How Well Do You Know Your Moon?

Phil Plait, who writes the wonderful Bad Astronomy blog over on Slate, noticed something interesting in a recent episode of The Simpsons:

Photo by 20th Century Fox Film Corp.
Photo by 20th Century Fox Film Corp.

To the untrained eye, nothing about this cartoon image is likely to seem unusual. But if you spend a lot of time looking at the sky, and you have the additional context that this scene occurred in the evening, then the Moon is actually quite revealing.

(For those of you thinking “it’s just a cartoon, don’t expect it to be accurate”, I realise that. But if you decide to treat it as though it must be accurate then it can be interesting to think about so bear with me.)

To understand why that is and how we know, we have to have a think about how we look at the Moon.

The Moon orbits the Earth in a plane that’s pretty well aligned with the plane of the solar system. This means if you drawn a line in the sky tracing the path of the Moon, you’ll also find the Sun and the planets roughly on that line. This is why we experience solar and lunar eclipses, which happen when the Sun and Moon are lined up particularly well with the Earth. Because the line is where eclipses happen, it’s called the ecliptic by astronomers.

Earth’s axis is tilted by 23.5° relative to this plane, but if you’re not too close to the equator (for example, if you’re in New Zealand or the USA) then you can say that if you projected the equator into the sky it would be in roughly the same position as the ecliptic. If you’re in the southern hemisphere, that means it’s to the north, and if you’re in the northern hemisphere, it’s to your south.

So, if you’re looking at the Moon from New Zealand, you must be looking roughly to the north. If you’re looking at it from the USA, you must be looking roughly to the south. This also means the Sun and Moon, moving east to west across the sky as the Earth spins, appear to move right to left from the southern hemisphere and left to right from the northern hemisphere.

When we look at the Moon, we’re seeing the same thing no matter where we are on Earth except for one thing: which way is “down”. The “bottom” of the Moon in the part that’s closest to the horizon. If you travel to the other hemisphere, and you’re familiar enough with the Moon, you may notice that it appears upside down. That’s because the direction of “down” has swapped – from roughly north to roughly south (or vice versa if you’ve travelled from north to south). If you want to see what the Moon looks like from the other half of the world, you have to bend over backwards (or lie on the ground). This is also why the Moon will appear to have rotated if you compare it when it’s rising to when it’s setting.

One more thing: the Moon orbits us in the same direction as we’re spinning, which means it moves across the sky slightly slower than the Sun. Each day, the Moon rises roughly 50 minutes later than the day before, so that over its 28 day cycle of phases this sums to 24 hours.

Now, getting back to that image from the Simpsons episode. That scene was apparently in the evening, and the Moon is low on the horizon. That means the Moon must either be about to set or have just risen. If it had just risen after sunset, then it was recently full (because a full moon rises at sunset and the moon rises later each day), which means its phase would be a waning gibbous. Waning refers to the fact that it is on its way from being full to being new, and a gibbous is the shape made by a circle with a crescent cut out from it.

In the picture, the Moon is obviously a crescent, so it can’t have just risen. If it’s just about to set after sunset, then is must just have been a new moon (because a new moon sets at sunset and the moon rises later each day), which means its phase would be a waxing crescent. Waxing refers to the fact that it is on its way from being new to being full, and the crescent refers to its curved shape.

Another thing we know about the Moon is that its lit side always faces the Sun. For example, the lit side of a full moon points right back at us, because from its perspective the Sun shines on it from behind us. If the Sun has just set, and the Moon is just about to set as well, then the lit side of the Moon must be facing the Sun. As the Sun sets in the west, this means the lit side of the Moon should also be facing west if it is a waxing crescent.

In the picture from the Simpsons, which we’ve established should be a waxing crescent, the lit side of the Moon is facing to the left. But remember, if you look at the Moon from the northern hemisphere you must be looking to your south, so west should be on your right. So if the waxing crescent moon is lit on its left, then you must be looking north to see it, which means you’re in the southern hemisphere.

Unfortunately, a lot of pop culture doesn’t get the Moon and its phases right. I know it’s such a tiny thing, and typically when they don’t get it quite right I can’t say I mind too much (although I often can’t help but notice), but I really love it when they put in that extra bit of effort to get it correct.

Almost all video games with day/night cycles where you can see the sky have the Moon orbit in 24 hours. Some of them include phases, although technically if your Moon always rises at sunset then it should always be full. I can forgive video games fairly easily though, I’m probably the only person who cares and I understand it could take significant development time to get proper lunar phases in. The only example I can think of that gets it right is Kerbal Space Program, where accurate celestial mechanics is an important part of the game.

Some books have issues with the Moon as well. Last year I was reading the book Ship of Theseus, and one scene describes the protagonist seeing the crescent moon rise as it gets dark. But crescent moons never rise as the Sun sets, light simply doesn’t work that way.

Movies often have trouble with it as well. The worst offender I’ve seen is the final scene from the movie Cloud Atlas:

Final scene from the movie Cloud Atlas
Final scene from the movie Cloud Atlas

Remembering that the lit side of a moon points towards its sun, and this applies even with multiple moons, that image implies some very strange things about the nature of light.

I’d love to see more media put more effort into getting this right. I know that one author who paid particular attention to this detail was J.R.R. Tolkien, who tried hard to get the phases of the Moon consistent with his dates when writing The Lord of the Rings and apparently did a pretty good job of it too.

My favourite example of well-documented in-depth world building doesn’t involve the Moon but I’d like to share it here anyway. My brother Jeremy (who’s currently working as a concept artist at Weta Workshop, a job he got soon after leaving Uni) worked on creating a deep and internally consistent fictional history to earn his masters degree in 2013. He ended up creating a fake National Geographic article from 1932 recounting the reporter’s visit to the settlement of Elkwood. The article only scratched the surface of all the thought he put into the work, if you want to see what he came up with you can read both the article and his exegesis explaining his research methods here: Creating Elkwood: building an alternate history

When fictional worlds are deep and internally consistent they become that much more enriched. If you know of any that have represented lunar cycles particularly well (or particularly poorly) let me know in the comments.


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