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Mutual Events of the Uranian Satellites

Background
Geometry
Details of events
Observing
Further information

Background

In 2007-2008, the planet Uranus will reach one of its annual equinoxes for the first time since 1966. As a consequence, its five major satellites orbiting in the equatorial plane will undergo a season of mutual eclipses and occultations very similar to the one that Jupiter's Galilean satellites undergo every 6 years. These events represent an unprecedented opportunity to conduct very precise astrometry of the satellites with modest aperture telescopes equipped with CCD cameras and also (a project for larger apertures) search for photometric signatures of albedo features (i.e. exceedingly bright or dark areas) in the north hemispheres of the satellites. Those hemispheres were in darkness when Voyager 2 flew by in January 1986.

Geometry

***Diagram of mutual event geometry***

This diagram explains the geometry of the events. The brown satellite is being occulted or passing into eclipse, and the blue disc is either another satellite (in the case of an occultation) or another satellite's shadow (in the case of an eclipse). The blue disc is larger than the brown satellite in this example, but it could be smaller. For each event there is an uncertainty Delta b in the impact parameter b (b affects how large the light drop is), and in the time of mid-event t_min. Indeed the fact that there is an a priori uncertainty in these quantities is one reason why it is valuable to do the observations.

The events

There are 321 events in total (Christou 2005). Some of these events occur when Uranus is at conjunction, or the satellite is transiting or too close to the limb of the planet. In addition, some events will be too ``shallow'' to detect i.e. the light drop will be too small or may not take place at all due to uncertainties in the satellites' position (the very reason to conduct astrometry for!). Taking all these factors into account, we estimate that around 150 events will be observable. This, however, assumes that for each one of those events there will be at least one willing observer at a location on the surface of the Earth from which Uranus will be sufficiently high in the sky (typically at least 15-20 degrees) at the critical time. Therefore, the number of observed events depends greatly on the number and geographical distribution of available observers.

Complete table of events
Data in table (text only)
README file (explanation of columns) for text only data

Each of the listed events may be reproduced graphically through Mark Showalter's online Uranus Viewer 2.1 engine.

Observing

Generally more events will be observable with larger aperture telescopes. But a few events should be accessible even to, e.g. a 20-25 cm equipped with CCD. Also, there are ways to improve one's chances of making useful observations. Better contrast can be obtained by using an I-band filter, as can be seen by comparing these V, R and I 2-sec exposures taken of Uranus and satellites with the 2-metre Faulkes Telescope North at around 14:05 UT on 15 Aug 2005. Counterclockwise from left to right: Titania, Umbriel & Ariel (merged), Miranda, Oberon.

***I-band image*** ***R-band image*** ***V-band image***

Further information

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Last Revised: 2007 March 19th
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