Uranus moons seen overtaking each other for first time - space - 18 May 2007 - New Scientist Space

"For the first time, astronomers have glimpsed one of Uranus's 27
moons passing in front of one of its siblings - a fleeting alignment
that can reveal information about the moons, such as their mass, that
cannot be gleaned in any other way.

Researchers hope this will be the first in a bonanza of data returned
from Uranus in the next year. That is because Uranus, which orbits the
Sun every 84 years, is in an ideal geometry to view its moons. Such
periods come around only once every 42 years, and the next will not
occur until 2049."

http://space.newscientist.com/article.ns?id=dn11891