Our Moon Rocks!

Last night's full moon photographed just after moonrise. Details: 400mm lens, f/5.6, ISO 800 and 1/125" exposure. Photo: Bob King

In yesterday’s blog, we learned why the far side of the moon is hidden from view. But there’s more to the story than that. A small portion of the lunar far side has been seen right here from Earth without the need to send astronauts into lunar orbit.

Astute sky watchers can actually see a total of 59% of the moon’s surface over time because the moon rocks! Well, not exactly the way you’re thinking, but a rocking movement of the moon called libration allows us to see it from slightly different angles at different times. We’re able to peer a little this way and a little that way around its edge to get a taste of the hidden side.

Because the moon's orbit is tilted with respect to Earth's, we see an extra "slice" at the top of its northern hemisphere (left of Earth in diagram) for half the month and an extra slice of the south during the other half. Illustration: Bob King

Libration comes in three flavors. The first, libration in latitude,  is best described as the moon shaking its head ‘yes’. The moon’s orbit isn’t flat but tilted 5 degrees from the plane of Earth’s orbit. During the approximately one month it takes the moon to circle Earth, the tilt lets us look ‘over the top’ for about two weeks and then ‘under the bottom’ for the next two.

The prominent crater Tycho provides an easy way for us to see libration in latitude over a month's time. In the photo at right, notice how much farther south of Tycho you can see during a favorable libration. Credit: John Chumack (left), Frank Barrett (right)

One of the best ways to see the moon nod is to keep an eye on the big and bright lunar crater Tycho located near the moon’s southern edge. Around the time of maximum southern libration, you’ll notice the crater ‘tips’ upward (northward) into the disk and appears to ‘open wide’. No telescope is needed – even binoculars will show its changing character. If you watch over a month’s time the moon will seem to ‘nod off’ and then ‘wake up’ again.

The moon is closest to Earth in its monthly cycle and moves fastest at perigee. When farthest from Earth at apogee, it moves slowest. Illustration: Bob King

Libration in longitude is the moon shaking its head ‘no’.  It results from the moon’s oval or elliptical orbit. If its orbit were perfectly circular, the moon would always move around the Earth at the same speed. In an elliptical orbit, it speeds up and slows down in accordance with Kepler’s Second Law of Planetary Motion, moving fastest (2,410 mph) when closest to Earth and slowest (2,160 mph) when farthest away.

Remember from yesterday why we can only see one face of the moon? It’s because the moon rotates on its axis once in the same time it takes to go once around the Earth. They’re in synchrony. Well, not quite. Though the moon’s rotation rate is constant, we just saw that its speed along its orbit varies during the month. This causes the moon’s rotation to sometimes get a bit ahead of itself or to fall behind of its orbital position, exposing more of the moon’s surface to the east and west. We get to  peek around the edges as it were.

Simulated views of the Moon over one month, demonstrating librations in latitude and longitude. Credit: Tom RuenÂ

You can easily observe this side-to-side motion by following a prominent feature along the east or west edges. Mare Crisium (Sea of Crisis) is perfect for the job, being located along the moon’s northwestern or upper right side. In a favorable libration, when the sea is tipped into the moon’s disk, it’s nearly circular. When not, it’s shaped like a bean. Here’s a nice set of photos so you can see for yourself.

One of the best lunar far side features that creeps around just enough to begin to appreciate its enormity is the Mare Orientale impact basin. This thing looks like a huge bullseye 600 miles across. At favorable times, we see it in profile along the moon’s eastern edge as a series of intriguing white and dark stripes. Other times it’s completely removed to the far side.

A final, razor-thin slice of otherwise invisible lunar cheese comes into view when the moon is rising and setting. Observers can see a smidge “over the top” of the moon at moorise and a smidge “under the bottom” near moonset. It’s called diurnal libration because it happens every day, but its effect is very small.

We see the moon from one side of the Earth and then the other between moonrise and moonset. This allows an observer to look a bit over and under the moon. The blue arrow shows the direction of Earth's rotation. Illustration: Bob King

The cause? Once again, it’s perspective – at moonrise we see the moon from one spot on our rotating planet. Some 12 hours later at moonset, the planet’s turned halfway around and we see the moon some 4000 miles from where we started at moonrise.

All of the motions involved in astronomy can sometimes give you a queasy stomach, but if you tease them apart one at a time, each can be understood individually and then formed into a whole.

2 Responses


    I like your astronomy information, it is very interesting. It is better than what we have in the AZ Republic or the rag we call it. Your explanations are easy to understand. Good Job !

    1. astrobob

      Hi Craig, thanks for your kind words. If you ever have an astro-related question, just click on the comment button.

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