Physics Photo of the Week

April 14, 2006
Occultation of the Pleiades - Click here for animation.

On the night of April 1, 2006 the moon passed in front of the Pleiades star cluster.  This occultation gave astronomers - professional and amateur - a good glimpse of the moon passing in front of many of the individual members of the Pleiades.  In the photo above, Alcyone, the brightest Pleiad, is about to be eclipsed after a fainter star.  The moon was a crescent, but the part of the moon visible in the photograph is the dark side of the moon (still the side facing the earth).  The reason we see the dark side is due to earthshine.  Earthshine is the illumination of the side of the moon facing the earth lit up by sunlight reflected back to the moon by the earth.  This image is a "stack" of four individual one-second exposures using a digital camera looking through a telescope. 

The moon is a bit blurred because the moon has moved relative to the stars during that time (1 minute).   The four images were aligned so that the stars are aligned, but the moon has moved during that time.  The lit portion of the moon is on the extreme right hand side - mostly cropped off the image.  The bright part of the moon is over exposed.

The image has been enhanced to show the colors.  The Pleiades are a young star cluster and the stars are characteristically blue-white.  We cannot see the familiar star pattern of the Pleiades due to the large image magnification through a telescope and due to the fact that a large portion of the star cluster is already covered up by the moon.  Please see Astronomy Picture of the Day for April 14, 2005 for the moon and the Pleiades at this time a year ago.

Occultations of stars by the moon are important to astronomers because the timing of the stars disappearing behind the moon can be timed very precisely and used to make corrections to the known orbital parameters for the moon.  If the stars are bright enough to be visible with a video camera, the disappearance of individual stars occurs completely within a 1/30 sec frame.  Visually the disappearance of stars behind the moon is very dramatic as the star very suddenly disappears.  Amateurs can help record the timings of occultations.  Please see the International Occultation Timing Associan (IOTA) website for details.

An animation of the Occultation is found at the next page.  The animated images are rather large, so please have patience.  Click here for the animation.

Special thanks to Brendan Kelley for assistance in setting up the apparatus for the photography.
Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.


Click here to see the Physics Photo of the Week Archive.

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