Thursday, April 06, 2006

The Moon in the northern sky

While on the couch in my apartment in the late-evening last night the light of the just-past First Quarter Moon fell upon my eyes. It told me a few things--that the Sun would be roughly where the Moon was now in a quarter-year (three months, or early July), and since my window faces northwest the moon was in the northern half of the sky. The "roughly" part had to be included because the Moon's orbit around the Earth is not precisely in the plane of the solar system (also known as the ecliptic). Instead it's tilted off about 5 degrees from the ecliptic, and it's why we don't have solar and lunar eclipses every month. Instead, we only get them roughly every six months: when the intersection of the plane of the moon's orbit and the ecliptic (also known as the nodes) line up with the line between the Earth and the Sun. We just had a solar eclipse just a week ago , so I know that the Moon should be nearly the full +5 or -5 degrees off the ecliptic.

I created a snapshot of the view from the couch with Stellarium and it shows the moon at +25 degrees declination, or a few degrees above the ecliptic, but not fully +5. Why? It's just past first-quarter, so it's heading back down to the ecliptic. When I plugged into Stellarium the previous night, you could see the Moon was at +27.5 declination, some +4 degrees above the ecliptic. Add the two weeks we are past the vernal equinox (where we'd expect the ecliptic to be near +23.5 declination), and it puts the Moon where we see it.

So, come early July, I can be lounging on my sofa in the late afternoon and the Sun should be streaming into my apartment and onto my face (apart from any leaves on the trees blocking it).

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