First Observation of a Uranian Mutual Event (Forwarded)

Armagh Observatory
Armagh, Northern Ireland

FOR FURTHER INFORMATION PLEASE CONTACT:
Apostolos Christou or David Asher
Armagh Observatory
Tel. 028-3752-2928

16 May 2007

First Observation of a Uranian Mutual Event

An international team of astronomers led by Apostolos Christou at Armagh
Observatory has made the first ever observation of one of the satellites of
the planet Uranus passing in front of another. The observation was made on
the night of 4th May by Marton Hidas and Tim Brown, of the Las Cumbres
Observatory Global Telescope, Santa Barbara, California, using the robotic
Faulkes Telescope South at Siding Spring Observatory, Australia. This work
involves a collaboration between scientists at Siding Spring, Las Cumbres,
Armagh and Cardiff University.

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.
Collectively, occultations and eclipses are called 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. Not
surprisingly, no-one had successfully recorded any mutual event involving
these extremely faint satellites, which are 3,000 million kilometres from
Earth.

But this situation changed this month, when the Faulkes telescope observed
the satellite Oberon (named after the "King of Shadows and Fairies" in
Shakespeare's "A Midsummer Night's Dream") occulting Umbriel (the "dusky
melancholy sprite" in Alexander Pope's poem "The Rape of the Lock"). As
Oberon's disc encroached upon Umbriel's, gradually blocking off Umbriels
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. It highlights the value of the North
and South Faulkes telescopes for recording rare, time-critical events. And
because the telescopes have an educational focus, the data will eventually
be used not just by astronomers but also by schools and schoolchildren
worldwide.

[NOTE: Images supporting this release are available at
http://www.arm.ac.uk/press/2007/uranus_event03.html ]