We like the moon, but it won’t always be close to us

Sit still for a moment. You’re still moving! I mean perfectly still. Don’t move at all.

Having trouble complying? That’s probably because, without launching yourself off this planet, you can’t. The planet you presently call home (unless someone puts this blog post in a time capsule or launches it out into space for other civilizations to discover and translate), is spinning right now. That’s what gives us the day/night cycle, as our fixed location relative to the Earth turns away from, then toward, our sun, Sol. If you were to float above the equator for an hour, you’d get to see roughly 1600km pass below you. Earth in turn is whipping around Sol at a rate of roughly 108,000km/h. And the sun isn’t exactly still either! It’s rotating in turn around the galactic centre point at a rate of roughly 220km/s — a second. That’s 792,000km/h.

To make the calculation even hairier, we also don’t know how fast our galaxy is flying away from the point of the Big Bang. We can only see so far into our own past via our best telescopes, before the opacity of the universe prevents us from seeing any further, so “where” the Big Bang happened is one of those questions that may never be answerable (though, a good answer is “everywhere”, since the relative dimensions of space are a byproduct of the initial event).

While we’re jamming about the cosmos at such breakneck speeds, we’ve got some neighbors that have achieved some small measure of stability in relation to us and our sun. Most of these neighbors accreted from the same gas disc that ignited our sun, though some of them may well have come from captured debris. Some of these stable objects are actually the result of large impacts in our solar system’s distant and shooting-gallery-like past. Our moon, for example, is widely believed to be the direct result of a massive impact very early in Earth’s existence by a body roughly the size of Mars — about one third the size of Earth.

The evidence for this theory is varied, but includes the fact that the moon shares very roughly the same composition of the Earth with the same oxygen isotopes. Save, that is, for the iron content, which would have mostly drained into the Earth’s core by the time of the impact, proving the Earth and Moon didn’t form concurrently from the same matter. There’s SOME iron on the Moon, but not nearly as much as here on terra firma.

From a computer simulation of such an impact, five hours after such a cataclysm you can see the ejecta still pluming out into space:

We know the Moon was once molten, and pretty much cooled to the point where volcanism ceased about 3 billion years ago (bya), so it had to happen sometime between the age of our solar system (roughly 4.6 billion years) and then. Obviously, it would have to have happened soon enough before the 3bya mark that volcanism would have had time to run its course, and late enough after the formation of the solar system that the Earth would have been about the right size and shape with the iron sinking down into the core (sans, of course, the material that crashed into it). Scientists used the ratios of Hafnium-182 and Tungsten-182 (what Hafnium-182 decays to), to determine that the Moon is very close to 4.527 billion years old, assuming that their measures of Hafnium-182’s half life is correct, giving us a rough time frame of about 70 million years after Earth’s accretion before the impactor went kersmash. As always, with new evidence, science refines its numbers.

Now, this impact didn’t exactly throw the moon into a perfect orbit around the Earth. That orbit is inclined about 5.1 degrees to the ecliptic — the 2D plane of Earth’s orbit around Sol — which is relatively close for the purposes of eclipses, but definitely not perfect. Also, it is slowly receding away from the Earth at a rate of 38 millimetres per year, and will eventually shear off of Earth’s orbit altogether (though scientists predict not for many billions of years). It is tidally locked with Earth, meaning it shows us the same face all the time. The fact that it is tidally locked with us, while we’re spinning, means it’s slowly leeching off our angular momentum, braking our spin and increasing its own speed. This action is responsible for the slow recession of the moon.

It’s got some eccentricities to the face it shows us, however — a wobble called “libration”. This image from Wikipedia illustrates:

Also, the moon’s orbit around Earth is elliptical. At its perigee (nearest point), it is roughly 364,397km away; at its apogee (furthest point), 406,731km. The difference in visible size is very obvious, as well, as this image illustrates:

This difference in size means that many solar eclipses do not reach totality. These annular eclipses are no less dramatic. But they do put the lie to the suggestion that the sun and moon are “exactly” aligned for eclipses. The variables are so many, that the “perfect case” scenario you see in infographics like this one from the BBC are so rare as to be nearly impossible.

That totality is possible at all, anywhere on the face of the planet, is a fun coincidence in our neighborhood’s history, but by no means a necessary condition for our universe, or our solar system, or life on our planet. And even this totality is not necessarily total. While the sun and moon are nearly perfect spheroids, the moon has a very pockmarked surface, leading to some spectacular displays during solar eclipses, known as Bailey’s Beads. While beautiful, this display betrays how imperfect even the most perfect eclipse can be. And I haven’t even touched on solar prominences and solar flares!

Pictures of Bailey’s Beads obtained from this site:

The moral of this science lesson is, of course, you must always be cautious with using the words “exact” or “perfect” around me (and my underlying linguistic prescriptivist nature!), or you might set me off on a science lesson. 🙂

Further reading, and images obtained from:

We like the moon, but it won’t always be close to us
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3 thoughts on “We like the moon, but it won’t always be close to us

  1. 1

    Excellent post, Jason. I think the majority of people really have an extremely difficult time wrapping their mind around concepts that involve timescales further than the next voting cycle, or distances further than the drive to Aunt Mildred’s.

    Everything is taken for granted as always being the way it will be, especially the Moon and the Sun.

    The moral of this science lesson is, of course, you must always be cautious with using the words “exact” or “perfect” around me (and my underlying linguistic prescriptivist nature!), or you might set me off on a science lesson.

    And don’t forget your towel! Towels are important, and you don’t want to get caught in the wrong circumstances without one!

  2. 2

    Hell yes! Glad to see you hitting the blogs again, dude. Don’t rush to blog about your hospital or chemo experiences though, I know how draining it can be to write and fret over your every little word. That said, would love to read it one day.

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