The Far Side of the Moon

This week’s guess the planet image is a photograph of the far side of the moon. Credit goes to the crew of Apollo Eight. They were the first humans to orbit the moon in December of 1968, bringing back some phenomenal images of its far side. Their most famous photograph shows the earth, rising above the limb of the moon, giving a unique perspective on our home planet. 

 For most of human history the far side of the moon was something of an enigma. The surface of our satellite always looks the same in the night sky, because the same hemisphere is always facing the Earth. For thousands of years astronomers had no way of viewing the far side, until the soviet Luna 3 spacecraft orbited the moon for the first time in 1959. It sent back humanity’s first view of this area which surprisingly looks quite different to the near side. 

Our side of the moon is dominated by the dark lunar “maria”, Latin for “seas”. These regions are actually dark planes covered by expanses of basaltic lava. The far side of the moon has far fewer maria, instead it is covered by brighter, heavily cratered terrain.

So why does one face of the moon always point towards Earth? 

The moons far hemisphere is sometimes referred to as its “dark side” however this is a misnomer. The moon spins on its axis as it orbits the Earth. This means that it experiences day and night, which can be clearly seen as it changes shape throughout the month. If the moon didn’t spin then we would see different sections of its surface as it gradually moved around us, however the moon is “tidally locked” with the Earth. This means that the rate at which it spins and that at which it moves through its orbit exactly match. It rotates at just the right speed to keep the same face pointed towards the Earth. This is no coincidence; the tidal forces which the Earth and the Moon exert on one another have gradually brought them into line.  

The same is true elsewhere in the solar system. Pluto and its large moon Charon are both locked to one another. The same side of Charon is always visible from Pluto, but unlike Earth’s moon the reverse is also true, and the same side of Pluto always faces its satellite. This means that Charon doesn’t move through the night sky in the same way that our moon does, but would always appear in the same place. It was long thought that mercury was tidally locked to the sun. However better observations revealed that this wasn’t the case, Mercury actually rotates three times for every two revolutions around the sun.

 This ratio is called the spin-orbit resonance. Everything in the solar system interacts gravitationally with its neighbours, so numerous resonances occur. Some are more complicated than others, and all produce different orbital arrangements. 

Image Credits: 

• Apollo Eight image of the lunar far side GPN-2000-001127. Public Domain image from NASA via https://en.wikipedia.org/wiki/File:The_Lunar_Farside_-_GPN-2000-001127.jpg
• "Earthrise" Photographed by astronaut Bill Anders from Apollo 8. via https://en.wikipedia.org/wiki/File:NASA-Apollo8-Dec24-Earthrise.jpg

More information about:

The Luna 3 spacecraft 

Apollo Eight

Orbital Resonance