Physics Photo of the Week

February 27, 2009

Comet Lulin
Photo and telescope assistance by Melanie Kemp.
About every year a telescopic or binocular comet visits the inner Solar System.  Two dedicated astrophotographers got up at 4:00 am on February 20, 2009 to set-up the telescope to obtain these photographs.  The temperature was about 16 degrees F with a strong north wind making observing very difficult and unpleasant.  This comet is moving quite rapidly between the constellations Virgo and Leo.  This week the comet can be viewed at a more appropriate time in the late evenings after 10:00 pm approaching the bright star Regulus in Leo, and is visible in binoculars.

The animated picture at right spans a 25 minute time frame.  From the scale of the picture (about 1/2 degree) the comet is moving at about 0.003 deg/minute.  This amounts to about 4.1 deg/day - 8 times the diameter of the full Moon.  The rapid apparent speed is enhanced because the comet is orbiting the Sun in the opposite direction from the Earth and it is now relatively close to the Earth - about 40% of the Sun-Earth distance.  As a result of Newton's laws of gravity and orbital dynamics, the comet's velocity is about the same as the Earth's speed, but in the opposite direction.

In order to view the structure of the comet (a thin tail to the left), several 30-second exposures were stacked (aligned and digitally added).  The brightness was also transformed to a logarithmic display (similar to the way our eyes respond).  Otherwise the central part of the comet would appear to be highly overexposed.

The comet is mostly a ball of diffuse gas and dust that evaporates from the small (kilometer-sized) nucleus of the comet due to the heat of the Sun.  The gas and dust are driven away from the comet by the radiation pressure from the Sun forming a tail.  A comet's tail is always directed away from the Sun.  The comet is now situated such that the Earth is between the comet and the Sun.  As a result the tail points mostly toward the far side of the comet.  As the tail particles are pushed further from the Sun, they still preserve the angular momentum around the Sun, but travel more slowly at the greater distances.  As a result the tails of comets are usually curved.  We cannot see the curve, because our vantage point is in the same plane as the curve.  The fact that the tail curves to the left allows us to see the tail at all.  Thus the particles in the left end of the tail in these photos are actually considerably further from the Earth and Sun.

On Monday
night (February 23, 2009) Pengye Su, George Pilzer, and Libba Miano, helped align the telescope and photograph the comet in color.  The weather again was very cold.  The monochrome CCD camera makes color photos by recording images through three color filters: red, green, and blue.  The three images are later assembled by digital superimposition using red, green, and blue-colored pixels respectively.  Because the whole process required about a half-hour the comet moved considerably during the imaging session.  For the alignment, stacking, and color assembly, the images were aligned to "follow the comet" rather than the stars.  As a result the stars left trails in the opposite direction.

In fall 2007 students helped photograph Comet Holmes (PPOW Dec. 7, 2007 and Nov. 1, 2007).  Other comets are featured in PPOW for Nov. 3, 2006 and Jan. 10, 2005.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature 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 

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photoprovider is forbidden.

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