Friday, December 26, 2014

My Best Look Ever at the Celestial Western Lunar Limb

How many times have we heard or read that the half Moon is the best time for lunar observing? I know the terminator is most striking at that time, but I really hate that advice. When asked when to best observe the Moon, I answer that it depends what you want to see. Only observing Luna near the half phase limits what can be observed, and I like to explore the entire disk, and when libration allows, sneak peeks of the far side.

On the night of December 6-7, the Moon was 99.8% illuminated when I started observing, but was already past full, so the celestial western limb was starting to show shadows as the Moon began to wane. With the Moon so recently past full, and a libration of over six degrees in latitude, I saw features along the limb that I had never seen before, or at least had never seen well.

I observed over the course of many hours, and watched as features near the terminator disappeared into lunar night. While atmospheric transparency was never great that night, toward the end of the observation it was terrible, and water vapor in the air started turning the images very soft.

The next time you think, "I'm not going to observe tonight because the Moon is full," remember that you might just see part of the lunar surface you've never seen before.

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