Moon phases

[HareBrain]

Over years of reading it's come to my attention that a great many fantasy authors, a certain S. King among them, don't get the behaviour of the moon right. (I'd hope the position is better in SF, but I don't read a lot of it.) So I thought I'd do a rough guide, so people know why to avoid things like "it was a lovely evening and a new moon was just rising".

First thing, the moon doesn't always come out at night. It rises roughly an hour later each day (because it orbits the Earth roughly every 28 days). This means that about half the time, it's not in the night sky at all, but the daytime sky.

Second, its phase (new moon, first quarter, full moon, etc) depends on where it is relative to both the sun and to us. At full moon, we see the whole disc lit because we are looking at the moon with the sun behind us -- the sun is shining over our shoulder. It's the same effect as if someone were standing behind you (but just off to one side) shining a powerful torch past you and onto a big ball.

For this reason -- the sun is behind us as we look at a full moon -- a full moon is necessarily opposite the sun in the sky. Therefore, a full moon only ever rises as the sun sets, and sets as the sun rises.

Similarly, a new moon is dark (unlit) because it's very close to the sun in the sky, and the sun is roughly behind it as far as our view is concerned. So it rises at sunrise and sets at sunset. (This proximity to the sun is why we only ever get a solar eclipse at new moon. Similarly, a lunar eclipse, when the Earth gets between the sun and the moon, can only happen at full moon.)

From the new moon, rising an hour later each day, the moon becomes a crescent as it waxes (grows larger), with its lit side facing the sun. So a waxing crescent moon will rise in the morning, and you will see it low in the western sky in the evening, following the sun down, with its right side lit if you are in the north.

Similarly, a waning crescent, with its left side illuminated (if you are in the north) will rise a couple of hours before the sun, and will be low in the eastern sky at dawn. It will set in the afternoon/evening.

The moon's position in the sky is also affected by the tilt of the Earth's axis. It works roughly opposite the sun, so is low in the sky in summer, and climbs high in winter.

I hope this helps some people, and I've explained it clearly enough. If not, let me know and I'll try again.

Thank you for listening to my displacement activity.

 

[Kerrybuchanan]

That’s actually very useful. Thank you for such a clear summary.

 

[The Judge]

I used to have bookmarked some pages with full details and illustrations/images of the moon's cycle, complete with in which part of the sky the moon could be seen, which I pinched from a primary school website, of all places -- I didn't understand it properly, but it was invaluable for helping me when I needed to have an idea of what the moon would look like at a particular time and where. Then my browser played silly b*ggers and I lost 10 years worth of bookmarks in one fell swoop, including that.

Anyhow, just a couple of things I can add to HB's moonshine...

• although the full moon only rises after the sun has set, it's as well to remember that it's not rising in pitch darkness but during twilight, so it's actually still quite light for some time with the full moon in the sky
• the way to remember that in the North the crescent of the waxing new moon is on the right of the moon's circle, is to think of it as kind of the shape of a curvy J, while the crescent of the waning old moon is on the left in kind of the shape of a C -- and, of course, J = Judge who is forever young. *cough cough*. Ahem, anyway J= Juvenile, ie young, while C = Crone, ie old.

 

[tinkerdan]

This reminds me of AirWolf.
I was watching those recently on one of the free channels.
In the first episode they this video of night at the cabin with dark pines and a full moon behind them.
Subsequent shows had the same moon every time there was night at his cabin. Which meant that the moon was always in the same place and always full.

A bit later in further episodes they would have the same video when they were somewhere else--one time out in the desert(a desert that was apparently full of mountains with pines.)

This was probably due to budget constraints.

As a struggling author I think I'd go for the budget constraints argument for any such discrepancies.

 

[HareBrain]

I've seen plenty of impossible moons in high-budget stuff. I just think "how the moon works" isn't something most people think about. My brother once told me he thought the dark bit of a crescent moon was caused by the Earth's shadow -- and he has a science degree! (Well, computer science.)

 

[tinkerdan]

How silly: everyone know the moon dragon feeds slowly on the moon until it reaches just the right spot for it all to slowly grow back again.

There are many things in the heavens and the earth that people make assumptions about and then manage to take that for granted.

The earth is flat; by the way.

 

[Phyrebrat]

That’s no moon...

 

[Andy Hauser]

Love this, thank you so much!

 

[CTRandall]

But how does it work in a binary star system? With three moons? And you're on the moon closest to a superearth planet?

 

[tinkerdan]

Gosh...let me get my sun goggles out.

But how does it work in a binary star system? With three moons? And you're on the moon closest to a superearth planet?

I'll take a look-see and let you know.

 

[Snuggs]

Over years of reading it's come to my attention that a great many fantasy authors, a certain S. King among them, don't get the behaviour of the moon right. (I'd hope the position is better in SF, but I don't read a lot of it.) So I thought I'd do a rough guide, so people know why to avoid things like "it was a lovely evening and a new moon was just rising".

First thing, the moon doesn't always come out at night. It rises roughly an hour later each day (because it orbits the Earth roughly every 28 days). This means that about half the time, it's not in the night sky at all, but the daytime sky.

Second, its phase (new moon, first quarter, full moon, etc) depends on where it is relative to both the sun and to us. At full moon, we see the whole disc lit because we are looking at the moon with the sun behind us -- the sun is shining over our shoulder. It's the same effect as if someone were standing behind you (but just off to one side) shining a powerful torch past you and onto a big ball.

For this reason -- the sun is behind us as we look at a full moon -- a full moon is necessarily opposite the sun in the sky. Therefore, a full moon only ever rises as the sun sets, and sets as the sun rises.

Similarly, a new moon is dark (unlit) because it's very close to the sun in the sky, and the sun is roughly behind it as far as our view is concerned. So it rises at sunrise and sets at sunset. (This proximity to the sun is why we only ever get a solar eclipse at new moon. Similarly, a lunar eclipse, when the Earth gets between the sun and the moon, can only happen at full moon.)

From the new moon, rising an hour later each day, the moon becomes a crescent as it waxes (grows larger), with its lit side facing the sun. So a waxing crescent moon will rise in the morning, and you will see it low in the western sky in the evening, following the sun down, with its right side lit if you are in the north.

Similarly, a waning crescent, with its left side illuminated (if you are in the north) will rise a couple of hours before the sun, and will be low in the eastern sky at dawn. It will set in the afternoon/evening.

The moon's position in the sky is also affected by the tilt of the Earth's axis. It works roughly opposite the sun, so is low in the sky in summer, and climbs high in winter.

I hope this helps some people, and I've explained it clearly enough. If not, let me know and I'll try again.

Thank you for listening to my displacement activity.

Perfect! Thank you. This will be incredibly useful in my first draft.