Alan Stahler: Harvest moon
At sunset today, the harvest moon will rise.
Play catch in the forest and the trees get in the way — the ball bounces this way and that. Send sunlight through the atmosphere and molecules of air get in the way, scattering photons of sunlight this way and that, making the daytime sky bright. Scattering blue the most, red the least, air molecules paint the sky blue.
With much of its blue scattered out, the rising or setting sun has an orangey tinge. Same with the moon. If there’s much dust or smoke in the air, the moon can even rise red. But that’s not what makes today’s moon the harvest moon.
Rising just over the horizon, the full moon looks huge. But it’s all in our heads — an optical illusion. Photograph the “huge” moon rising, the “normal” moon some hours later, and see if they’re not identical. So seeming hugeness is not what makes today’s moon the harvest moon.
The full moon rises when the sun sets. Full moonlight can be helpful, after sunset, if you’re working outdoors.
Imagine riding a carousel. Friends stand just off the carousel, watching you go ‘round.
To get some exercise, our friends start walking around the carousel, in the same direction it’s turning. As we come around, we see them in front of the hot dog stand.
Next time we come around, we see the hot dog stand, but no friends — they’re walking in the same direction we’re turning, so the carousel has to turn a bit more, before we catch up with them, in front of the balloon stand.
We go around again. We pass the hot dog stand; we pass the balloon stand. The carousel turns a bit more, and we finally see our friends in front of the ice-cream stand.
Planet Earth is a carousel, turning once a day. The moon, like our friends, orbits around the Earth, in the same direction we’re turning. Night after night, we have to turn a bit more to see the moon rise – moonrise is delayed, from one night to the next.
How much of a delay is there, from one night to the next? That depends.
If the ground around the carousel were perfectly flat, all the way around, every stride would take our friends forward the same distance, so the delay, one turn to the next, would be the same.
But the ground around the carousel is hilly. After passing the ice cream stand, they have to climb to get to the corn dog stand. Each stride takes them a little way forward, and a little way up. They don’t get as much horizontal distance from each stride. In the time it takes our carousel to go around, they get only half-way to the corn dog stand. So the carousel doesn’t have to turn, quite so much, for us to see them again — there’s less of a delay, from last turn to now.
In some parts of the moon’s orbit, it can cover a lot of space (a lot of “ground”), so we have to turn quite a bit to catch up with it. That means a long delay — as much as 80 minutes or so — between the time of last night’s moonrise and tonight’s.
In other parts of its orbit, the moon moves at an angle to our path, as if climbing a hill. It makes only a little forward progress, one night to the next. We don’t have to turn so much to catch up, and there’s only a short delay — as little as 20 minutes — from last night’s moonrise to tonight’s.
The geometry of the solar system dictates that we get the shortest delay in full moonrise at the time of the autumn equinox. One night after another, the moon rises with minimal delay, lighting the fields as farmers bring in their crops. With the autumn equinox coming next week, this today’s moon is that of the harvest.
Al Stahler’s science programs can be heard on KVMR (89.5 FM). He brings an enjoyment of science and nature to students of all ages — kid to adult — and may be reached at firstname.lastname@example.org.
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