Are we lighter in the daytime?

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Re: Are we lighter in the daytime?

#101  Postby Pulsar » Jan 01, 2015 11:04 pm

Veida wrote:If you average the daytime maximum and the nighttime maximum over a year, however, they would be the same - right? (Ignoring variations in tilt.)

Yes, more or less. The averages will even out almost completely after a saros, when the Sun and Moon return to the same configuration.

Veida wrote:There would be no difference between the minimums at day and night at fall and spring equinox. The strength of the daytime minimum would be larger in summer and smaller in winter.

Yes, although the effect of the Moon is stronger than that of the Sun.
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Re: Are we lighter in the daytime?

#102  Postby Veida » Jan 01, 2015 11:48 pm

Pulsar wrote:Yes, although the effect of the Moon is stronger than that of the Sun.


Yes, I was only talking about the effect from the sun - since we're discussing day vs. night.

The tidal force from the moon is roughly twice that of the sun. Which, by the way, is a strong hint that the tidal force is in fact a second-order effect as the gravitational force between the Earth and the Sun is larger than the gravitational force between the Earth and the Moon.
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Re: Are we lighter in the daytime?

#103  Postby Macdoc » Jan 02, 2015 12:24 am

Why do you think there would be solar tides on a tidal locked world....if there were oceans it would be a standing wave for the solar influence overlaid by the moon's rotation around the planet causing lunar tides.
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Re: Are we lighter in the daytime?

#104  Postby Veida » Jan 02, 2015 11:00 am

Macdoc wrote:Why do you think there would be solar tides on a tidal locked world....if there were oceans it would be a standing wave for the solar influence overlaid by the moon's rotation around the planet causing lunar tides.

Are you asking me?

On a tidal locked world, that always show the same side towards the sun, there wouldn't be tides caused by the sun. There would be tidal forces but as they would be constant at all points there wouldn't be any changes of water levels or ground that had to do with those forces.

(Assuming that the orbit is circular or almost circular.)
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Re: Are we lighter in the daytime?

#105  Postby DavidMcC » Jan 02, 2015 2:58 pm

Veida wrote:
DavidMcC wrote:
Elsewhere you said the following, which seems to be related to what you say above.
Were it not for the earth's spin about its own axis, the night time effect would be to increase our weight, but, as it is, conservation of angular momentum changes that, as with the tides.


Care to explain that? How would our weight increase at night if the Earth didn't spin about its axis?

(It wouldn't. I'm asking only in the hope that you'll explain yourself so I/we can pinpoint where you went wrong.)

I don't think I did go wrong, Veida. Without the earth's rotaion, all that would matter would be the combined pulls of earth, sun and moon, which would combine differently at night (when the sun's pull would add to that of the earth, but the moon's pull would depend on where the moon was (it can be above the horizon, or below)).


Ah. I think I see where you went wrong. You seem to think that the weight difference due to tidal forces is a first order effect from gravitation.

It isn't. It is a second order effect, that is due to the gravitation from e.g. the sun not being uniform - it varies slightly in strength between dayside and nightside, and it varies slightly in direction between dawnside and duskside.

All first-order effects are cancelled out due to the Earth being in free fall.

"The sun isn't uniform"?? :lol:
An important thing I missed out in this recent "debate" is that there is an equatorial bulge in the oceans, caused by the earth's rotation about its own axis, and that this bulge is tilted up by the moon's gravitional pull. That tilt is the reason why any one point on the earth's surface passes through two maxima and minima every 24 hours. I explained this a long time ago, in an old thread, but you might have missed it.
Last edited by DavidMcC on Jan 02, 2015 3:26 pm, edited 1 time in total.
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Re: Are we lighter in the daytime?

#106  Postby DavidMcC » Jan 02, 2015 3:00 pm

... BTW, there's obviously as lot of misleading stuff on the internet, that gets thrown at me.
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Re: Are we lighter in the daytime?

#107  Postby Thommo » Jan 02, 2015 3:30 pm

DavidMcC wrote:
Veida wrote:Ah. I think I see where you went wrong. You seem to think that the weight difference due to tidal forces is a first order effect from gravitation.

It isn't. It is a second order effect, that is due to the gravitation from e.g. the sun not being uniform - it varies slightly in strength between dayside and nightside, and it varies slightly in direction between dawnside and duskside.

All first-order effects are cancelled out due to the Earth being in free fall.

"The sun isn't uniform"?? :lol:


Here, let me help you.

