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A spot on Uranus

— No, it's not some astronomer's idea of a rude joke: Rather, the Hubble Space Telescope has captured a rare view of one of Uranus' satellites, Ariel, floating over the planet and casting a shadow on the cloud tops.

Just how rare is it? Such an event is only possible only every 42 years, due to Uranus' bizarre sideways rotation.

NASA / ESA / UW-Madison
In this picture of Uranus, the bright spot is the moon
Ariel, casting a shadow on the planet's cloud tops.

We can see moons transiting planets like Saturn and Jupiter relatively often, because those moons move straight through the solar system's ecliptic plane. But because Uranus and its moons turn sideways, from "north" to "south," the planet's equator has to line up precisely with the sun to provide the view seen by Hubble last month. That occurs twice during Uranus' orbit, which takes 84 Earth years.

Earthlings have never seen this phenomenon before, said Lawrence Sromovsky, a senior scientist at the University of Wisconsin-Madison's Space Science and Engineering Center. Smorovsky and his colleagues happened upon the transit while they were using Hubble to study Uranus' atmosphere.

"The technology wasn't there to see this the last time it happened," he explained.

Smorovsky told me it could well happen again in October, when his team will be watching Uranus using the Keck Telescope in Hawaii. This is a good time to study how Uranus' cloud climate is changing, because the sideways planet's north and south poles are both visible and illuminated at the same time, he said.

If an observer were able to watch Ariel pass over from Uranus' cloud tops, they'd experience the event as a solar eclipse unlike any on Earth, Smorovsky noted. "The sun looks like a bright star, so the shadows are sharper than they are on Earth," he said.

This graphic accompanying today's image release from the Space Telescope Science Institute illustrates the differences between Uranian and earthly eclipses. Note the fuzzy dark spot on the Sahara, which represents the umbra and penumbra of a solar eclipse.

Another graphic shows how the orientations of the Uranian moons change over time - and you'll definitely want to check out these cool online videos showing the movement of the moons back in 1994.

Now, about those rings around Uranus...