Wednesday, March 04, 2009

Coming Around: The Moon as Signpost

We've been here fact, every month we can use the moon as the first stepping-stone toward our three-dimensional experience of the cosmos.

Last night, she was right behind us, a perfect half-moon. As ever, this means the moon was "beside" us in space, following along behind the earth in its (earth's) orbital path. We were there, where the moon is, roughly three hours ago....


Just after sunset, we view the sky from the very early edge of the "night" side of the earth (see rotation of earth in the diagram). As we look from here, on the trailing terminator of the earth, we see the leading terminator of the moon, following along behind us. (in the diagram above, the earth's orbit around the sun keeps us moving toward the top of the image).

Combined with our observation of the crescent a few days back, we begin to "feel" the moon swinging out around earth, now at its extreme side-swing, and about to head around behind us.

While every month offers this chance to see the phases of the moon as a handle toward feeling its 3D motion around earth, anchored by the monthly moment when the "moon is behind us" in space, this night of looking behind us also offers a chance for a longer-term orientation to the star field. This month (late winter), as we look back at the moon, we see Orion just up to the left, and Sirius beyond. Gemini is up there as well, with Leo rising in the east. Just noticing these landmarks is Step One toward appreciating the annual path of the earth around the sun, and the ever-changing view that affords us from the night-time side of the planet.

BTW, I really don't know why we call this "first quarter"; if it IS a quarter (a quarter way through the month; we see a quarter of the moon lit?), then the full moon should similarly be called a half moon....I insist on seeing this as the half moon. So there.