Is there a dark side of the Moon?

Is there a dark side of the Moon?

What it looks like? Why we are not able to see it?

The MoonSource: NASA

Exploring these enduring questions is very common in human culture. For decades people were wondering on the mystery of this unknown lunar hemisphere. It even permeated to the literature of Jules Verne, as well as popular music. But does the dark side of the Moon really exist?

At the same time YES and NO. Yes, because the part of moon surface that constantly faces away from Earth is surely present. It was photographed by numerous spacecraft. And no, because the existence (or non-existence) of any part of any celestial body that is not visible from a certain point of view is always relative and it is a philosophical issue. Is the dark side of the Moon really always dark, or it is just dark because it’s never illuminated by the sunlight for an observer placed here, on Earth? Of course, the latter is true. When Sun, or another source of light, shines on it, it is fully visible.

To explore this matter, however, we have to make some assumptions. The first is the nature of lunar visibility. The Moon does not have any own sources of light or radiation. It is a rocky, cold globe with no, or almost no, thermal emission. So why we can observe it? This question is tricky. Lets remember that the phase of the Moon changes: sometimes we see the Full Moon, sometimes the half of its surface, and occasionally – no Moon at all. This led ancient sky watchers to the conclusion that the moonlight was really the light reflected from the Sun that at that particular time had been located opposite to the Moon. The reflection of this light takes about two seconds to reach us. More interesting fact is that the Moon absorbs also solar radio emission and then reflects it back, but after several hours. So it is also “visible” on radio waves.

The visibility of the Moon on Earth is not complete – actually, we can admire only about a half of its surface. Some people believe that this happens due to the lack of the moon’s spin around its own axis. The Moon is not rotating and we can only see the part which is facing the Earth. But this picture is incorrect and Moon has its spin. However, its motion is very special. Scientists would say that the Moon is in synchronous rotation with Earth. This means it’s spinning around its axis, but with such speed that its near side, which we call “visible”, is always facing the Earth. The effect of synchronous rotation, or captured rotation, is common in celestial mechanics. It is caused by tidal locking: gravity of one body produces a tidal force acting on the second. This force distorts the shape of the second body slightly so that it becomes elongated towards the first body. In the case of large spherical bodies the tidal distortion produces a slightly elongated spheroid.

After a long time some amounts of the material are displaced, and the gravitational pull exerts a torque on the second body. This tidal drag causes the slow loss of kinetic energy of both bodies and their movement slows down leading eventually to tidal locking. It also applies to other celestial bodies in the Solar System including Earth: the gravitational forces of the Sun and Moon cause gradual slowing down of Earth’s rotation, so the days and nights become slightly longer and in a very distant future Earth and Sun may be also in synchronous rotation. But lengthening of the Earth’s day due to the tidal effects of the Moon is much more efficient: average day is now longer by about 1.7 milliseconds than a century ago.

Lunar Farside, As Seen by Apollo 8
Lunar Farside, As Seen by Apollo 8 (Source: NASA)

Now we know that one hemisphere of the Moon is always invisible. This conclusion is only approximately true. We can observe more than half (about 59%) of the Moon’s surface around its edges due to the phenomenon called libration. Libration is a word describing several effects which allow us to „peek” at celestial body around the edge. One of them is geometrical libration due to the shift in viewing direction. Other one occurs because the orbiting motion of the Moon is not steady and at some times it is faster or slower than average, allowing us to see some additional lunar longitudes.

So is the dark (or, more scientifically speaking, far) side of the Moon ever illuminated? Of course it is. Sunshine falls there at any time when the Moon is located just between Sun and Earth. At the same time we have so-called New Moon because the usually visible lunar hemisphere is now lost in the sunlight. We cannot observe it from the surface of our planet, but we can see it indirectly through spacecraft located beneath far side of the Moon. One of humans’ first views of the dark side of the Moon dates back to the Sixties, when American Apollo 8 mission orbited the lunar globe. It was also photographed by the Soviet Luna 3 probe in 1959. Far side is significantly different from the visible one: it is covered by numerous impact craters and relatively few lunar “seas”. There is also the second-largest impact feature in the entire Solar System, the South Pole-Aitken basin.

The Earth-Moon System
The Earth-Moon System – as seen by Galileo spacecraft (Source: NASA)

One possible explanation for these differences is a higher concentration of heat-producing elements on the near side. This theory has been already confirmed by geochemical maps obtained by lunar probes. However, in general, far side of the Moon seems to be nothing unusual – perhaps apart from the fact that it provides very convenient potential location for large radio telescopes and radio interferometers as it would always be shielded from possible radio interferences generated on Earth.

Author: Elzbieta Kuligowska

References & further reading:

Earth’s Moon: Overview (NASA)
Dark Side of the Moon (Bad Astronomy)
Libration of the Moon
What and where is the dark side of the moon? (HowStuffWorks)