See Occultation of Uranus' Moons, Overtaking Each Other for the First Time

Astronomers have received pictures showing two of Saturn' 27 moons, one passing in front of the other. This is considered very important for scientific calculations, as it is the only way that the mass of the moons can be approximated.

With the help of the Faulkes Telescope South in Australia, astronomers observed the 1500-kilometre-wide moon Oberon passing in front of the 1200-km-wide Umbriel, an event named occultation, when 30% of the light reflected by Umbriel was blocked by Oberon.

Planet Uranus is the seventh planet from the Sun, a gas giant made up mostly of hydrogen, (83%) and helium (15%). For the moment, the planet has an ideal position to allow clear observations of its moons. These periods come around only once every 42 years out of the 94 years it takes Uranus to circle the Sun and the next will not occur until 2049.

"At best in a human lifetime, you can observe it a maximum of three times - the first time when you're about two years old, then again in your mid-40s, then assuming that your eyes still hold, you can observe it in your late 80s, if you phase your life correctly," says Apostolos Christou, an astronomer at Armagh Observatory in Ireland.

The strange thing about Uranus is the fact that it has an axial tilt of ninety-eight degrees, which makes part of its orbit one pole to face the Sun continually while the other pole faces away. However, its moons are in orbit above the equator and this makes them visible from Earth.

This event could prove itself useful in mapping the two biggest moons, as to observing how the brightness of these satellites changes over time, which might suggest that frost from Uranus' surface is evaporating and resettling elsewhere on the moon.