# Is one side of the Earth faster at night and slower at daytime?

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## Main Question or Discussion Point

I'm reading Physics for Entertainment by Yakov Perelman and in it he says under "When we move around the sun faster" that at midnight the speed of the rotation of the earth is added to that of earth's translation effectively saying we move faster at night than during the day.

He further goes on to say "Since any point travels about the earth at half a kilometer a second, the difference there between midday and midnight speeds is about a whole kilometer a second."

Does this happen because the gravity of the sun? Is this just a phenomenon of any object, like a wheel (I heard someone explain it on a quora but I wasn't sure if it was related)?

Anyways I don't understand please explain in easy way for me or tell me what to study so I understand it.

A excerpt of the book:

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scottdave
Homework Helper
Tracing a point on a wheel is a pretty good analogy to what this is referring to. Your scanned article does not make any reference to how the entire solar system is also moving through space, as well. But understand this first type of motion first.

I see the next chapter was the wheel demonstration saying there's a translation of movement and a rotation of movement and the translation shows a stopping with the wheel meeting the ground. I guess the translation of stoppage in the orbit would be the gravity of the sun stopping it and maybe also the moon's pull makes the night side spin faster? Is that correct?
Or does the stoppage have to do with the circular orbit?

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Nugatory
Mentor
guess the translation of stoppage in the orbit would be the gravity of the sun stopping it and maybe also the moon's pull makes the night side spin faster? Is that correct?
No, there's no gravity involved here (except that of course the sun's gravity is what keeps the earth in orbit). All that's going is that the earth is rotating so people standing at different points on the surface of the earth are moving at different speeds relative to one another. Imagine that you are floating in space above the North Pole, looking down at the earth turning underneath you. Two people standing on the equator on opposite sides of the earth will be moving in opposite directions (just as the tip of the hand of a clock is moving to the left at 6:00 but to the right at 12:00).

If you're standing on the equator at midnight, the earth's rotation is moving you in the same direction as the earth's orbit around the sun so your total speed is the speed of the center of the earth around the sun, plus the rotation speed. Meanwhile the earth's rotation is moving a person on the other side of the earth (where it is high noon) in the opposite direction from the earth's orbit around the sun, so we have to subtract the rotational speed instead of adding it.

That makes sense, so there's no stoppage here at all? It's simply a matter of placement in relation to earth during the transition?

sophiecentaur
Gold Member
@shawnr when you have got your head around the Earth and a wheel, look at this movie which shows a view of the Moon's combined orbits round the Sun and the Earth.

Janus
Staff Emeritus
Gold Member
@shawnr when you have got your head around the Earth and a wheel, look at this movie which shows a view of the Moon's combined orbits round the Sun and the Earth.
The diameter of the Moon's orbit is 768000 km and it takes ~29.5 days to complete 1 orbit with respect the Sun. Thus it takes 14.75 days to move 768,000 km from leading the Earth in its orbit to trailing it. The Earth's orbital speed is ~30 km/sec, and thus would have moved 38232000 kn along its orbit at in that time. this is nearly 50 times the width of The Moon's orbit. The Moon's heliocentric path can't "back track" over itself.

Janus
Staff Emeritus
Gold Member
That makes sense, so there's no stoppage here at all? It's simply a matter of placement in relation to earth during the transition?
The center of the Earth orbits the Sun at ~30 km/sec
A point on the equator moves relative to the center of the Earth at ~ 0.46 km/sec due to the Earth's rotation.

Thus you get the following (with the sun located below the image):

The point of the equator nearest the sun has its rotational speed subtracted from the orbital speed of the Earth in order to get its speed relative to the Sun, while the far side has its rotational speed added to the orbital speed,

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