Moon Shot

<chirp, chirp> “Moire here.”

“Hi, Mr Moire, it’s Jeremy. Hey, I’ve been reading through some old science fiction stories and I ran across some numbers that just don’t look right.”

“Science fiction can be pretty clunky. Some Editors let their authors play fast and loose on purpose, just to generate Letters to The Editor. Which author and what story?”

“This is Heinlein, Mr Moire. I know his ideas about conditions on Mars and Venus were way off but that was before we had robot missions that could go there and look. When he writes about space navigation, though, he’s always so specific it looks like he’d actually done the calculations.”

“OK, which story and what numbers?”

“This one’s called, let me check, Gentlemen, Be Seated. It’s about these guys who get trapped in a tunnel on the Moon and there’s a leak letting air out of the tunnel so they seal the leak when one of the guys —”

“I know the story, Jeremy. I’ve always wondered if it was Heinlein or his Editor who got cute with the title. Anyway, which numbers bothered you?”

“I kinda thought the title came first. Anyway, everybody knows that the Earth’s gravity is six times the Moon’s, but he says that the Earth’s mass is eighty times the Moon’s and that’s why the Earth raises tides on the Moon except they’re rock tides, not water tides, and the movement makes moonquakes and one of them might have caused the leak. So why isn’t the Earth’s gravity eighty times the Moon’s, not six?”

“Read me the sentence about eighty.”

“Umm … here it is, ‘Remember, the Earth is eighty times the mass of the Moon, so the tidal stresses here are eighty times as great as the Moon’s effect on Earth tides.‘ I checked the masses in Wikipedia and eighty is about right.”

“I hadn’t realized the ratio was that large, I mean that the Moon is that small. One point for Heinlein. Anyway, you’re comparing north and east. The eighty and the six both have to do with gravity but they’re pointing in different directions.”

“Huh? I thought gravity’s pull was always toward the center.”

“It is, but it makes a difference where you are and which center you’re thinking about. You’re standing on the Earth so the closest center to you is Earth’s and most of the gravity you feel is the one-gravity pull from there. Suppose you’re standing on the Moon —”

“One-sixth, I know, Mr Moire, but why isn’t it one‑eightieth?”

“Because on the Moon you’re a lot closer to the center of the Moon than you were to the center of the Earth back on Earth. Let’s put some numbers to it. Got a calculator handy?”

“Got my cellphone.”

“Duh. OK, Newton showed us that an object’s gravitational force is proportional to the object’s mass divided by the square of the distance to the center. Earth’s radius is about 4000 miles and the Moon’s is about a quarter of that, so take the mass as 1/80 and divide by 1/4 squared. What do you get?”

“Uhh … 0.2 gravities.”

“One-fifth g. Close enough to one-sixth. If we used accurate numbers we’d be even closer. See how distance makes a difference?”

“Mm-hm. What about Heinlein’s tidal stuff?”

“Ah, now that’s looking in the other direction, where the distance is a lot bigger. Earth-to-Moon is about 250,000 miles. Standing on the Moon, you’d feel Earth’s one‑g gravity diminished by a factor of 4000/250000 squared. What’s that come to?”

“Umm… the distance factor is (4000/250000)² … I get 250 microgravities. Not much. Heinlein made a good bet with his characters deciding that the leak was caused by a nearby rocket crash instead of a moonquake.”

“How about Heinlein’s remark about the Moon’s effect on Earth?”

“Same distance but one eightieth the mass so I divide by 80 — three microgravities. Wow! That can’t possibly be strong enough to raise tides here.”

“It isn’t, though that’s the popular idea. What really happens is that the Moon’s field pulls water sideways from all directions towards the sub‑Lunar point. Sideways motion doesn’t fight Earth’s gravity, it just makes the water pile up in the center.”

“Hah, piled-up water. Weird. Well, I feel better about Heinlein now.”

~~ Rich Olcott

One thought on “Moon Shot

  1. Quick random thought. Earth’s radius is 4,000 miles, or some 20 Mft. A typical sea level tide outside of funnels like the Bay of Fund is a few feet. That’s a ratio of about a million, so I have no trouble with 3 µ gravities. The tides would be higher if Earth rotated slower.


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