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Sunday, May 24, 2015

Earthsky Tonight – Jan 6 2010: Quarter moon, Spica

Courtesy of EarthSky
A Clear Voice for Science

The star Spica is near the moon as it reaches its last quarter phase.
January 6 Night Sky

Our sky chart shows the sky scene for before dawn on Thursday, January 7. Notice that the last quarter moon couples up rather closely with Spica, the brightest star to light up the constellation Virgo the Maiden. Twenty-four hours before – on Wednesday, January 6 – the moon shone closer to the ringed planet Saturn.

The moon changes its position daily because it never sits still in front of the background stars. Relative to the stars, the moon moves eastward at about 1/2 degree per hour or 13 degrees per day. For reference, the moon’s diameter spans about 1/2 degree, and a fist at an arms length is approximates 10 degrees.

Before dawn on Thursday, the moon will reach its last quarter phase at precisely 4:39 a.m. Central Time (5:39 a.m. Eastern Time; 3:39 a.m. Mountain Time; 2:39 a.m. Pacific Time). At this juncture, the moon will be half-illuminated in sunshine and half-engulfed in its own shadow. The terminator – the line dividing the lunar day from the lunar night – shows you where it’s sunrise on the moon.

Follow the terminator to the upper edge of the lunar disk to find the moon’s north pole, and to the lower edge to locate the moon’s south pole.

Written by Bruce McClure

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