There’s no real reason most of us need to know what the Moon will look like on any particular day at any particular hour next year. No reason other than intellectual curiosity, that is. So if you have a healthy supply of that, then you’ll enjoy NASA’s latest contribution to staring at the internet and wondering where the time went.
Actually, that might be a little unfair. Like almost everything NASA produces, it is instructive. This animation of the Moon throughout the year does a good job of illustrating the idea of lunar libration, and how it shows us a slightly different portion of the Moon throughout the year.
There are actually two animations, both in 4K. One is an animation of the view from the Earth’s northern hemisphere, and one is from the south. Each one’s about five minutes long.
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Most of us know that the Moon is tidally locked to the Earth. That means the side that faces opposite Earth was unseen for most of human history. It’s only thanks to the space race that we finally saw it. In October 1959 the USSR’s Luna 3 uncrewed spacecraft was the first to take pictures of the Moon’s far side and transmit photos back to us.
So with half of the Moon’s surface out of sight, that means from Earth we can only ever see 50% of the surface, right? Not exactly.
Thanks to lunar libration, we can actually see a little more than 50%. Not all at once, obviously. But over time, we can see up to 59% of our satellite. The Moon has a slight north-south wobbling as well as a slight east-west wobbling, and those actions change what portion of the Moon we can see at any time.
This short animation shows the effect of libration.
The animations shows clearly the effect of both latitudinal and longitudinal liberation. If you focus on the Tycho Crater, the large crater on the bottom of the Moon, it really makes the point.
The Moon’s rotation helps explain its libration. Yes, even though the Moon is tidally locked, it still rotates. We can’t tell because its rotation takes as long as its orbit around Earth: 27.32 days. That’s called synchronous rotation, and many moons in the Solar System display the same behaviour.
In the animation we can clearly see the Moon growing larger and then shrinking. This is because the Moon’s orbit has an apogee and a perigee; points when it’s furthest from Earth then closest to Earth. Though the Moon’s rate of rotation stays the same, its orbital speed changes. At perigee its orbit is quicker and at apogee its a little slower.
Those changes in orbital speed mean that longitudinal librations are at their maximum about one week after perigee and one week after apogee. After perigee (closest point), the Moon’s rotation falls behind its orbit, and after apogee (furthest point) the orbit can’t keep up with rotation. That exposes about another 8 degrees of the Moon to view.
There’s also latitidunal libration due to the Moon’s axial tilt. Compared to Earth’s orbital plane, the ecliptic, the Moon is tilted about 5 degrees in its orbital plane, and its equatorial tilt adds another 1.5 degrees of tilt, approximately.
When the Moon crosses the Earth’s ecliptic, that’s called a node. That happens twice a month, and when the Moon travels north across the ecliptic, that’s called the ascending node, and when it travels south, it’s called the descending node. The Moon’s latitudinal librations are most pronounced during the week after each node.
So after the Moon crosses the ecliptic to the north, a little more of the Moon’s south pole region is exposed. After it crosses to the south, a little more of the north pole is exposed. At certain times, more than 7 degrees beyond the poles are visible. Simple, right?
Your position on the Earth also affects which portion of the Moon you can see. But that doesn’t have much to do with the Moon itself. If you prefer to see more of the Moon’s north pole region, then move to the Earth’s northern hemisphere.
Luckily for us, it doesn’t matter where we live on Earth anymore. We have an almost unlimited view of the Moon thanks to telescopes and satellites.
But knowing these details of the Moon and its motion and its relationship with Earth does bring the Moon more “alive” and makes us think about things a little more deeply.
Mission accomplished, NASA.