Does the moon orbit around the earth's equator?

November 27, 2007 8:52 PM

We had a full moon a few days back, and it seemed - to me - to be floating very much higher in the sky than a full moon I watched last summer, which I think was at a much lower angle. But if the moon rotates around the equator, then wouldn't it always be the same height in the sky, for any particular position on earth?

I understand the earth/sun relationship, and how the tilt of our axis causes the sun to be (apparently) higher and lower in the sky at different times of the year, but does the moon also follow a similar pattern?

I guess this is a no-brainer for astronomy buffs, but I'm having trouble looking this up on the web, not knowing the correct terminology. Can somebody shed some (moon)light on this for me?

posted by woodblock100 to science & nature (11 comments total)

I'm definitely no astronomy buff, but what makes you think the moon rotates around the equator?
posted by Dee Xtrovert at 8:54 PM on November 27, 2007

The moon doesn't rotate around the equator — it's inclined by about 20 to 30 degrees. So, no, it won't necessarily appear at the same height in the sky; in fact, it stays within about 5 degrees of the sun's path (the ecliptic).
posted by teraflop at 8:57 PM on November 27, 2007

I think these lunar phase illustrations will help. but I get a little dizzy looking at them myself.
posted by jessamyn at 8:58 PM on November 27, 2007 [1 favorite]

From Wikipedia: The Moon differs from most satellites of other planets in that its orbit is close to the plane of the ecliptic, and not to the Earth's equatorial plane. The lunar orbit plane is inclined to the ecliptic by 5.1°, whereas the Moon's spin axis is inclined by only 1.5°.

So in simple turns, the moon doesn't rotate around the equator, just like you had surmised.

This is one of those things that's more easily explained (at least to me) by those great models most planetariums have in the lobby, or animation. It takes way too many words to describe...
posted by pupdog at 9:00 PM on November 27, 2007

lunar phase illustrations

Phases I get ... this is different.

The moon doesn't rotate around the equator — it's inclined by about 20 to 30 degrees.

Well there you go ... that's it. It seems pretty counter-intuitive that it would be in such an orbit, but that will of course account for the variability in apparent height in the sky at different times of year.

Thanks for the quick answers ...
posted by woodblock100 at 9:09 PM on November 27, 2007

I'm guessing that the fact the moon orbits in the ecliptic plane is evidence that the moon was captured by the earth after both were formed.
posted by rdr at 9:35 PM on November 27, 2007

I'm guessing that the fact the moon orbits in the ecliptic plane is evidence that the moon was captured by the earth after both were formed.

It's actually thought that a very large meteor hit the very-young earth, breaking off a huge chunk that settled into orbit.

Just be glad we weren't around then.
posted by chrisamiller at 9:53 PM on November 27, 2007

A little more than halfway down on this page is a diagram of the showing the relationship of the ecliptic to the celestial equator (and showing the earth tilted on it's axis).
posted by oneirodynia at 10:20 PM on November 27, 2007

Related: libration, the group of processes whereby slightly more than half the Moon's surface is visible from Earth.

Earth's moon is one evidence for impacts of planetary-scale objects in the early solar system. Earth's large iron core and thin atmosphere (contrast Venus) are thought to be consequences of the same impact. Mercury and Mars both have hemisphere-sized craters, Venus has really slow and retrograde rotation, the asteroid belt, well, is, all theorized to arise from various planetary body impacts in the era of heavy bombardment. It is possible to take this idea too far, however.
posted by eritain at 10:23 PM on November 27, 2007

Oy. Forgot to mention: Not only the Moon's existence, but its composition (metal-poor, low-density minerals) argues for the impact's having happened to differentiated protoplanets and not just to random chunks of chaotic circumsolar matter.
posted by eritain at 10:25 PM on November 27, 2007

The 5.1 degree inclination of the moon's orbit to the ecliptic explains why we don't have a solar or lunar eclipse every month. Most months the moon is either above or below the ecliptic so that the moon, earth and sun don't line up in the same plane.
posted by JackFlash at 10:41 PM on November 27, 2007