Recently, China's Chang'e 4 probe passed a milestone - 1000 days on the far side of the moon.
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The far side of the moon is often mistakenly called the dark side of the moon. However, to all you Pink Floyd fans out there, there is no such thing.
There is always a side of the moon that faces the Earth, called the near side, and a side that always faces away, the far side.
The far side of the moon gets sunlight.
It receives nearly the exact same amount of sunlight as the near side of the moon, the side we can see.
A lunar day, the time it takes the moon to rotate on its axis, which on Earth is 24 hours, is the same as a lunar year, the time it takes to go around the Earth.
Much like it takes 365.2564 days for the Earth to go around the sun.
This is all because the moon is tidally locked to the Earth.
Over millions of years, the gravity between the Earth and moon has slowed the rotation of the moon to reach the point where its rotation (day) and orbit (year) are happening at the same rate. And unless something dramatically happens, it will stay like this.
Relative to the stars and rest of the sky, the moon does one orbit around the Earth every 27.29 days. Since the orbit around the Earth is the same as its rotation, this means every spot on the moon has about two weeks of continuous sunlight and daytime, followed by two weeks of continuous nighttime and darkness. Even the far side of the moon.
As the moon goes through its movement, the way the moon looks when it is lit by the sun or in darkness changes, and so does our view of it. This is known as the phases of the moon.
When we get a full moon, the side that is facing us, the near side, is completely illuminated by the sun. The other side, the far side, is indeed completely dark. However, about two weeks later, as the moon keeps going around the Earth, this all changes.
Now the side facing us is dark, in what we call the new moon phase. However, the other side of the moon, the far side, is now in full sunlight. It is not dark at all.
We then have phases in between when only half the side we see is lit by the sun, or just a small part, a crescent moon.
If you were in a space ship on the other side of the moon, you would see the phases of the moon on the far side.
You would have a full moon, a new moon and the phases in between.
It takes the moon about 29.53 days to go through the phases. Keen eyes would see this is two days longer than how long it takes the moon to go around.
Since the Earth and moon are still going around the sun during this time, the Moon has to catch up a bit in its phases.
It is an elegant system that is not really dark.
- Brad Tucker is an astrophysicist and cosmologist at Mount Stromlo Observatory, and the National Centre for the Public Awareness of Science at ANU.
