# Earth movement

1) the earth moves around the sun. What is the force causing this movement around the sun? I know that there is a gravitational force pulling it inward, but what force keeps it from falling toward the sun?

2) The earth is constantly spinning. This causes the day/night that we see. What is the force causing this revolution?

3) The earth tilts on its axis about 23 degrees. This causes the seasons. What is the force causing this tilt?

4) Why does it take 365 days for the tilt in question 3 to complete 1 period

Please tell which question you are answering. thanks

## Answers and Replies

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russ_watters
Mentor
1) the earth moves around the sun. What is the force causing this movement around the sun? I know that there is a gravitational force pulling it inward, but what force keeps it from falling toward the sun?
The earth is falling toward the sun - it just always misses! That's what an orbit is.

Another way to describe an orbit is that the tangential velocity is high enough that the earth accelerates toward the sun without ever getting closer to it.

Or, if you prefer, an orbit is what happens when the acceleration due to gravity is balanced by the "ficticious" centrifugal force.
2) The earth is constantly spinning. This causes the day/night that we see. What is the force causing this revolution?
By Newton's first law and/or conservation of energy, a force is not required to keep a moving object moving.
3) The earth tilts on its axis about 23 degrees. This causes the seasons. What is the force causing this tilt?
Again, no force.

Force causes acceleration. If there is no acceleration, there is no need for a force.
4) Why does it take 365 days for the tilt in question 3 to complete 1 period
The tilt does not precess in 365 days - it takes thousands of years. What happens in 365 days is that the earth orbits the sun. Draw yourself a diagram if you can't see why the seasons happen with a fixed tilt.

thanks for your response:

For the orbit around the sun
1) The earth has a tangential velocity which is constant. Since there is no acceleration, no force is needed to maintain this tangential velocity. However what brought the earth to this tangential velocity? Also is the tangential velocity decreasing as the earth does not travel in a vaccum(stuff runs into the earth)?

2) What caused the rotational velocity of the earth (the one that causes night/day) to begin?

3) Thanks for explaining the tilt concept. Is it known what causes this tilt, or it's movement over thousands of years

thanks

tony873004
Gold Member
#1. The stuff Earth runs into is insignificant compared to Earth's mass. Even if it weren't, it can hit Earth from the front or the rear, or above, or below, resulting in no significant bias.

#1 & 2. These are open to debate. The formation of the solar system is not completely understood. Earth's tangental velocity is the perfect speed for a (or an almost) circular orbit. How Earth's orbit circularized is open for debate. Perhaps it formed in a circular orbit, or perhaps it originally had a good deal of eccentricity and its orbit circularized later due to its interactions with solar system debris, or to a lesser degree, with the tides it pulls on the Sun (which is probably easily-computable, allowing it to be ruled out).

#2. If Earth had no rotational velocity, that would more-require an explanation than Earth having rotational velocity. During formation, Earth was getting bashed left and right, up and down, by objects ranging in size from microscopic to objects rivaling Earth's mass. It would be a weird lottery-style chance if after all that, Earth's spin ended up perfectly stationary. The bias, as demonstated by most other planets in the solar system, is a counter-clockwise (as viewed from above), prograde (same as its orbital direction) rotation. This was probably imparted by the momentum of the stuff that bashed together to form the Earth.

#3. Again, a tile of 0.0000 degrees would more-require the explanation. Because Earth is not perfectly spherical, this gives a handle for other solar systems to tourque it around over long time scales, so the tilt precesses.

The main reason things in the Solar System move as they do is as a result of the formation of the system itself.