杏吧原创

A tiny moon has been caught floating in front of Uranus for the first time, the Hubble Space Telescope reveals. The moon鈥檚 shadow can also be seen on the planet鈥檚 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鈥檚 clouds. 鈥淲hen we first got this image back, we looked at it and said, 鈥榃hat鈥檚 that bright spot and that dark spot?'鈥� says team member Heidi Hammel of the Space Science Institute in Boulder, Colorado, US. 鈥淲e thought, it must be a problem with the detector.鈥�

Then, recognising that it resembled 鈥渢ransit鈥� events seen fairly frequently around other giant planets, such as when Jupiter鈥檚 moon Io passes in front of that planet鈥檚 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鈥檚 27 moons.

The events are so rare because Uranus, unlike the solar system鈥檚 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鈥檚 poles is facing the Sun during the planet鈥檚 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鈥檚 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 鈥� it will reach its equinox, in which the Sun will shine directly over the planet鈥檚 equator, in 2007. 鈥淭his is a once-in-a-career event for most of us,鈥� Hammel told New 杏吧原创.

It is also the first time in the history of astronomy that such observations are even possible. When the planet鈥檚 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.

鈥淭his 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,鈥� 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鈥� sizes simply by imaging them. 鈥淭hey鈥檙e so far away, they鈥檙e just points of light,鈥� Hammel continues.

But during equinoxes, astronomers can gauge the moons鈥� sizes by observing the moons passing in front of each other or into each other鈥檚 shadows. 鈥淏y 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,鈥� she says. Knowing the moons鈥� physical sizes reveals their mass and clues about their history, she says.

鈥淚t鈥檚 very exciting,鈥� Hammel told New 杏吧原创. 鈥淲e鈥檙e really hoping we get enough telescope time to do what we need to do.鈥