“Summer ends, and Autumn comes, and he who would have it otherwise would have high tide always and a full moon every night.” –Hal Borland
Of course you know the danger that would befall us if the Earth ever got too close to the Sun, as the Perry Bible Fellowship shows, atop. But have you ever stopped to think about the Moon in our skies, and what would happen if the Earth and Moon were closer together than they actually are?
While photos such as this — from the Galileo spacecraft — accurately show the relative size and illumination of the Earth and Moon, it’s a trick of perspective that hides the actual vast distances between these two objects.
If I were to insist on showing you the Earth and Moon, including the Earth-Moon distance shown to scale, it would look very different!
The Moon is, on average, about thirty Earth-diameters away from our planet. This tremendous distance helps explain why the Moon appears to be the same size in our sky regardless of whether it appears directly overhead — which it does close to midnight on the full Moon — or on the horizon, when it’s another full Earth-radius away.
But what would happen if the Moon weren’t so (literally) astronomically far away from us?
In other words, what would happen if the Moon were closer to us?
Well, if the Moon were, say, only half the distance away from us as it is today, we would immediately notice a few large differences.
Yes, the Moon would appear twice as large in the sky, and yes, it would be about four times as bright due to its proximity. But that’s not really a major change.
But there is something that would change majorly.
Right now, on Earth, the difference between high and low-tides affects the ocean height by a substantial amount. The Moon (and to a lesser, about 30% effect, the Sun) gravitationally pulls on the Earth slightly more strongly in the direction closest to it, and slightly less strongly in the direction farthest from it.
This causes the liquid part of the Earth — the oceans — to form two bulges, which are responsible for the high-and-low tides as the Earth rotates. This is why there are two high tides and two low tides each day; each point on the Earth needs to pass through both high points and both low points to make a complete rotation about its axis.
With the Moon at our current, 30-Earth-diameter distance, this means that the difference between high tide and low tide can be anywhere from about 5-to-8 feet (1.5 to 2.4 meters), depending on where the Sun, Moon and Earth are relative to one another. (Yes, there are other slight variations to do with latitude etc., but I’m ignoring those.)
But if the Moon were just half-the-distance it is to us now?
You might think that the difference between high and low tide would be twice as large. Or, you might remember that Newton’s Law of Gravitation is an inverse-square-law force, and so you might think the tides would be four times as large.
The way tidal forces work, it turns out, means that the tides would be eight times as large as they are now, or that each day, the difference between high and low tides would be about 52 feet, or 16 meters.
In other words, every high tide would bring catastrophic worldwide tidal waves to coastal cities everywhere.
And if the Moon were only one-fourth the distance it is right now, those waves would be 400-footers instead of 50-footers.
And if you were willing to bring it a little closer — just under one-eighth the current distance — the Moon would pass through geostationary orbit, and would always appear at the same point in the Earth’s sky.
Any closer than that, and the Moon would actually appear to rise in the West and set in the East, because it would be orbiting the Earth faster than the Earth itself could spin!
But things would get a whole lot worse for the Moon than they would for us. Because while the Moon’s tidal forces on Earth can severely impact our oceans and coastal areas, the reverse — the Earth’s tidal forces on the Moon — would begin to tear the Moon apart!
Only the core of the Moon — because it’s so much denser than the rest of the Moon — would remain. And even that would only survive until the core was closer than about 2 Earth radii. And at that point, the tidal forces would tear apart even a solid ball of iron.
And if you dared to get much closer than that — to low-Earth orbit — you’d start to notice severe differences from the horizon to directly overhead.
An object orbiting maybe 200 kilometers (120 miles) above the Earth’s surface would appear 10 times larger when it was directly overhead vs. when it first crested the horizon! This would apply to any rock, any component of a ring, anything at all, even our current satellites in orbit.
But 200 kilometers is closer above the surface than you’d ever want anything to be, because at that low-altitude, it will start to experience atmospheric drag. And once that becomes at all significant, you know what’s next.
It will re-enter our atmosphere, moving at some five miles per second (8 km/sec), until the catastrophic heat from re-entry burns it up.
So be pleased about the Moon and — like Rodrigo Andolfato — make sure you enjoy tonight’s Moon/Jupiter conjunction!
But be thankful that the Moon is as far away as it is, and resist the temptation to bring it closer and give it a hug. The consequences would be catastrophic for humans on Earth, but could be fatal to the existence of the Moon itself!