Expert: Courtney Seligman - 7/30/2010

Question
It's generally understood that people weigh less in the morning than at night. I've read this is due to the body's loss of water vapor during sleep.

When the earth faces the sun (morning), does the sun's gravity contribute to the the lower weight reading?

If so, by roughly how much?

Answer
I'm afraid that if it did, you couldn't tell, because the effect is so small; and it actually works opposite the way you're supposing, so it doesn't contribute to the lower weight at all.

What effect there is, is due to the tides that the Sun raises in the body of the Earth and at its surface, but it's about 20 million times less than your weight; and at sunrise (and sunset) it makes you seem to weigh more, not less (it would make you seem to weigh less near noon and midnight).

As it happens, my website has an incomplete page discussing the similar (but twice as large) effect for the Moon's tides, so if you'd like to wade through the math (or just skip the math and look at the results at the end), see http://cseligman.com/text/moons/earthmoontides.htm

By the way, in addition to the loss of water vapor, you lose some weight because the carbon dioxide you breathe out weighs more than the oxygen you breath in, while you're sleeping; and even this weight difference, though small compared to the loss of water vapor, is hundreds of thousands of times greater than the tidal effects of the Sun and Moon.