The dark side of the moon has captured imaginations since we first stepped foot there over 40 years ago. From Pink Floyd to the recent Transformers movie, popular culture has theorized what is on the side of the moon that is in perpetual darkness. So, what is the dark side of the moon? Are there really alien robots on the other half of the moon, or have we shed a little more light on the subject?
Well, it turns out that there actually isn’t a “dark side.” Like the Earth, the moon has both a day and a night. It is better to say that the moon has a “day side” and a “night side” than a “dark side.” Sometimes, the side facing the Earth is the day side, and other times it is the night side: this is why we observe moon phases. Both sides of the moon receive sunlight.
Caption: Phases of the moon.
The reason it seems that only half of the moon receives sunlight is because the Earth only views half of the moon. It is more accurate to say that the moon has a near side and a far side. The near side is always facing Earth and the far side is always facing away, because the moon revolves around the Earth at the same rate that it rotates on its own axis. It takes 29 days for the moon to rotate around its axis, and it also takes 29 days for the moon to orbit around the Earth. If the moon did not rotate, or if it orbited slower around the Earth, then we would be able to see the other side of the moon, but because these rotations happen at the same rate we always see the same side. This term is known as synchronous rotation.
While only one side of the moon faces Earth, we still have an idea of what the far side looks like thanks to NASA’s Lunar Reconnaissance Orbiter Camera, or LROC, Wide Angle Camera, or WAC. The LROC was the first space craft to take high resolution photos of the far side of the moon last March, finally giving scientists a complete picture of the moon.
Caption: The lunar far side, seen by the LROC WAC.
Credit: NASA/Goddard/Arizona State University
There may not be aliens on the far side of the moon, but there are still many questions about it that we are still in the dark about, like why does the crust appear thicker on the far side? As NASA continues to explore the far side of the moon, people will no doubt continue to be fascinated by the “dark” side and continue to look for answers to illuminate its mystery.