We are used to seeing the same side of the moon and referring to the side which doesn’t face us as the ‘dark side’.
It isn’t dark, it’s just that our satellite is tidally locked to Earth, so it always shows us the same face. We only see the same half of the moon.
Except…
That’s not quite true!
As the moon orbits our planet, it ‘wobbles’ a little bit, which you can see in the gif, below. The wobbling is caused by the moon traveling slightly faster when it gets close to us, and slower when it is farther away.
This slight swaying means we see little bits of the dark side in a series of what are called librations.
In the rest of this article, we’re going to discover the detail behind lunar librations and how, as an astronomer, we can look out for them.
How Much of the Moon Do We See?
Because the Moon is tidally locked to Earth, we should technically only see the same 50% of its surface all of the time.
However, due to lunar librations and the moon’s slightly changing appearance, we see as much as 59% of the surface during the lunar month.
The extra detail which points our way is often in darkness and so invisible to us but there are a few days each month when a ‘favorable libration’ reveals craters that are, technically, on the far side of the moon.
Three Types of Libration
Libration is a catch-all term for three different causes of the moon’s wobble.
Longitude Libration
The Moon wobbles from east to west, and that wobble is called longitude libration. Longitude libration occurs because the moon moves faster when it is in its perigee (closest to Earth) and slower at apogee (furthest from Earth).
These changes in speed cause the moon’s orbital and rotational speeds to go out of sync, showing us more of the moon’s polar regions than what we would normally see.
Latitude Libration
The moon’s orbit is slightly tilted which makes it appear to wobble along its north-south polar axis. An observer on Earth will see more of the northern hemisphere at some points of the lunar cycle and more of the southern hemisphere on other days.
During the entire cycle, the latitude libration has the effect of making the moon look like it is nodding a ‘yes’.
Diurnal Libration
Things get slightly more complicated here because this type of libration also includes the fact that Earth itself is moving in space, and the observer is on the planet’s surface, not at its core.
What we as an observer sees of the moon from Earth’s surface changes from moonrise to moonset. During moonrise (in the east), we see more of the moon’s eastern side and during moonset (in the west), we see more of the western side.
This is a minor effect in the overall scheme of things and not one considered when astronomy tables set out libration timings.
When to See Librations
Librations happen all the time; they are a constant feature of the moon’s orbit around our planet. However, they are more pronounced on certain days and only some of these are visible to us because others expose parts of the moon’s surface which is in darkness.
In this section, we’ll look at how to find out when the best librations are happening in the month and how to read a libration map.
Lunar Libration Almanac
To look for ‘extra’ craters during a libration, we need to know when they are happening.
We cover them ourselves inside the Virtual Astronomy Club every month, click the link to become a member.
Elsewhere, you can find them inside astronomy magazines, Sky & Telescope is our favorite example.
On the internet, AstroPixels has a table for this year (opens in a new tab) that shows all the lunar libration data for the year. Use columns 7-9 for the relevant data (explained here). This is detailed but not as intuitive as a map of the moon showing where the libration is most prominent.
How to Read Lunar Libration Images
Once you have a date in mind for observing the libration, pop over to this website from NASA (opens in a new tab) and enter the relevant date and time of your observation.
Click on the main lunar image at the top of the page to download a large map of the moon for that moment. In the bottom left-hand corner, you’ll see a small version of the moon with crossing blue lines and a blue and yellow dot, like the one below. This tells you where the moon is librating.
To understand the image above note that the intersection of the two blue lines represents where the moon’s center is when there is no libration.
The blue dot you see is the center of the moon’s surface from our perspective on Earth.
To see where the libration is most noticeable, join a line from the intersecting blue lines straight through the blue dot and onto the edge of the chart. This is shown as a red dashed line in the image above. Where the line you’ve drawn meets the edge is the point of greatest libration.
The size of the libration is represented by the distance between the intersecting blue lines and the blue dot, as shown by the green bars in the example above.
For completeness, the yellow dot shows where the sun is overhead.
Locations to Observe at Liberation
There are thousands of craters on the moon – see exactly how many here, but only a few are visible from the dark side. Two of the best features to try and find see are more of Mare Crisium and the crater Endymion.
Mare Crisium
- Size: 345 miles or 556 km in diameter, 68,000 square miles or 176,000 square km of surface area
- Location: 17.0 °N 59.1 °E. Northeast of Mare Tranquillitatis in the Crisium basin. Learn more about lunar coordinates.
- Features: Mare Crisium has a flat base and is distinguished by many “wrinkled” mini-craters all around its boundary. From a small telescope, it is seen as a dark spot on the moon’s edge.
Crater Endymion
- Size: Depth – 2.6 km, Diameter – 77 miles to 125 km
- Location: 53.6 °N 56.5 °E
- Features: Crater Endymion resides to the east of Mare Frigoris and the north of Lacus Temporis. It looks slightly oval when you zoom in a little.
Summary
Lunar librations allow amateur astronomers and space enthusiasts to see more of the moon than we normally do.
Only a few of the lunar features from the dark side of the moon are seen during these librations, which is what makes them so challenging and worth looking forward to!
Which feature will you try spotting first?
