July 20th is World Jump Day. I am certain the website’s claim of 562,000,000 registrants is baloney, but what do you think of the whole idea? ( I vaguely remember Cecil talking about this a long time ago, but I couldn’t find his column.)
Well, according to the website, there are now 582,157,300 umpers, an increase of some 20 million over an hour of time. So the number is definitely wrong. Besides that, jumping won’t work, due to Newton’s 3rd Law (equal and opposite). Cecil’s link is here. So, don’t bother jumping, and it’s probably a hoax anyway.
And now, 2 hours later, there are 582,157,376. Registration really hit a speedbump.
It’s probably an e-mail havesting site.
[crude mexican stereotype character]
World Jump Day? Is that for real?! Madre de Dios, I jope so, Senor! 
[/cmsc]
But there’s an interview on the site with a real live German scientist. I was skeptical too, but I saw the clip, and he really DID have a German accent, so that proves it’s real. Plus, they have charts and graphs!
Seriously though, I think it would push the earth off course (though probably not measurably) for the moment when the jumpers were in the air, and then the earth would return to its regularly scheduled orbit when they landed. This is ignoring air effects, which complicate things.
A jump day where you’re just supposed to jump a lot, for fun - on a trampoline, on the ground, with a jump rope - that’s an idea I could get behind.
The website seems to be skewed very much to the western hemisphere. It doesn’t offer a sign up option to jumpers in any countries east of GMT+2 (thus omitting a huge chunk of the world’s population in India and China).
It won’t even do that; considering the earth as a system (necessarily including all those objects gravitationally bound to it, such as… humans), the centre of mass of the system cannot be moved by forces acting entirely within the system.
Sure, parts of the earth-system are going to move away from each other momentarily, but the centre of mass of the system of earth+humans will therefore continue in orbit completely as normal.
Ahh–but could you alter the rotation or orientation of the earth? (a la reaction wheels on satellites/space probes that change the object’s orientation)
Why is this discussion in GD?
[beavis & butthead]
Heh-heh . . . Heh-heh . . . He said “orientation”!
[/b&b]
Only if you go and live somewhere else, or at least go all the way around and back to where you started. Again, the effect is going to be immeasurably insignificant, even if you got the entire population of the earth to do it with you - because the planet hugely outweighs us; perhaps someone could do a back-of-the-envelope calculation on the ratio of masses between humans and [everything else].
::takes out envelope::
6 billion humans @ 50kg = 300 billion kg = 300 million tonnes
Earth diameter ~ 8000mi ~ 13 000 km = 13 million metres. Radius ~ 6.5 million metres. Volume = 4/3 p r[sup]3[/sup] ~ 4 r[sup]3[/sup] = 6.5 cubed times 10[sup]18[/sup] cubic metres, about 2.5 x 10[sup]20[/sup] m[sup]3[/sup]. Assuming an average relative density of 3, a not untypical value for rock, that’s 7.5 x 10[sup]20[/sup] tonnes.
Earth’s mass exceeds humanity’s by a factor of about 2.5 x 10[sup]12[/sup] - for every kilo of humanity, there are 2.5 million tonnes of Earth.
All that coding with no preview, but I forgot to multiply by four when calculating the volume. Ten million tonnes per kilo of human.