With the help of NASA's Hubble Space Telescope, we captured a rare sight in the solar system - a glimpse of Uranus as one of its moons, Ariel, and its shadow traverse the disk of the planet.
The sight has never been observed before because Uranus travels a large, 84-year orbit around the sun and the orientation of the planet on its polar axis is sideways. The combination of sideways orientation and large orbit make the planet's seasons last decades and keep unexposed regions of Uranus in frigid darkness for as long as 42 years.
In addition to the view of Ariel and its shadow, the new images of the planet show that the southern hemisphere is brighter than the northern hemisphere. This asymmetry in cloud structure, probably arises from the sluggish response of Uranus to solar heating due to its extremely low temperatures.
On Earth, seasonal changes are more modest because the Earth's rotational axis is tilted much less (only 23.5 degrees compared to Uranus' 97 degrees). The planet's southern and northern hemispheres are alternately tipped toward and away from the sun as the Earth moves in its solar orbit. Summer in the United States occurs when the northern hemisphere is tilted toward the sun.
Uranus is now approaching its 2007 equinox when the sun hovers directly over the Uranian equator. It is only near the planet's equinox when scientists can observe the shadows of the moons in its orbit as they pass between the planet and the sun.
The last time Uranian equinox occurred was in 1965, when Lyndon B. Johnson was president, a first class postage stamp cost a nickel, the Sound of Music was a cinematic smash - and the Hubble Space Telescope was but a distant vision for astronomers.
Although Ariel, named for a mischievous airy spirit in Shakespeare's "The Tempest," is only one-third the size of Earth's moon, it casts a much larger and sharper shadow than our moon does during a solar eclipse on Earth. This is a result of Uranus being 20 times the Earth's distance from the sun, which makes the sun appear 10 times smaller than Ariel, while our moon appears to be about the same angular size as the sun.