The first
ever observation of one of the satellites of the planet Uranus passing in front
of another has been made. The observation was made on the night of 4th May using
the robotic Faulkes Telescope South at Siding Spring Observatory, Australia.
When one
satellite passes in front of another, the phenomenon is known as an
occultation; when one moves into the shadow of another it is an eclipse.
Occultations and eclipses are known as mutual events. These provide a means to
determine the positions of the satellites with exceptional precision, better
than any optical telescope, but they are rare. In the case of Uranus, a season
of mutual events occurs just once every 42 years, each individual event lasting
just a few minutes. At the time of the last Uranian mutual event season, man
had yet to walk on the Moon. No one had successfully recorded any mutual event
involving these extremely faint satellites, which are 3,000 million kilometers
from Earth.
But this
situation changed when the Faulkes telescope observed the satellite Oberon
occulting Umbriel. As Oberon's disc encroached upon Umbriel's, gradually
blocking off Umbriel's light, the combined brightness of the moons dropped by
about a third.
Measurements
of such changes in brightness, and comparison with models of the satellites'
motions, allow astronomers to work out the masses of the moons and the effects
of the shape of Uranus on their orbits, and to model their surface features.
The current Uranian mutual-event season is expected to lead to some of the
greatest advances in the study of the Uranian system since the flyby of the Voyager
2 spacecraft in 1986.
This
observation kicks off a campaign extending from now into 2008 to observe the
entire mutual event season.
-- Armagh Observatory and SPACE.com Staff
Credit: NASA/JPL-Caltech
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