Rare Uranian Eclipse Captured by Hubble
A never-before-seen image of an astronomical alignment of a Uranian moon, Ariel, as it traverses the face of the giant gas planet.
Credit: NASA, ESA, L. Sromovsky (University of Wisconsin, Madison), H. Hammel (Space Science Institute), and K. Rages (SETI)

The shadow cast by a moon as it drifted through space above the blue-green cloud tops of Uranus was recently captured for the first time by NASA's Hubble Space Telescope.

To an observer on Uranus, the passage of the planet's icy moon Ariel would have appeared as a solar eclipse and the Sun would be completely blotted out. However, the effect would not be nearly as dramatic as a total eclipse involving Earth and its moon. The Sun appears much smaller on Uranus than on Earth because the giant planet is located much further away. Therefore, a total eclipse involving its moon completely blocks out not only the Sun's body, but also its corona.

Though such "transits" by moons across the disks of their planets are commonplace for other gas giants like Jupiter, the satellites of Uranus orbit the planet in such a way that they rarely cast shadows on the planet's surface. The moons of Uranus orbit the planet above the equator, so their paths align edge-on to the Sun only every 42 years.

In order for a Uranian transit to occur, the Sun must shine directly over the giant planet's equator; the next time this will happen again is in 2007. The last time such a Uranian equinox occurred was in 1965, but telescopes at the time were not sharp enough to view the transits.

Named after a mischievous airy spirit in Shakespeare's "The Tempest," Ariel is only one-third the size of Earth's moon. The moon is the nearest large satellite to Uranus.

As Uranus approaches equinox, there will be additional eclipses by its large moons Umbriel, Titania, Oberon, as well as many smaller moons.

Because our moon is moving slowly away, it is only at this moment in geologic history that a total eclipse can happen on Earth. Millions of years from now, our moon will be too small in the sky to completely block out the Sun.