Once In A Blue Moon

I must confess that I am puzzled by people who ask me if I am excited about a “blue moon”—commonly understood to mean a second full moon in the same calendar month. It’s nice and big and bright, but it means the worst possible conditions for stargazing. The full moon is so bright that it washes out most anything else in the sky, it’s in the sky all night long, and it’s not even a good time to look at the moon itself. With the sun shining directly down on the center of the moon’s visible face, the illumination is flat. There are no shadows, and therefore no easy way to see height variations in lunar topography. To me, the ideal time to view the moon is when it is in the first quarter phase, half-illuminated and high in the southern sky at sunset.

This gorgeous image shows you why. The terminator is the dividing line between the dark and illuminated portions of the surface. If you were standing on the moon’s surface near the terminator, the sun would be low on your horizon, casting long shadows. The depths of craters and the heights of mountains are much more apparent here. Near the moon’s limb (the curved portion of the illuminated part), those topographical variations are much less obvious.

Of course, half of the visible lunar surface is dark at first quarter! To see the other half, you need to wait until third quarter, but this phase is highest in the sky just before dawn. This is for the early risers!

Another gorgeous image here; look near the terminator to find a crater with a bright spot in the center of its dark floor. This is a central peak poking up high enough to catch the sunlight. Features near either limb are best seen in a crescent phase.

And then there are the times when there is no moon in the sky at all, either because it has already set or not yet risen, or because it is in a new moon phase, when the sun is behind it from our point of view and it is not in the night sky at all. Those are the nights to see the “dim fuzzies”, those glorious objects mostly beyond the solar system. Two of my personal favorites are M13, a globular cluster that is part of our Milky Way Galaxy:

and M51, the “Whirlpool Galaxy”, a relatively nearby (23 million light years away) member of our Local Group of galaxies.

Nights with a full moon are great times to catch up on your reading!

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