Do we weigh less at night?

03 May 2009

Question

My question is do we and objects weigh less at night time? I could imagine that an additional gravitational upward pull of the sun during day time will work to a certain degree against the downward pull of the earth.

Answer

We put this to Mark McCaughrean, Professor of Astrophysics at the University of Exeter. Your question is the question specifically whether the Moon and the Sun, the positions of them in the sky make us weigh more or less and they do a little bit. Typically the overall variation due to the positions of the Sun and the Moon might be as small as about 10 or 15 mg which is less than an aspirin so it really isn't going to make much difference. You certainly won't feel that difference in weight.

It's somewhat complicated. People have written in suggesting well, the Moon can pull to one side depending where it is and of course the Moon isn't always out at night, it's out in the daytime just as much as it is out at night but people don't tend to look. So there's a little effect there but there are much bigger effects at work.

The one that people haven't written about which is actually quite important for most people and as to do with atmospheric pressure, the atmospheric pressure will change from day-to-day, people know about highs and lows and that the typical change during a night to the high pressure zone move into a low pressure zone might actually change the weight of a person by as much as 6 grams so not 10 or 15 milligrams but 6 grams. Which is the weight of a pencil, for example. And that's all to do with the buoyancy of the air when the pressure goes up.

But the biggest way of losing weight, if you really want to lose weight quickly is to move location, move to somewhere nearer the equator where the centrifugal force or more correctly the centripetal acceleration of the earth reduces your weight because the earth is spinning and at the equator it's spinning the fastest and also going to higher altitude. You can lose as much as 300 or 400 grams that way, about half a per cent of your body weight. So if you really want to lose weight astronomically then move to Mexico City, near the equator and at high altitude. On the other hand it's probably not the best place to go at the moment...

Comments

I have read the answer, and understand it, but I think the answer misunderstands the question. If inertial mass is equivalent to gravitational mass, then does the inertial mass of a body on the surface of an orbiting planet add to the bodies "weight" when the body is on the day side, and detract from it at night?

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