# Thread: Moon speed and Earth motion

1. Hello

Does the centrifuge acceleration on Moon caused by Earth's circular motion around the Sun affect geocentric Moon's velocity?

In other words, if our planet didn't revolve around anything, would Moon's speed around it be still 3682km/h?

2.

3. Originally Posted by termina
Hello

Does the centrifuge acceleration on Moon caused by Earth's circular motion around the Sun affect geocentric Moon's velocity?

In other words, if our planet didn't revolve around anything, would Moon's speed around it be still 3682km/h?
The Moon's orbital velocity(ignoring tidal effects form the Sun) varies from 3497 kph to 3907 kph due to the eccentricity of its orbit. The only effect the Sun has is it tidal effect on the Moon's orbit which can actually increase or decrease the eccentricity depending on the Earth-Sun-Moon alignment.

4. The Moon's orbital velocity(ignoring tidal effects form the Sun) varies from 3497 kph to 3907 kph due to the eccentricity of its orbit. The only effect the Sun has is it tidal effect on the Moon's orbit which can actually increase or decrease the eccentricity depending on the Earth-Sun-Moon alignment.
Hum...Ok!

And what about centrifuge force on Moon from Earth's revolution?

If a tiny ball is stuck thanks to a glue on the border of a spinning disk, the quicker the disk spins, the greater the centrigufal force F exerting on the ball becomes.

but after reaching a precise rotational speed, the glue won't be strong enough to firmly fix the ball, and the latter's movement relative to the disk would be affected.That occur when F centrifugal > F glue,

where F centrigual=ball mass.(disk velocity˛/disk radius)

So i tried to apply the same reasoning to Earth motion on Moon
:

we know Earth exerts 1.98E20 N on the Moon
and the centrifugal force on Moon due to Earth's revolution velocity:

Fc= Moon's mass*(Earth velocity˛/Earth orbital radius)
Fc=4.35E20 N

But here F(earth-moon) < F centrifugal !!!

So that means the Moon would have to move farther from the Earth (if it'd escape from it)?????

5. Originally Posted by termina
The Moon's orbital velocity(ignoring tidal effects form the Sun) varies from 3497 kph to 3907 kph due to the eccentricity of its orbit. The only effect the Sun has is it tidal effect on the Moon's orbit which can actually increase or decrease the eccentricity depending on the Earth-Sun-Moon alignment.
Hum...Ok!

And what about centrifuge force on Moon from Earth's revolution?

If a tiny ball is stuck thanks to a glue on the border of a spinning disk, the quicker the disk spins, the greater the centrigufal force F exerting on the ball becomes.

but after reaching a precise rotational speed, the glue won't be strong enough to firmly fix the ball, and the latter's movement relative to the disk would be affected.That occur when F centrifugal > F glue,

where F centrigual=ball mass.(disk velocity˛/disk radius)

So i tried to apply the same reasoning to Earth motion on Moon
:

we know Earth exerts 1.98E20 N on the Moon
and the centrifugal force on Moon due to Earth's revolution velocity:

Fc= Moon's mass*(Earth velocity˛/Earth orbital radius)
Fc=4.35E20 N

But here F(earth-moon) < F centrifugal !!!

So that means the Moon would have to move farther from the Earth (if it'd escape from it)?????
What holds the Earth in its orbit?, the Sun's gravity. This same gravity acts on the Moon.

What's the force of the Gravity from the the Sun on the Moon:

So it is the Sun's gravity that holds the Moon in orbit.

Now since the Moon is sometimes a little closer to the Sun than the Earth and sometimes further (by about 1/400th the Earth-Sun distance), The Sun's gravity is sometimes a little weaker and sometimes a little stronger. It is this difference that results in the tidal force I mentioned above and changes the shape of the Moon's orbit slightly.

6. The Earth and Moon actually revolve around a common point close to the Earth but since we are on the Earth and it has most of the mass, it appears that the Moon is revolving around the Earth. As pointed out, the Sun controls the orbit around it of both of them.

7. Originally Posted by Cyberia
The Earth and Moon actually revolve around a common point close to the Earth but since we are on the Earth and it has most of the mass, it appears that the Moon is revolving around the Earth. As pointed out, the Sun controls the orbit around it of both of them.
That's very true, however I think termina was referring to how the Earths rotation was affecting the moons orbit. Correct me if I'm wrong, but as I understand it as the moon orbits Earth it creates a title bulge in the Earth that follows slightly behind the moons orbit, and this title bulge is slightly closer to the moon and creates a very slight gravitational drag on the moon, which over millions of years causes the moons orbital speed to slow down very slowly over time and this in turn allows the moon to move into ever higher orbits. In other words it's getting further away from Earth at the rate of a few centimeters per year.

8. Originally Posted by Cyberia
The Earth and Moon actually revolve around a common point close to the Earth but since we are on the Earth and it has most of the mass, it appears that the Moon is revolving around the Earth. As pointed out, the Sun controls the orbit around it of both of them.
Actually, the barycenter of the earth-moon system is below the earth's surface by about 1700 miles.

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