Friday, April 16, 2010

Earthshine and Aristarchus

It was cloudy all day here in Tucson, but around sunset the clouds moved out leaving behind a starry sky.  I decided to take out the Stellarvue 90mm Triplet to check out the crescent moon for a few minutes.  As I headed outside around 8:15 PM MST (0315 UT 4/17/2010) one of the first things that caught my attention was how bright the earthshine was on the moon tonight.  The moon is sitting just below the Pleiadas star cluster and together with Venus the western sky is lovely.

Wikipedia has a detailed entry regarding earthshine that is worth the read.  "Earthshine is most readily observable shortly before and after a New Moon, during the waxing or waning crescent phase. When the Moon is new as viewed from Earth, the Earth is nearly fully lit up as viewed from the Moon. Sunlight is reflected from the Earth to the night side of the Moon. The night side appears to glow faintly and the entire orb of the Moon is dimly visible.  It is also known as the Moon's ashen glow or as the old Moon in the new Moon's arms."

Using a 10mm eyepiece at 63x, I was impressed with how sharp the terminator and moon features were.  I started to look around the portion of the moons face that was illuminated by the prominent earthshine, and I suddenly noticed a distinct bright glow where I did not expect to see anything other than the earthshine.  Of course, as any self-respecting amateur astronomer, I was torn between hoping I was witnessing a transient lunar phenomenon, and knowing that the glow was more likely a result of a brighter area on the moon.  Just to be sure I was not imagining things, I had Beth come out to take a look and she saw the bright spot as well.  It was not quite starlike, rather it was a diffuse glow much like a small cloud or nebula superimposed on the moon.  The image to the right is from the freeware program Virtual Moon Atlas (which I highly recommend) and represents the phase of the moon tonight.  If you click on the thumbnail and look closely at about 10 o'clock just in from the lunar limb, you can see an area that is slightly brighter than its surroudings.  In the telescope, the spot was much brighter than in the image.

Thanks to Virtual Moon Atlas, I quickly ascertained that the bright spot I was seeing was likely the crater Aristarchus.  Aristarchus is a prominent lunar impact crater that lies in the northwest part of the Moon's near side. It is considered the brightest of the large formations on the lunar surface, with an albedo nearly double that of most lunar features. The crater is located at the southeastern edge of the Aristarchus plateau, an elevated area that contains a number of volcanic features, such as sinuous rilles. This area is also noted for the large number of reported transient lunar phenomena, as well as recent emissions of radon gas as measured by the Lunar Prospector spacecraft. 

A Google search on 'Aristarchus earthshine' turned up quite a bit of information that confrimed that I was in fact obsrving the crater Aristarchus illuminated by earthshine.  One of the interesting facts I turned up was that Aristarchus was the subject of the first earthshine photographs ever taken from lunar orbit, aboard the Apollo 15.  The image to the left is from the NASA report titled Lunar surface properties as determined from earthshine and near-terminator photography.  According to Gerald North's book Observing the Moon, William Herschel mistakenly believed that Aristarchus was a volcano erupting on the moon.   The two photos below are both of Aristarchus and were also taken from Apollo 15, but under full solar illumination.   Both show how bright and reflective the crater is.

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