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#ZeroGDay: the day gravity stopped working

Planetary alignment


OK, not really. Not even a bit. The Jovian–Plutonian gravitational effect is one of those things that pops up now and then to get a bunch of gullible people excited and acting like idiots.

This all began way back in 1976, on April 1st. While talking on a radio program, Moore told everybody that there was a planetary alignment that would put Pluto and Jupiter directly behind the Earth, and that if you were to jump at exactly 9:47 AM, the combined effects of the two planets gravity would be felt by you as a decrees in Earth’s gravity. He stated that those who followed his directions, would have a momentary feeling of weightlessness. I’m sure many of you have already taken note of the date, and realized that this was an astronomer with a sense of humor. This hasn’t stopped many others from repeating the claims, sure, the date of the event has changed, but the retellings keep pretty much all the other details intact.

Now, even if you weren’t aware of the origin of this tale, it’s still pretty dumb to believe it. Sure, you could point at the moon and how it’s gravity effects our oceans, then look at how much bigger Jupiter is. It’s almost believable that such a massive planet could have some kind of an effect on us. Gravity is a tricky thing though, you see, the further you get away from the mass, the weaker the gravity becomes. I could go into the math here, and show you how from this distance, Jupiter and Pluto have about as much gravitation effect on you as a mid-sized car parked on the other side of the road, but I won’t bore you with the numbers.

Instead, I’ll draw you a picture:


Here we see the sun, the Earth, Jupiter, and Pluto.

At 9:47 AM, you are on the side of the Earth facing the sun. At this same time, all planets are supposed to be on the same side of the sun, with Jupiter and Pluto (even though it’s no longer a planet) aligning perfectly with the Earth. This would put them opposite you, facing the dark side of the Earth.

Now, if the two planets were to exert any gravitational force on you, wouldn’t they pull you towards them? And since towards them is also towards the Earth, wouldn’t you actually experience greater gravitational pull than normal? Instead of a few moments of weightlessness, the added pull of the Jovian–Plutonian gravitational effect would actually pull you down to the Earth with crushing force.

Still not convinced, let’s try another one. Jupiter is 0.001 the mass of the sun. The Earth is 1 AU from the sun. Jupiter is close to 5 AU from the sun. This means that Jupiter is also about 4 AU from the Earth. Now, wouldn’t something a thousand times more massive and four times closer to the Earth have a much greater effect than Jupiter does? Even adding in the exponentially smaller mass of Pluto over the almost 30 AU distance, you still wouldn’t get nearly the effect that we get from the sun.

That said, if you jump up and down at noon, when the sun is directly overhead, do you float at all? No? Then why would something much smaller and much more distant do any more for you?

Zero G Day isn’t even going to happen. It’s not even going to be Slightly Less G Day. It’s just going to be another Saturday. That doesn’t mean that there isn’t a reason to get excited though. You’ll want to grab your camera so you can document all the people who are making fools of themselves and falling for a 38-year-old prank.

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