Christoph, Utrecht asked:
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.
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 isnít going to make much difference. You certainly wonít feel that difference in weight.
Itís somewhat complicated. People have written in suggesting well, the Moon can pull to one side depending where it is and of course the Moon isnít always out at night, itís out in the daytime just as much as it is out at night but people donít tend to look. So thereís a little effect there but there are much bigger effects at work.
The one that people havenít 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 thatís 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 itís 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 itís probably not the best place to go at the moment...
Surely you would weigh least at midday when the gravitational pulls of Earth and Sun were closest to direct opposition.
No, that wouldn't work because of the (more significant) gravitational attraction from the other fatties in the room with you... chris, Tue, 28th Apr 2009
I would think you would weigh more at night because of the position of the Sun and Earth. Ignoring the moon at the moment, the mass of the Earth and Sun are grater than you and would have the largest of the gravitational affects in this theory. The Sun and Earth pulling on you at the same time would make you heaver at night when both are lined up in the same direction from you.
At first glance this seems to make sense. Weight is a measurement of how much force you're applying to the earth's surface by standing on it, and this force is the effect of how much gravity is pulling you "down" towards the earth. When the sun is directly overhead, you'd expect it would lift you slightly, and when it's on the other side of the earth, you'd expect it to pull you down slightly.
The answer is no (Ö on average Ö depending on your definition of night.)
At 100Kg, what difference in weight to expect ? Is it measurable for the ordanary person ? Dr Guz, Fri, 1st May 2009
Counting only the change over a day, 8.5 grams (3oz). So you're lighter at noon and midnight and heavier at sun rise and set. How much heavier. Just using gravitational force, compare sun set to noon. The difference is the radius of the earth. The other parts of the formula cancel out. (Gm/r^2) / (Gm/(r-r)^2) becomes r^2 / (r-r)^2. The formula relies on SI units so use meters. Sun is 149600000000 meters away. Earth is 6378000 meters to the center.
Oops , of course, 100kg is my mass. But to the sun, that only weighs 60 grams. So ...
earth being a satellite of the sun isnt d gravitational force of d sun which is providing d necessary centripetal force zero? sbXI, Sat, 5th Feb 2011
Make sure to drink the water while in Mexico City, to weigh even less! Bob, Mon, 2nd Nov 2015
Another term is due to Earth's orbit around the Sun: At midnight the orbit tends to throw us off Earth away from the Sun, whereas at noon the orbit tends to compress us toward the center of Earth away from the Sun. Ron, Mon, 8th Feb 2016