Are we lighter in the daytime?

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Are we lighter in the daytime?

#1  Postby jamest » Oct 24, 2011 7:32 pm

At night (especially the middle of the night), we have the gravitational effects of the earth and sun combining to pull us towards the earth's centre. However, during the day (especially midday), the sun's gravitational pull [on us] is away from the earth's centre.

... So, maybe I'm being a dumbo, but doesn't this mean that we should be noticably lighter in the daytime, than at night? If this is indeed the case, then how much lighter are we?
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Re: Are we lighter in the daytime?

#2  Postby home_ » Oct 24, 2011 7:41 pm

Try Moon, not Sun.

Sea tide, but it happens on both sides of Earth.

There is some difference in gravitational pull, but it's very small. Maybe you could try calculating it? ;)
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Re: Are we lighter in the daytime?

#3  Postby chairman bill » Oct 24, 2011 7:56 pm

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Re: Are we lighter in the daytime?

#4  Postby jamest » Oct 24, 2011 7:58 pm

home_ wrote:Try Moon, not Sun.

Yes, should have mentioned the moon too.

There is some difference in gravitational pull, but it's very small. Maybe you could try calculating it? ;)

The problem is, I keep eating in the daytime. :shifty:

You say the difference is small, but if we consider what happens to the oceans, then I would have expected a noticable change in our weight throughout the day.
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Re: Are we lighter in the daytime?

#5  Postby campermon » Oct 24, 2011 8:24 pm

jamest wrote:
home_ wrote:Try Moon, not Sun.

Yes, should have mentioned the moon too.

There is some difference in gravitational pull, but it's very small. Maybe you could try calculating it? ;)

The problem is, I keep eating in the daytime. :shifty:

You say the difference is small, but if we consider what happens to the oceans, then I would have expected a noticable change in our weight throughout the day.


The effect is very small and the oceans are very big. All those very small effects add up to a measurable effect in the very big oceans.

:thumbup:
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Re: Are we lighter in the daytime?

#6  Postby chairman bill » Oct 24, 2011 8:29 pm

As the link I posted makes clear, we do get a weight change, but it's pretty small, and dwarfed by the effects of changes in atmospheric pressure
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Re: Are we lighter in the daytime?

#7  Postby Scar » Oct 24, 2011 9:07 pm

What you have to understand is that gravity is a very very weak force. Consider that a body the size of the earth can't really keep you from jumping up with relative ease.
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Re: Are we lighter in the daytime?

#8  Postby jamest » Oct 24, 2011 9:17 pm

campermon wrote:
The effect is very small and the oceans are very big.

I would have thought that the size of the oceans was irrelevant. The gravitational effect per unit of volume, so to speak, is the same throughout and suffices to produce the overall effect of the tides. Similarly, If humans/animals were turned into a sea of liquid mush, I would expect to see a similar tidal effect (moreso, since we are heavier than water). Again, the gravitational effect per unit of volume would be the same throughout and would suffice to produce significant tidal effects. Since this gravitational effect per unit of volume is significant enough to cause the tides, I cannot understand why it makes hardly any difference to the weight of a unit of volume.

I'm just expressing my surprise, btw. I'm obviously not saying that the effect is not very small. I believe you, but don't gettit.
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Re: Are we lighter in the daytime?

#9  Postby jamest » Oct 24, 2011 9:19 pm

chairman bill wrote:As the link I posted makes clear, we do get a weight change, but it's pretty small, and dwarfed by the effects of changes in atmospheric pressure

Please explain what you mean regards the changes in atmospheric pressure. Thanks.
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Re: Are we lighter in the daytime?

#10  Postby chaggle » Oct 24, 2011 9:23 pm

In my case yes. Soon after I get up every morning I lose several pounds.
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Re: Are we lighter in the daytime?

#11  Postby jamest » Oct 24, 2011 9:45 pm

Scar wrote:What you have to understand is that gravity is a very very weak force. Consider that a body the size of the earth can't really keep you from jumping up with relative ease.

Yes, we can jump - but only a few feet.

We say that gravity is weak yet the effects of it from the sun suffice to cause the planets' orbits. Similarly, the moon & sun cause the tides - the significant movement of a body of weight in a given direction. If that direction is away from the earth, then one would expect that there would be a noticable decrease in the weight of a body per unit of volume.


I've had another thought too, just before you all run off. The tidal flows must produce stress points upon locales of the earth's crust, which has to bear the weight of the water. So, I was wondering whether tidal effects had any geological significance? Would it be insane, for instance, to wonder if tidal effects had any relation to plate tectonics? Or, whether tidal effects were instrumental in triggering earthquakes?

I don't really know much about this stuff. I'm just thinking out loud. Thanks for any input.
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Re: Are we lighter in the daytime?

#12  Postby jamest » Oct 24, 2011 9:47 pm

chaggle wrote:In my case yes. Soon after I get up every morning I lose several pounds.

You mean, you go straight down to the cafe to buy a good old English breakfast? :grin:
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Re: Are we lighter in the daytime?

#13  Postby chairman bill » Oct 24, 2011 9:55 pm

jamest wrote:
chairman bill wrote:As the link I posted makes clear, we do get a weight change, but it's pretty small, and dwarfed by the effects of changes in atmospheric pressure

Please explain what you mean regards the changes in atmospheric pressure. Thanks.


From the linked-to site. 3rd paragraph.

