I can still remember the astonishment—the absolute and overwhelming wonder—I experienced the first time I found out what those twinkly little dots in the night sky really are. They are stars, of course, but what came as a real surprise to me was that each one of them is a massive burning ball of hot gas, just like the one we call the Sun. The only difference is that the ones we call stars are so unfathomably far away that they look to us like tiny white-yellowish specks in the sky.
Just a few months ago I heard about the far side of the Moon, and my world was rocked again. In my 29 years on this planet, nobody had told me about it! I learned history, mathematics, and physics in school… but a little more astronomy to shed some light on the beauty and elegance of our universe would also have been appreciated.
I wondered if other people had also missed out on the big news, so I did what any sensible inhabitant of the 21st century would do at a pivotal moment in his existence… I went on social media. Rather than spilling the metaphorical beans right away, I posed a question:
1: Have you heard about the “far side of the Moon?”
2: Do you know what that originally means?
Post yes/no answers only. Thanks!
There was an overwhelming surplus of yeses, but I realized some people might have been somewhat misled. This sentiment was voiced by my friend Joe Skilton, who tactfully pointed out:
Yes. But most people think they know and don’t 😉
My lovely sister sent me a text message inquiring:
What is that about? I’ve heard about the dark side of the moon, but I don’t know specifics. I know it’s the title of an album, but I’m not a fan of The Doors.
Sister apparently isn’t a fan of Pink Floyd either. However, all this led me to believe that many might not know about the astronomical implications (and the grandeur!) of the far side of the Moon. As I recently discovered, this refers to the hemisphere of the Moon we never see from planet Earth. Yes, there is such a thing.
Earth rotates around its own axis while concurrently revolving in an elliptical orbit around the Sun. It takes us (that is, Planet Earth) 24 hours to spin a full revolution and 365 days to travel a complete lap around the Sun. As result of this difference in duration, we experience day and night 365 times every year.
The Moon also rotates at the same time that she revolves around the Earth. However, there is a fundamental difference. It takes the Moon 27 days to make a full turn around her axis, which is the same amount of time it takes her to travel around the Earth—a peculiar and lovely phenomenon known as synchronous rotation. As a result, only one hemisphere of the Moon is ever visible from Earth… and it’s always the same one.
Let’s imagine it’s January 1, and the Moon is at point A on her journey around our planet. By January 8 (which happens to be my birthday!), our satellite has traveled ¼ of the path around her orbit, and rotated ¼ of a revolution. As a result, the Moon is displaying to us the same hemisphere we saw 7 days ago. The same math applies at every single step along the way.
Every tiny displacement is compensated by a proportionally tiny rotation, until the cycle is completed 27 days later. Then it begins all over again. As a result, only one hemisphere of the Moon is ever visible from Earth… and it’s always the same one. This hemisphere is referred to as the near side of the Moon. Its counterpart, which we never ever see from Earth, is known as the dark side.
As fantastic as this sounds, it is no cosmic coincidence. At the beginning (that is, when the Moon was formed some 4.5 billion years ago, give or take a few million years) her motion was more accelerated. The omnipresent effect of Earth’s gravity on the Moon has slowed her rotation down to its current pace. Nowadays the Moon moves in perfect synchrony around the Earth, a graceful dance that has taken millions of years to refine.