(S)he said the gravitation from the sun is not uniform. And it isn't, it varies as a function of radial distance.

You're welcome.
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Re: Are we lighter in the daytime?

#108  Postby DavidMcC » Jan 02, 2015 3:37 pm

Thommo wrote:
DavidMcC wrote:
Veida wrote:Ah. I think I see where you went wrong. You seem to think that the weight difference due to tidal forces is a first order effect from gravitation.

It isn't. It is a second order effect, that is due to the gravitation from e.g. the sun not being uniform - it varies slightly in strength between dayside and nightside, and it varies slightly in direction between dawnside and duskside.

All first-order effects are cancelled out due to the Earth being in free fall.

"The sun isn't uniform"?? :lol:


Here, let me help you.

He said the gravitation from the sun is not uniform. And it isn't, it varies as a function of radial distance.

You're welcome.

OK, but so what? The sun may be essential to us, as we orbit it, but it is a minor player when it comes to tides, and it only has a small impact on the tilting up of the equatorial rotational bulge, which is mainly caused by the moon.
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Re: Are we lighter in the daytime?

#109  Postby Thommo » Jan 02, 2015 3:46 pm

DavidMcC wrote:
Thommo wrote:Here, let me help you.

He said the gravitation from the sun is not uniform. And it isn't, it varies as a function of radial distance.

You're welcome.

OK, but so what? The sun may be essential to us, as we orbit it, but it is a minor player when it comes to tides, and it only has a small impact on the tilting up of the equatorial rotational bulge, which is mainly caused by the moon.


So your patronizing tone was completely misplaced, Veida is not the one misunderstanding things. Your answer misunderstood both the post and the context of the post.

Both the sun and the moon exert tidal forces which cause bulges on both the nearest and furthest points of the Earth. They do this on a rotating Earth and they would do it on a tidally locked Earth. Nobody is discussing the relative magnitude of the tidal force from the moon and sun (and yes, it is indeed larger in the case of the moon) because the question was "Are we lighter in the daytime", which relates to the force of gravity due to the sun, not the force of gravity due to the moon (which is overhead equally much in the day and at night).

Instead of further shifting the goalposts (the equatorial bulge was not under discussion) you should be spending more time understanding what has come before, it is exceptionally peculiar that your response to finding out you've belittled someone based on your own misunderstanding is "so what?".
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Re: Are we lighter in the daytime?

#110  Postby DavidMcC » Jan 02, 2015 3:55 pm

Thommo wrote:
DavidMcC wrote:
Thommo wrote:Here, let me help you.

He said the gravitation from the sun is not uniform. And it isn't, it varies as a function of radial distance.

You're welcome.

OK, but so what? The sun may be essential to us, as we orbit it, but it is a minor player when it comes to tides, and it only has a small impact on the tilting up of the equatorial rotational bulge, which is mainly caused by the moon.


So your patronizing tone was completely misplaced, Veida is not the one misunderstanding things. Your answer misunderstood both the post and the context of the post.

Both the sun and the moon exert tidal forces which cause bulges on both the nearest and furthest points of the Earth. They do this on a rotating Earth and they would do it on a tidally locked Earth. Nobody is discussing the relative magnitude of the tidal force from the moon and sun (and yes, it is indeed larger in the case of the moon) because the question was "Are we lighter in the daytime", which relates to the force of gravity due to the sun, not the force of gravity due to the moon (which is overhead equally much in the day and at night).

Instead of further shifting the goalposts (the equatorial bulge was not under discussion) you should be spending more time understanding what has come before, it is exceptionally peculiar that your response to finding out you've belittled someone based on your own misunderstanding is "so what?".

How can the equatorial ocean bulge not be relevant to gravity variation, since it is tipped up from the equator mainly by the gravitational pull of the moon
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Re: Are we lighter in the daytime?

#111  Postby tuco » Jan 02, 2015 4:02 pm

Not sure, but we are taller in the morning.
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Re: Are we lighter in the daytime?

#112  Postby Thommo » Jan 02, 2015 4:03 pm

DavidMcC wrote:How can the equatorial ocean bulge not be relevant to gravity variation, since it is tipped up from the equator mainly by the gravitational pull of the moon


Because they are totally separate effects. There's a standing effect from the rotation of the Earth and a separate effect from the tides. If there was no moon there would still be a an equatorial bulge, but no lunar tide. If the Earth was not rotating there would be no equatorial bulge, but there would still be a lunar tidal change in surface gravity, with reductions on the points closest to and furthest away from the moon.

