmosphere just didn't do the calculations and were pleasantly surprised. BTW, moons' shadows on Jupiter are observed and photographed all the time.

[verity]

I appreciate the clarification. lol

[RadioAstronomer]

I hope so.

[exit82]

Excellent comment. A liquid through the nose shooter.

[Awgie]

Uranus got mooned!

[Drammach]

Cassini-Huygens has also transmitted many excellent photos of satellites and their shadows on Saturn.. and other phenomena..

It's just the sheer idiocy of the quotes in the article..
Are these people astronomers or middle-school students?
"Geeze, we thought maybe Hubble was broken or something".. Duh..

These "astronomers" didn't do their homework..

[Lonesome in Massachussets]

These "astronomers" didn't do their homework..

Worse than you think. They called it a transit, when clearly it's an eclipse. Sheesh.

The JPL free ephemeris website http://ssd.jpl.nasa.gov/?sat_ephem claims that they produce ephemerides of Areil 0.02 arcsecond accuracy from 1980 to 2010. The diameter of Uranus is ~3.6 arc seconds viewed from Earth, so they can predict it's location to about one part in 1/180 of the diameter of Uranus. They clearly would have predicted this if they had done the math.

[martin_fierro]

I am really angry with Saturn and Uranus. Neptune and Jupiter are OK though.

Venus and Mars Are Alright Tonight.