Is Something Lighter on the Side of the Earth Facing the Sun?

[Willem F Esterhuyse]

The Earth is accelerating and the acceleration vector points in the direction of the Sun. Is something lighter on the side facing the Sun than the same thing on the side facing away from the Sun? If not, why not. F = m(g-a) so total a is smaller on the side facing the sun.

[Lorentz Jr]

  

1 hour ago, Willem F Esterhuyse said:

The Earth is accelerating and the acceleration vector points in the direction of the Sun. Is something lighter on the side facing the Sun than the same thing on the side facing away from the Sun? If not, why not.

EDIT: Misread that...

Edited February 2, 2023 by Lorentz Jr

[Janus]

If the Earth were stationary (held suspended above the Sun by some magical string), then yes, an object on the Sun side would weigh less.

But it is not, it is in orbit, which is a free fall trajectory.   That means everything on Earth is also following that trajectory. 

In other words, if you suddenly removed the Earth, and left behind only the objects resting on its surface, those objects would continue to orbit the Sun on their own.

Now let's simplify things.  Assume you have only three objects., The Earth, and objects on its Sun side and the opposite side. We will then magically remove the Earth's gravity's hold on the two objects.

What would happen?  The Earth will continue along its orbit as usual.  The Sun side object will follow it own independent orbit.  It is closer to the Sun than the Earth's center, but starts with the same velocity as the Earth's center. It would need a higher speed to maintain this orbital distance, so it will begin to "drift" in nearer to the Sun.

On the other hand, the object on the opposite side, being further from the Sun, and also starting off with the same speed, is moving too fast for its distance from the Sun, and will start to drift away from the Sun.

Add the Earth's gravity back in, and you get the result that both objects are a bit lighter than objects sitting on the day/night line.  This is known as "tidal effect".

 

 

[Lorentz Jr]

16 minutes ago, Janus said:

everything on Earth is also following that trajectory. 

Earth is revolving about its own axis in the same sense as its orbit around the Sun (except for the tilt, of course), so objects on the Sun side are orbiting more slowly than ones on the dark side.

Edited February 2, 2023 by Lorentz Jr

[Willem F Esterhuyse]

I disregarded centripetal force.

[studiot]

19 hours ago, Willem F Esterhuyse said:

The Earth is accelerating and the acceleration vector points in the direction of the Sun. Is something lighter on the side facing the Sun than the same thing on the side facing away from the Sun? If not, why not. F = m(g-a) so total a is smaller on the side facing the sun.

 

2 hours ago, Willem F Esterhuyse said:

I disregarded centripetal force.

 

What effect are you expecting here ?

Hopefully you realise that you cannot shield against gravity. So the mass of the Earth's body does not shield againsts the Sun's gravity.

And what about the Moon ?

[Willem F Esterhuyse]

48 minutes ago, studiot said:

What effect are you expecting here ?

The acceleration vector due to the Suns gravity is in the opposite direction at the sunny side than on the dark side. Centripetal force is towards Earth on the sunny side (and if I'm correct cancels the force from the Sun exactly), while centripetal force is away from Earth on the dark side. Thus the object would be lighter on the dark side.

49 minutes ago, studiot said:

Hopefully you realise that you cannot shield against gravity.

Yes.

50 minutes ago, studiot said:

And what about the Moon ?

The Moons gravity was neglected.

[studiot]

12 minutes ago, Willem F Esterhuyse said:

The acceleration vector due to the Suns gravity is in the opposite direction at the sunny side than on the dark side. Centripetal force is towards Earth on the sunny side (and if I'm correct cancels the force from the Sun exactly), while centripetal force is away from Earth on the dark side. Thus the object would be lighter on the dark side.

Yes.

The Moons gravity was neglected.

Thank you for answering.  +1

I asked this way to help you work out for yourself, which is by far the best way.

Perhaps you would like to rethink the embolded part of your reply in the light of the fact that you understand that nothing shields the bodies on the Earth's surface, no matter which side they are on ?

If you still can't see why  what you said is wrong I will explain in greater detail.