This is however, still changing the subject away from your misunderstanding, which is that without the Earth's rotation around its axis the weight experienced on the far side of the Earth would increase, when in reality it would still decrease.
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Re: Are we lighter in the daytime?

#113  Postby Macdoc » Jan 02, 2015 4:05 pm

. If the Earth was not rotating there would be no equatorial bulge,


Solar bulge would be a standing wave but small and vary with the distance from the sun.
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Re: Are we lighter in the daytime?

#114  Postby Thommo » Jan 02, 2015 4:11 pm

Macdoc wrote:
. If the Earth was not rotating there would be no equatorial bulge,


Solar bulge would be a standing wave but small and vary with the distance from the sun.


That's a different effect, in a different plane. The equatorial bulge is the bulge around the equator in the plane of the Earth's rotation about its axis, which is tilted from the plane of the Earth's rotation about the sun by around 23 degrees.

The tidal effect due to the sun is not the equatorial bulge and I don't think it's a standing wave either.
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Re: Are we lighter in the daytime?

#115  Postby DavidMcC » Jan 02, 2015 4:17 pm

Thommo wrote:
DavidMcC wrote:How can the equatorial ocean bulge not be relevant to gravity variation, since it is tipped up from the equator mainly by the gravitational pull of the moon


Because they are totally separate effects. There's a standing effect from the rotation of the Earth and a separate effect from the tides. If there was no moon there would still be a an equatorial bulge, but no lunar tide. If the Earth was not rotating there would be no equatorial bulge, but there would still be a lunar tidal change in surface gravity, with reductions on the points closest to and furthest away from the moon.

Thommo, the equatorial bulge is a measure of the contact force between a body on the earth and the earth - ie, its weight. The bulge is thus a diagnostic for weight.
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Re: Are we lighter in the daytime?

#116  Postby Thommo » Jan 02, 2015 4:21 pm

DavidMcC wrote:
Thommo wrote:
DavidMcC wrote:How can the equatorial ocean bulge not be relevant to gravity variation, since it is tipped up from the equator mainly by the gravitational pull of the moon


Because they are totally separate effects. There's a standing effect from the rotation of the Earth and a separate effect from the tides. If there was no moon there would still be a an equatorial bulge, but no lunar tide. If the Earth was not rotating there would be no equatorial bulge, but there would still be a lunar tidal change in surface gravity, with reductions on the points closest to and furthest away from the moon.

Thommo, the equatorial bulge is a measure of the contact force between a body on the earth and the earth - ie, its weight. The bulge is thus a diagnostic for weight.


And - and this is important, so please follow closely - it's the same all the way round the equator (after averaging out for the variations due to the phase of the sun and moon), making no difference to your weight in the day as compared to your weight in the night. It's a totally separate effect than the tidal forces, with the tidal force due to the sun decreasing your weight in the middle of the day and the middle of the night and slightly increasing it roughly half way in between.
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Re: Are we lighter in the daytime?

#117  Postby campermon » Jan 02, 2015 4:24 pm

The tide comes in...the tide goes out...

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

#118  Postby DavidMcC » Jan 02, 2015 4:27 pm

Thommo wrote:
Macdoc wrote:
. If the Earth was not rotating there would be no equatorial bulge,


Solar bulge would be a standing wave but small and vary with the distance from the sun.


That's a different effect, in a different plane. The equatorial bulge is the bulge around the equator in the plane of the Earth's rotation about its axis, which is tilted from the plane of the Earth's rotation about the sun by around 23 degrees.

The tidal effect due to the sun is not the equatorial bulge and I don't think it's a standing wave either.

It must be, in spite of various authors and media claiming that the lunar bulge is one-sided. It would only be one-sided if the earth was not spinning on its axis, and did not have angular momentum to conserve. As it is, the lunar pull tips up the plane of the bulge.
The earth's oceans are like a large gyroscope, tipping when interfered with.
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Re: Are we lighter in the daytime?

#119  Postby twistor59 » Jan 02, 2015 4:28 pm

campermon wrote:The tide comes in...the tide goes out...

:popcorn:


The tide is high but I'm holding on....
A soul in tension that's learning to fly
Condition grounded but determined to try
Can't keep my eyes from the circling skies
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Re: Are we lighter in the daytime?

#120  Postby Macdoc » Jan 02, 2015 4:31 pm


The tidal effect due to the sun is not the equatorial bulge and I don't think it's a standing wave either


where did I say it was equatorial?

If you don't like standing wave then permanent feature on a world tidal locked to its star.
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