I was out on a sunny day a few weeks ago waiting for a bus, and as per usual I have my head up in space (I would say up in the clouds but that is too low for me). I noticed the Moon up in the sky, just past first quarter, and I was thinking about the angle of the illuminated side and how it related to some of the positions of the Earth and Sun in space. I wondered what information we could gather from the way it looked. It led me to this ‘illuminating’ post.
I realized that if you draw a line from the top of the crescent to the bottom of it, that line would be perpendicular to a line from the Sun to the Moon and consequently from the Sun to the Earth. But why was this line angled (as per the drawing below)?
For one, I noticed that this imaginary line would point south, which instantly became a great navigation tool. Go try it for yourself, most of the time it will very accurately point south. I say most of the time because it’s not as accurate when the Moon is close to the horizon.
This is true for those in the northern hemisphere, but what about for the rest of the world?
I looked at some photos of the Moon taken by photographers in the southern hemisphere and along the equator (isn’t the internet great?), and the findings were quite interesting. Both the position in the sky and the angle of the Moon changed.
The imaginary line from one side of the crescent to the other was pointing north when viewed in the southern hemisphere, and when at the equator the line was straight. This makes sense, since it seems one side points north and the other south. So the trick for southern observers is that you can use the Moon to find north and get your bearings the same way as I do on the other side of the planet.
Then a true realization hit me. For an observer standing on the Earth, the apparent angle of the Moon’s crescent could be used to give a rough estimate of the observer’s latitude! All you have to do is make a line through the Moon perpendicular to the horizon and a line from one side of the crescent to the other. The angle between these two lines will give you you’re rough position latitudinally on the Earth.
So there you have it. Not only can you navigate with the Moon, you can use it to figure out where you are on the Earth. Astronomically speaking, the assumptions you make are that the Earth is round and that the Sun is infinitely far away so it’s light falls parallel to the Earth and Moon. The Earth is round for sure, but the Sun isn’t infinitely far away, though it is a safe assumption to make since the Sun is so large and distant.
It’s amazing how an understanding of astronomy and a bit of math can combine to make some pretty cool navigational tools, and tell us where we are on the Earth. This is true science, using what you see to make predictions about your world.