The Moon Does Not Go Around the Earth

Moon RiseOn Thanksgiving night, I was looking at the moon. I find it endlessly fascinating, but probably not in the way that most people do. Everyone knows that the earth goes around the sun and the moon goes around the earth. But they are really screwed up about this. People watch the sun as it rises in the east and sets in the west and they know that what is really happening is that the earth is rotating on its axis and so it just appears that the sun is rising and setting. And that is more or less correct.

But then they look at the moon and they see that it rises in the west and sets in the east and know what is really happening is that the moon is rising in the east and setting in the west. And that is more or less complete wrong. The moon orbits the earth in the same direction that the earth rotates. So if the earth stopped rotating, the moon would rise in the west and set in the east — very slowly.

This is why watching a lunar eclipse can be a freaky experience. You are watching the moon go across the sky toward the west. And then the shadow of the earth begins happening on the east edge! It is counter intuitive. And if you continue to watch the eclipse, you will see it gets more weird. An eclipse can last almost two hours during which time, the moon will appear to move across the sky quite a distance.

Wittgenstein famously noted that the sun going around the earth and earth turning on its axis create the same observational evidence. And I think it is interesting that people may know that the earth goes around the sun, but they don’t really. It is just a little factoid that has been drummed into their heads. And we know this because of the way that they confuse what is going on with the moon. I know this because I used to teach planetary astronomy. Most students thought that the phases of the moon were due to the earth’s shadow and so were confused about how the moon could stay full all the way from moon rise to moon set.

My trick for teaching this subject was for my students to start thinking about the moon the way they do the sun. Forget what you’ve been told: the moon does not go around the earth. At least, the moon doesn’t go around the earth in any given night. It moves about 6° in that time, so it isn’t a big enough effect to worry about. The effect of the rotating earth totally swamps the effect the moon’s orbit.

We modern humans tend to greatly overestimate how much we know. We are told simple facts such as the moon going around the earth, but it confuses us more than anything. This might bee a good definition of a factoid. It is something people know, which is true. But it is not only useless to them, it actually confuses them by causing them to think that the moon travels across the sky because it orbits the earth.

And look at the kind of questions you can answer if you assume that the moon does not orbit the earth! What is the phase of the moon during a solar eclipse? (New.) Can you see a lunar eclipse at dawn? (Yes.) And most of all, you won’t go your whole life thinking that the phases of the moon are caused by the earth’s shadow.

Afterword

Orbits are actually more complicated than this. Both the earth and the moon revolve around their shared center of gravity. That’s also true of the sun and the rest of the solar system, but the sun is so large that it doesn’t move much. Of course, that’s only true from our perspective. Our solar system is moving roughly 500,000 mph around the center of our galaxy. There is no such thing as absolute speed, which is what Einstein was talking about a century ago.

2 thoughts on “The Moon Does Not Go Around the Earth

  1. Military school wasn’t for me, but reckoning a ship’s location by stellar observation was so damn cool. I usually was only off by a mile or so, pretty good for an amateur, deadly for a pre-GPS/radar navigator. Ships still carry tables of star/planet/moon/sun stuff they could use to find where they are, if all else goes wrong (or if you’re in the lifeboat!)

    If you ever heard the Shackleton story, about the Antarctic explorer who had to sail in an open boat some 700 miles to get rescue for his crew left behind, the real hero was the navigator, Frank Worsley. (http://en.wikipedia.org/wiki/Frank_Worsley#Voyage_of_the_James_Caird) Dude did sextant readings in basically, a canoe, and calculated the location using info in his head. That is some attention to detail, right there!

    • I look at the stars every night that it is clear. Yet I still get confused. I mostly do it just so I can find the Big Dipper and thus the North Star. Of course, since I usually know what direction north is, it isn’t that hard. But I find it shocking that there is so much that modern man “knows,” but only because he’s been told it. That is not a good situation.

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