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Are you writing correctly about the moon?

Since I keep seeing things in stories like a full moon rising in the middle of the night, or a “waning” moon showing up at sunset, or the points of the crescent moon pointing toward the sun in the sky, I figured it was time to write this brief guide. Because none of those things are astronomically possible if you are writing about planet Earth as we know it.

This doesn’t matter very much, I know. But the small section of the population who does notice moon phases, and/or are astronomy buffs, will appreciate your being accurate about it. So here is the quick-and-dirty guide to how the moon works:

The waxing moon (the phase in which it’s gradually getting more illuminated—“bigger,” we say) begins as a very thin crescent at sunset, just above the sun in the west. Its back, its illuminated edge, will face the sun; the points will point away from the sun.

That’s how it always works, of course: the sun illuminates the moon, so the bright side of the moon is the side facing the sun, and the points of the crescent therefore always point away from the sun.

As the waxing moon gets brighter, it rises a little later each night, so that it’s a bit farther to the east at sunset than it was the previous night. Waxing moon can thus be seen in the afternoon too, before sunset, but not in the morning and not for very long in the night; it sets sometime during the night.

The full moon always rises right at sunset. Sun in the west, full moon in the east, shining at each other from opposite horizons, sun fully illuminating face of moon.

And then we are in waning moon phase. After the full, the moon starts rising after sunset, later and later each night as it becomes less illuminated (“smaller”). Waning moon can thus be seen in the morning too, till it goes fully dark and starts over again. (That's the new moon, which rises and sets with the sun, which is why you can’t see it.)

This page has some good tables and explanations on moonrise and moonset too, written by a proper astronomer.

We have some lovely waxing moon going on right now, by the way!


( 2 comments — Leave a comment )
May. 4th, 2017 02:28 pm (UTC)
re: writing abt the moon
Great summary!

Now, why do TV shows and movies almost always show a full moon? There's tons of incorrect stuff ging on there as well.

The bf, once upon a time a physics major heading for grad school in astronomy, is still rather keen on astrometry and the space program. I check w/ him abt any astrometrical issues on TV and in films. His biggest "beef" is when they show the face of the full moon incorrectly.
May. 4th, 2017 05:12 pm (UTC)
Re: writing abt the moon
Huh, I never noticed the moon's face actually being wrong! I'm not quite that adept. :) I wonder why they'd do that? Do they flip it upside-down or something? I remember Neil deGrasse Tyson saying it bugs him when in movies people look up at the stars, and the constellations are all wrong. I don't know the sky quite well enough to notice that either, but I can see how it'd be annoying, and surely an easy thing to get right.

( 2 comments — Leave a comment )