The Moon, Jupiter, and the plane truth

Contributed by
Oct 16, 2007

I happened to look out the kitchen window while doing the dinner dishes on Monday (October 15), and what did I see? The gorgeous crescent Moon underneath the bright "star" of Jupiter.

I ran out and took a bunch of images; the one above is arguably the best. I didn't even notice Antares, the star to the right of the Moon (on the other side of the tree), when I took the shots!

As I was taking the pictures, I pondered how far below Jupiter the Moon was. Holding up my fist as a gauge, I reckoned they were about 5 degrees apart. Hey, I thought. The Moon must be at its most southernly ecliptic latitude!

I think things like this because I am a geek. But face it, you are too. You read my blog.

And so, because you're dying to know, here's what I was thinking:

The Earth orbits the Sun (with me so far?). Over the year, that means it looks like the Sun makes a big circle in the sky relative to the stars. We call that path the ecliptic. The major planets all orbit the Sun in roughly the same plane, so they stick to the ecliptic too. Not exactly on it, but pretty close.

The Moon, however, orbits the Earth in a plane tilted to the ecliptic by about 5 degrees (think two hula hoops, one jammed inside the other, and then tilted a little bit). That's why we don't get a solar eclipse every month: the two planes (ecliptic and Moon orbit) need to intersect when the Moon happens to be new, between the Earth and the Sun. Otherwise, it might miss the Sun's position by as much as +/- 5 degrees.

So when I saw the Moon and Jupiter on Monday, I knew the Moon had to be dipped down as low below the ecliptic as it can go. I looked up the ecliptic latitude of the Moon for that night, and bango! -5 degrees. Cool.

Science! It makes things predictable.

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