Astronomy Essentials

What is lunar libration?

Two images of the halfmoon next to each other.
Lunar libration lets us see more than 50% of the moon. Photos by Manuel Castillo Vela/ EPOD.

It’s common knowledge that our moon has a near side and a far side. And most people know that half of the moon always faces Earth, and one half always points away. Does this mean we can only see 50% of the moon’s surface from Earth? No. Over time, it’s possible to see as much as 59% of the moon’s surface, due to a combination of motions – in particular, a slight north-south rocking and east-west wobbling of the moon – known as lunar libration.

Libration in longitude

Libration in longitude is the moon’s east-west wobble. This sort of libration is a product of the moon’s elliptical (elongated) orbit. Although the moon’s rotation, or spin, goes at a nearly constant rate, its orbital speed varies, going fastest at perigee (moon’s closest point to Earth) and slowest at apogee (moon’s farthest point from Earth).

At perigee or apogee, there is no libration of longitude.

Maximum librations are seen about one week after perigee and one week after apogee, revealing (depending upon the month) up to 8o of longitude on the moon’s back side, along the eastern and western limbs, respectively.

Following perigee, the moon’s rotation can’t keep pace with its orbit, so a slice of the moon’s back side slips into view along the moon’s east (right) limb; following apogee, the moon’s rotation outpaces its slower orbit, causing a sliver of the moon’s back side to emerge along the west (left) limb.

Moon at perigee and apogee: 2001 to 2100

Map of the surface of the moon, grey with dark patches.
View larger. The yellow lines define the near side of the moon, and the space between the yellow and green lines outline the far side of the moon that is visible from Earth, given favorable lunar librations. Image via WikiMedia Commons.

Libration in latitude

Libration in latitude is the moon’s north-south nodding. It results primarily from the approximate 5o tilt of the moon’s orbital plane with respect to the ecliptic (Earth’s orbital plane).

Add to that, the approximate 1.5o tilt of the moon’s equator to the ecliptic, and you have the inclination of the moon’s equator to the plane of its orbit around Earth at some 6.5o (5 + 1.5 = 6.5). Consequently, during the month, you can see about 6.5o of latitude beyond the moon’s north pole, and a fortnight later, 6.5o past the south pole.

Twice a month, the moon crosses the ecliptic (Earth’s orbital plane) at points called nodes. When the moon crosses the ecliptic from south to north, it’s called an ascending node; and when the moon crosses the ecliptic from north to south, it’s called a descending node.

There is no libration of latitude when the moon is at its ascending node or descending node.

Maximum librations happen about one week after the moon crosses either node. The moon’s southern limb is most exposed about one week after the moon crosses its ascending node, and its northern limb is maximally exposed about one week after the moon crosses its descending node.

In other words, when the moon swings farthest north of the ecliptic, the lunar south pole points most toward Earth. On the other hand, when the moon goes farthest south of the ecliptic, it’s the lunar north pole that points maximally toward our planet. At particularly favorable librations, we can see nearly 7o beyond either pole.

Nodes of the moon: 2001 to 2100

2 images side by side of part of the moon, including the Tycho crater.
The prominent crater Tycho provides an easy way for us to see libration in latitude. In the photo at right, notice how much farther south of Tycho you can see during a favorable libration. Images via John Chumack (left)/ Frank Barrett (right)/ AstroBob.

Other types of lunar libration.

Your position on Earth also has some, but significantly less, bearing on latitudinal libration. If you reside at a far northern latitude in the Northern Hemisphere, you see further north on the moon than does someone in the Southern Hemisphere. Of course, the reverse is also true: someone in the Southern Hemisphere sees more of the moon’s southerly features.

Your position also influences longitudinal libration – though once again, only marginally. At moonrise, you can make out a little more of the moon’s east (or top) limb; and at moonset, a little more of the moon’s west (and now at top) limb.

So, as you stand on Earth’s surface, it’s true that you see only 50% of the moon at any one time. And yet, all told, lunar libration – the north-south and east-west oscillations of the moon – reveal 59% of the lunar terrain.

Animated gif of a rocking moon, showing a simulation of what we see of the moon's surface over a month.
Simulated views of the moon over one month, demonstrating librations in latitude and longitude. These rocking and wobbling motions as seen from Earth enable us to see 59% of the moon’s surface over time. Image via Tomruen

Bottom line: A slight north-south rocking and east-west wobbling of the moon – known as lunar libration – let’s us see as much as 59% of the moon’s surface. That’s true even though one side of the moon always faces Earth.

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Posted 
May 25, 2021
 in 
Astronomy Essentials

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