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Q & A: Gravitational pull from the sun

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Most recent answer: 07/24/2018
Q:
At noon, the sun and the earth pull the objects on the earth's surface in opposite directions. At midnight the sun and the earth pull these objects in same direction. Is the weight of an object, as measured by a spring balance on the earth's surface, more at midnight as compared to its weight at noon?
- Kells (age 18)
Pakistan
A:
Kells,

Let's begin by imagining that the pull of the sun is equal at all points on the Earth.  This is the force responsible for keeping the Earth (and consequently each and every one of us) going around the sun each year.  If you can visualize the astronauts in the space station, both the space station and the astronauts are orbiting the Earth due to the gravitational attraction to the Earth.  They don't feel their weight because the floor beneath them is going around the Earth with them; you need to be pushing against the floor to measure your weight.  You, the Earth, and the Sun comprise the same kind of system.  Because the Earth is revolving around the Sun at the same rate you are, you don't feel the extra "pull" from the Sun.

However, it is not the case that the Sun's pull is constant everywhere on the Earth.  As you know, the gravitational pull of the Sun drops off as you go farther away from it.  Then shouldn't that mean you should weigh differently at noon and midnight?  The answer is actually no; at noon, it's true that you are pulled toward the Sun and off the Earth, but the Earth is pulled toward the Sun as well. At midnight, the Sun pulls the Earth from under your feet but it pulls you towards the ground about the same amount. These effects are equal, so you do weigh the same at noon than at midnight.

It is rather interesting though that you will weigh a bit more at 6 am and 6 pm than you do at midnight and noon.  If you draw a line from your feet to the center of the Earth and from the center of the Earth to the center of the Sun at 6 am or 6 pm, you'll make a right triangle (because the Sun is at the horizon).  A component of the force from the Sun actually pulls you a bit downward, and in a sense, presses you harder against the ground.

Imagine, if you will, the tides in this entire picture, now.  Water is pulled up off the ground on the points of the surface of the Earth that are in line with the sun (which is at midnight and noon), but pulled down onto the ground on the points of the surface of the Earth that are perpendicular to that line (which is at sunrise and sunset).  The Sun is still very very far away, and so there is little difference between the force of its pull at the different points of the Earth, so the tidal forces of the Sun are small.  The moon, however, exerts a larger tidal force because it's closer to the Earth.

If you'd like to learn more about tidal forces, there's a very well-written article on Wikipedia with very instructive diagrams:

Thanks for the good question,
Jim

p.s. As Jim said, the average pull from the Sun doesn't affect our weight here. The tidal force, the difference between the pull on different parts of the earth, does, but to a first approximation it's the same at noon and midnight. However, the tidal effects on the side of the earth closer to the sun are just a tad bigger than those on the other side. So the answer is that yes, you weigh more at midnight than at noon, but by an extremely small amount./ Mike W.

(published on 01/24/2011)

Follow-Up #1: speed of gravity from sun

Q:
I have pondered the question of the speed of gravity ever sinse freshman college. I am familiar with all the arguments that quote it at the speed of light to instant. Do we have scientific instruments accurate enough to measure the sun's pull 8 minutes before noon (time for the sun's light to reach us) when the sun is really directly overhead, and the again when the light is directly overhead. I know to find a difference and prove Einstine wrong would send the scientific community into a tailspin, but wouldn't it be fun!!!! Lee Scherer
- Lee Scherer (age 81)
Stewartville, MN USA
A:

Nice question. You do realize that in treating the Sun as moving overhead, rather than the earth as spinning, you're using a peculiar coordinate system? General Relativity allows peculiar coordinate systems, but I can't mentally handle ones much different from the standard flat space. So let's be Copernicans and pick a coordinate system where the sun stands still and the earth spins as it orbits. In this coordinate system the only traveling done is for your spot on the earth to spin over to a different spot with a little different gravitational field from the sun. The field isn't traveling, so there's no propagation delay.

Mike W.


(published on 07/18/2013)

Follow-Up #2: Does the earth rotate around the "visual" position of the sun?

Q:
I cannot find a reference to whether the Earth circles the actual or the visual position of the Sun. I think this question was answered during an eclipse when the gravitation anomaly in 1954 - ie the Allais Effect was seen to occur before the visual eclipse. Can anybody help?
- Tom Hollings (age 71)
Leeds
A:

Since  light waves and gravity waves travel with the same speed, c, the earth will track the 'visual' gravitational position which is the same as 'visual' optical position.

LeeH


(published on 07/24/2018)

Follow-up on this answer.