 

Remember also that the Moons gravitational influence at the Earth's surface is greater than that of the Sun.

 

But yes, all other things being equal, a body on the dark side experiences a slightly smaller pull due to the Sun compared to the a body on the light side because it is further from the Sun. But theSun's pull does not change direction, it is the Earths pull that pulls in opposite directions on the dark and light sides.
So the pull from the Sun on the light side opposes the pull from the Earth, whereas the pull from the Sun reinforces the pull from the Earth on the Dark side.

[dimreepr]

26 minutes ago, Willem F Esterhuyse said:

The acceleration vector due to the Suns gravity is in the opposite direction at the sunny side than on the dark side. Centripetal force is towards Earth on the sunny side (and if I'm correct cancels the force from the Sun exactly), while centripetal force is away from Earth on the dark side. Thus the object would be lighter on the dark side.

How would you test for this affect, surely scales would experience the same effect.

Edit cross posted with studiot.

Edited February 3, 2023 by dimreepr

[swansont]

55 minutes ago, dimreepr said:

How would you test for this affect, surely scales would experience the same effect.

Edit cross posted with studiot.

A scale that compared gravitational attraction, e.g. balance scales, yes. But e.g. a spring scale measures the normal force, which would vary with the acceleration.

Not that we necessarily have devices sensitive enough.

1 hour ago, studiot said:

But yes, all other things being equal, a body on the dark side experiences a slightly smaller pull due to the Sun compared to the a body on the light side because it is further from the Sun. But theSun's pull does not change direction, it is the Earths pull that pulls in opposite directions on the dark and light sides.

In the earth reference frame it does. Toward the center vs away from the center.

[studiot]

1 hour ago, swansont said:

In the earth reference frame it does. Toward the center vs away from the center.

That isn't or is what I said, depending upon how you look at it.

 

Maybe my phrasing could having been better, but I was trying to avoid switching frames from heliocentric to geocentric in mid description.

So as I said the Sun's gravity does not change direction at any one place in the Earths orbit. It is always directed towards the Sun, whichever side of the Earth you are on.

[swansont]

24 minutes ago, studiot said:

That isn't or is what I said, depending upon how you look at it.

 

Maybe my phrasing could having been better, but I was trying to avoid switching frames from heliocentric to geocentric in mid description.

So as I said the Sun's gravity does not change direction at any one place in the Earths orbit. It is always directed towards the Sun, whichever side of the Earth you are on.

I was using the reference frame that Willem described. Which seems reasonable in answering a question they posed (and which doesn’t raise an issue of shielding gravity)

[Janus]

22 hours ago, Lorentz Jr said:

Earth is revolving about its own axis in the same sense as its orbit around the Sun (except for the tilt, of course), so objects on the Sun side are orbiting more slowly than ones on the dark side.

For the Sake of this example, we ignore the Earth's own rotation, as it not a factor as far as tidal forces go and unnecessarily complicates the scenario.

[studiot]

9 minutes ago, swansont said:

I was using the reference frame that Willem described. Which seems reasonable in answering a question they posed (and which doesn’t raise an issue of shielding gravity)

Was he ?

" The acceleration vector always points in the direction of the Sun"

What acceleration vector ?

And what frame would that be please.

[swansont]

3 hours ago, studiot said:

Was he ?

" The acceleration vector always points in the direction of the Sun"

What acceleration vector ?

And what frame would that be please.

“Centripetal force is towards Earth on the sunny side (and if I'm correct cancels the force from the Sun exactly), while centripetal force is away from Earth on the dark side”

(emphasis added)

Seemed obvious to me. YMMV

[Tutoroot]

The side facing away is cooler and darker, and experiences night. Because the Earth is constantly spinning, the line between day and night is always moving around the planet.

Edited February 10, 2023 by Tutoroot

[Genady]

3 hours ago, Tutoroot said:

The side facing away is cooler and darker, and experiences night. Because the Earth is constantly spinning, the line between day and night is always moving around the planet.

Ahh, I got the pun.