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Just another full moon

Almost-full moon rises over I-210 in Pasadena

Almost-full moon rising over I-210 in Pasadena at 8:00 p.m., July 24, 2010.

It was about 8:00 p.m. Saturday night. Jane and I were driving home from having just seen Salt in Pasadena. This beautiful moon was rising above the Foothill Freeway, with dark blue earth shadow just below it.

I realized in this one picture was a whole astronomy lesson.

The first thing I realized is that this moon had to be several hours away from exactly full.

How did I know that? Notice that the moon is a few degrees above the line of the earth’s shadow. If you were to see the rising full moon at exactly the moment when the moon is directly opposite the sun in the sky (full moon) it should be directly on that line.

Upon checking with an ephemeris, the moon seen is actually 22-1/2 hours away from full. It will travel about 22 diameters before reaching that moment of exactly “full.” It certainly appears closer than 22 diameters, but two things are in effect: one is the slight motion blur making the moon appear bigger, and two is that it will move in a line that’s quite a diagonal in this photo.

The next thing I realized is that about one-half orbit ago (about 15 days), this moon was eclipsing the sun in a total eclipse. That means that now this moon must again be within a few degrees of crossing the ecliptic.

That puts this moon as close to directly opposite the sun as it can be, short of actually passing through the shadow of the earth (a lunar eclipse).

At the time Jane snapped this picture, the freeway had just turned from due east to a few degrees toward the south. That would match nicely with the sun having just set a few degrees to the north. Not as far north as last month, since we’re now about one month into summer.

All fun realizations from a simple beautiful full moon rising. The geometry of the solar system is right there in the sky to see.

4 comments to Just another full moon

  • Ed Greenberg

    “The geometry of the solar system is right there in the sky to see.”

    Of course, it’s a lot easier for us to look and say, “there, I see proof of what I know” than it was for the ancients to say, “there, I see this, therefore I know this new thing.”

    Thanks for the post.

  • Cindy Sandino

    Thank you for the astronomy lesson. I read somewhere that the moon is never really “full.” Do you think that’s true?

  • Pretty much true! When the moon is exactly, precisely, opposite the sun from the earth, it’s in the center of a total lunar eclipse.

    A fellow amateur astronomer once expressed this thought by saying “there’s always a terminator.” He meant there was always somewhere on the moon where you could see shadows, meaning it’s not completely full.

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