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Author Topic: Does the moons orbit round earth speed up as it gets closer to the sun?  (Read 5990 times)

Offline acecharly

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Any thoughts

Cheers Ace


 

Offline Soul Surfer

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I am not quite sure what you are getting at ace could you expand a little on your explanation.
 

Offline CliffordK

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The orbital velocity varies with the eccentricity of the orbit, with the body travelling fastest at periapsis, or the point in the orbits when they are closest. 

Does Earth's orbit around the sun significantly alter the eccentricity of the moon's orbit?  I wouldn't think so.

Earth is closest to the sun during the Northern Hemisphere winter, and thus travelling fastest.  Technically, the moon would be travelling fastest at the full moon at the earth's perihelion, just not necessarily orbiting around the Earth fastest. 

However, to really consider velocity, one should not only consider the moon's orbit around the Earth, and the earth's orbit around the sun, but one must also consider the sun's orbit around the milky way, and the milky way's movement through space (as measured by the red-shift of CMBR. 

Thus, the true maximum velocity would be when Earth's orbit around the sun is maximally tangential with respect to the movement with respect to the CMBR, as well as when the moon's orbit around the Earth is also maximally tangential with respect to the movement with respect to the CMBR (at full moon?).
 

Offline RD

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Does Earth's orbit around the sun significantly alter the eccentricity of the moon's orbit?  I wouldn't think so.

how much is "significantly" ?  ...

Quote
Evection (Latin for carrying away), in astronomy, is the largest inequality produced by the action of the Sun in the monthly revolution of the Moon around the Earth. ... It can be considered as arising from an approximately 6-monthly periodic variation of the eccentricity of the Moon's orbit and a libration of similar period in the position of the Moon's perigee, caused by the action of the Sun
https://en.wikipedia.org/wiki/Evection
 

Offline acecharly

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What i mean is the moon feels gravity from both the sun and the earth, the moon is falling into the gravity dent of both the earth and the sun.

 If you imagine the moon at 4 different points in its orbit around the earth. 1st directly in line with the sun but behind the earth, secondly at the side, thirdly directly between the earth and sun and fourthly at the side again before returning to the first point. So what im saying is as the moon moves between points 2 and 3 it is being pulled towards the sun, it then hits point 3 directly between the sun and earth and starts then to move away from the sun so does the sun slow it down as it returns to point 4. Like a pendulem in some way, how it starts at the top of its swing and accelerates until it is at its lowest point the deccelerates until it stops (allthough obviously the moon carries on).
 

Offline Soul Surfer

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The moon's orbit is in fact very complex.  As anyone who has tried to predict an eclipse soon finds out.

It can be considered to be orbiting the sun just as much as it orbits the earth because the centre of curvature of its orbit round the sun is always towards the sun and never towards the earth.

It also orbits in the same sense as the earth does the sun (i.e. if you look from the side where the earth is going clockwise the moon is going clockwise round the sun.

This means that when the moon is further away from the sun it is overtaking the earth i.e moving at a faster orbital speed around the sun and when it is closer to the sun the reverse is true and the moon is moving slower in its orbit around the sun.

The moons orbit like the earth's is elliptical and slightly inclined to the plane of the earth's orbit (the ecliptic) also the positions of the perigee (moons closest position to the earth) and nodes (where the moon's orbit crosses the ecliptic) change steadily through the years.  The perigee moves 40.6 degrees each year and the nodes 19.3 degrees each year.
 

Offline acecharly

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Thanks cliff and soul...great answer thanks
 

Offline Soul Surfer

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O I forgot to say that the moons orbit radius is approximately 1/400 of the earth's orbit radius which  means that the approximate difference in attraction from the sun when the moon is closer to the sun than the earth and when it it is furthest away is 1/200 squared ie 1/40,000 so the sun's field is essentially uniform and can be neglected.

The moon's orbital speed is about 1Km/sec the earth's orbital speed around the sun 30km/sec so the moons orbital speed varies between 29 and 31 km/sec around the sun.
 

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