Q. 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. Christoph, Utrecht



We put this to Mark McCaughrean, Professor of Astrophysics at the University of Exeter.

A. 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...
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Re: Are we lighter in the daytime?

#14  Postby epepke » Oct 24, 2011 10:03 pm

jamest wrote:
campermon wrote:
The effect is very small and the oceans are very big.

I would have thought that the size of the oceans was irrelevant.


Yes in the sense of no. The area of the oceans doesn't matter much, but the average depth of the ocean over a local area does. Consider a water column a meter square. (The area doesn't matter all that much as long as it's small compared to curvature , but it's easier to think about.) The reduction in tides affects every cubic meter in the column. The total effect can be measured in the weight of the difference in the water column between high tide and low tide (or, technically, half of that). So a very small difference in weight over each cubic meter in the water column can add up to a difference of a few meters in the height of the water column.

So, the average depth of the ocean is 3790 meters. To see a 1 meter rise, the reduction in weight only has to be 1/3790.

Of course, this is a ridiculously oversimplified picture and does not account for many real-world aspects, but at least it shows the basic idea.
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Re: Are we lighter in the daytime?

#15  Postby klazmon » Oct 24, 2011 10:03 pm

jamest wrote:
campermon wrote:
The effect is very small and the oceans are very big.

I would have thought that the size of the oceans was irrelevant. The gravitational effect per unit of volume, so to speak, is the same throughout and suffices to produce the overall effect of the tides. Similarly, If humans/animals were turned into a sea of liquid mush, I would expect to see a similar tidal effect (moreso, since we are heavier than water). Again, the gravitational effect per unit of volume would be the same throughout and would suffice to produce significant tidal effects. Since this gravitational effect per unit of volume is significant enough to cause the tides, I cannot understand why it makes hardly any difference to the weight of a unit of volume.

I'm just expressing my surprise, btw. I'm obviously not saying that the effect is not very small. I believe you, but don't gettit.




The calculation is simple FFS. Newton worked out how to do this over three hundred years ago!

Gravitional force of the Sun on a person. F = G Mm/r2.

Assume a spherical person (physics joke there) weighs 60kg.

F = 6.67 x 10-11 x 2 x 1030 x 60 / (2.25 x 1022) = approx .35 Newtons.

Similarly, force of gravity due to the Earth on a 60 kg mass at earth's surface approx 590 Newtons.

So the effect of the Sun is small.

Tidal forces are a different thing again. The tides are caused by the gradient in force across a linear distance. Hence tidal forces vary by 1/r3
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Re: Are we lighter in the daytime?

#16  Postby jamest » Oct 24, 2011 10:04 pm

chairman bill wrote:
jamest wrote:
chairman bill wrote:As the link I posted makes clear, we do get a weight change, but it's pretty small, and dwarfed by the effects of changes in atmospheric pressure

Please explain what you mean regards the changes in atmospheric pressure. Thanks.


From the linked-to site. 3rd paragraph.

Q. 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. Christoph, Utrecht



We put this to Mark McCaughrean, Professor of Astrophysics at the University of Exeter.

A. 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...

Thanks Bill. I hadn't considered the effects of the buoyoncy of the air. Interesting.

Is the air prone to tidal effects too? Or is it so light that such effects are beyond being negilgible?
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Re: Are we lighter in the daytime?

#17  Postby jamest » Oct 24, 2011 10:08 pm

epepke wrote:
jamest wrote:
campermon wrote:
The effect is very small and the oceans are very big.

I would have thought that the size of the oceans was irrelevant.


Yes in the sense of no. The area of the oceans doesn't matter much, but the average depth of the ocean over a local area does. Consider a water column a meter square. (The area doesn't matter all that much as long as it's small compared to curvature , but it's easier to think about.) The reduction in tides affects every cubic meter in the column. The total effect can be measured in the weight of the difference in the water column between high tide and low tide (or, technically, half of that). So a very small difference in weight over each cubic meter in the water column can add up to a difference of a few meters in the height of the water column.

So, the average depth of the ocean is 3790 meters. To see a 1 meter rise, the reduction in weight only has to be 1/3790.

Of course, this is a ridiculously oversimplified picture and does not account for many real-world aspects, but at least it shows the basic idea.

I think I understand that. Makes sense. Thanks.
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Re: Are we lighter in the daytime?

#18  Postby klazmon » Oct 24, 2011 10:11 pm

jamest wrote:
Thanks Bill. I hadn't considered the effects of the buoyoncy of the air. Interesting.

Is the air prone to tidal effects too? Or is it so light that such effects are beyond being negilgible?



http://en.wikipedia.org/wiki/Atmospheric_tide
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Re: Are we lighter in the daytime?

#19  Postby jamest » Oct 24, 2011 10:23 pm

klazmon wrote:
jamest wrote:
Thanks Bill. I hadn't considered the effects of the buoyoncy of the air. Interesting.

Is the air prone to tidal effects too? Or is it so light that such effects are beyond being negilgible?



http://en.wikipedia.org/wiki/Atmospheric_tide

Seems like everything is tidal. The moral of the story seems to be that if you've got a beer belly, then keep your back to the sun.
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Re: Are we lighter in the daytime?

#20  Postby jamest » Oct 24, 2011 10:31 pm

chairman bill wrote:
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...

So, Santa isn't fat. He's just geographically challenged?
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