The earth and moon are locked in a gravitational embrace. The moon, strictly speaking, does not orbit the earth. Rather, the earth and the moon each orbit around a common point. That point is called the barycenter (‘bary’ is Greek for ‘heavy’) because it is the center of mass of the two-body system. The moon’s mass is about 1/80 of the earth’s and it is about 30 earth-diameters away. The barycenter of the earth-moon system, it turns out, is located about 1000 miles beneath the earth’s surface.

Here’s a .gif I found on Wikipedia that illustrates the dance:


That’s how it goes for us and our lunar partner. Round and round and round. Our moon is quite large for a planetary satellite. It is in fact bigger than the dwarf planet Pluto! The Sun is much more massive than the Earth, about 333,000 times bigger, so the barycenter of the earth-sun system is much closer to the center of the sun itself even though the sun is almost 12,000 earth-diameters away. The sun accounts for about 99% of all the mass in our solar system. Jupiter, interestingly, is massive enough and far enough away that its barycenter is about half a million miles above the sun’s surface. That’s not much when you figure Jupiter is nearly half a billion miles away.

This odd dance we do only shows us one side of the moon. We know the earth rotates on its axis once a day. The moon rotates on its axis once a month. The earth revolves around the sun in one year. The moon revolves around the earth in one month. That synchronicity of the moon’s rotation with its revolution means we see the same face all the time. The moon is said to be tidally locked to the earth. The so-called dark side of the moon is not really dark at all, we just don’t get to see it illuminated like we do the near side. So it’s best to call it the far side of the moon as it is always further from us than the side we see.

The moon disappears from our view once a month as it travels between us and the sun and the near side is no longer sunlit. That means the far side would be in a full moon phase if we could see it. No humans saw the far side of the moon until a Soviet spacecraft sent back pictures in 1959. We actually do see a little more than half of the moon’s surface from earth because the moon’s orbit is not a perfect circle. Its slightly elliptical shape causes a little more of one side and then the other to appear in our view now and again. The moon appears to wobble east-west. This is called libration and is something you can observe yourself. Get a moon map and a pair of binoculars and watch the Mare Crisium and note how far from the edge it appears. Watch carefully for several days and you will be able to see it ‘wobble.’ A full moon, I should note, is very bright, so if you look at it with binoculars wear your sunglasses. Waxing crescent and first quarter phases are good times to find the Sea of Crises, just west (from our perspective) of the famous Apollo 11 landing site at Tranquility Base in the Mare Tranquillitatis.


The moon also has a nodding north-south libration but that is due to its inclined orbit. The moon does not lie in the same plane as the sun and the earth but is tilted about five degrees. This is the reason we don’t have eclipses every month. But that’s the subject of my next post.

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