There is no real reason why most of us need to know what the moon will look like on any given day at any given hour in the next year. No reason other than intellectual curiosity. So when you have a healthy supply of it, you'll enjoy NASA's latest post staring at the internet and wondering where the time went.
Actually, that could be a little unfair. Like almost everything NASA produces, it is educational. This year-round animation of the moon does a good job of illustrating the idea of lunar vibration and how it shows us a slightly different part of the moon throughout the year.
There are actually two animations, both in 4K. One is an animation of the view from the northern hemisphere and one from the south. Each is about five minutes long.
Most of us know that the moon is tide-bound to the earth. This means that the side facing the earth has been invisible to most of human history. It was only thanks to the space race that we finally saw it. In October 1959, the unscrewed Luna 3 spaceship of the USSR was the first to take photos of the other side of the moon and send photos back to us.
The first photo of the far side of the moon, taken by the Soviet (Russian) spaceship Luna 3 on October 7, 1959. The right three quarters of the disk are the opposite side. A = Mare Moscoviense, B = Tsiolkovsky crater with a central peak, C = Mare Smythii (on the near side edge) and D = Mare Crisium (near side). This is the wide angle view. Photo credit: Roscosmos
So if half of the moon's surface is out of sight, we can only see 50% of the surface from Earth, right? Not exactly.
Thanks to the lunar vibration, we can actually see just over 50%. Not all at once, of course. However, over time we can see up to 59% of our satellite. The moon has a slight north-south wiggle as well as a slight east-west wiggle, and these actions change which part of the moon we can see at any given time.
This short animation shows the effect of the libration.
The animations clearly show the effect of the liberation in width and length. When you focus on Tycho Crater, the big crater at the bottom of the moon, it really makes sense.
The rotation of the moon explains its calibration. Yes, even though the moon is tide-locked, it is still spinning. We can't tell because its rotation is as long as its orbit around the earth: 27.32 days. This is known as synchronous rotation, and many moons in the solar system exhibit the same behavior.
In the animation we can clearly see the moon getting bigger and then shrinking. This is because the moon's orbit has an apogee and a perigee. Points if it is furthest from Earth and then closest to Earth. Although the rotation speed of the moon remains the same, its orbital speed changes. Its orbit is faster at perigee and a little slower at apogee.
Comparison of the apparent size of the moon at the lunar perigee apogee. Photo credit: The original uploader was Tomruen at English Wikipedia. – Transferred from en.wikipedia to Commons by Mike Peel with CommonsHelper., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=8627371
These changes in the orbital velocity mean that the longitudinal vibrations are at their maximum about one week after perigee and one week after apogee. After the perigee (closest point), the rotation of the moon falls behind its orbit, and after the apogee (furthest point) the orbit cannot keep up with the rotation. This makes another 8 degrees of the moon visible.
There is also a latitidunal libration due to the axial tilt of the moon. Compared to Earth's orbit, the ecliptic, the Moon is tilted about 5 degrees in its orbit, and its equatorial tilt adds about another 1.5 degrees of tilt.
This diagram shows a simplified view of the moon's orbit and angular position in relation to the earth's orbit (ecliptic). The note states: “The relative sizes and angles of the earth and moon are to scale. The relative distance between the earth and the moon is not to scale. “Photo credit: By Peter Sobchak – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=35889221
When the moon crosses the earth's ecliptic, it is called a knot. This happens twice a month. When the moon moves north across the ecliptic, this is known as the ascending node. As it migrates south, it is known as a descending knot. The longitudes of the moon are most pronounced in the week after each knot.
After the moon crosses the ecliptic to the north, a little more of the moon's south pole region is exposed. After it crosses south, a little more of the North Pole is exposed. At certain times, more than 7 degrees are visible above the poles. Easy right?
Your position on earth also affects which part of the moon you can see. But that has little to do with the moon itself. If you want to see more of the Moon's North Pole region, move to the Earth's northern hemisphere.
Fortunately, it no longer matters where we live on earth. Thanks to telescopes and satellites, we have an almost unlimited view of the moon.
But when we know these details of the moon and its movement and its relationship to the earth, the moon becomes "more alive" and we think a little deeper about things.
Mission accomplished, NASA.