# Moons orbit

hey,
can somebody plz help me examine the truth of the statement

"the moon moves in a near-circular orbit around earth. because the earth is so much more massive, it own motion is not appreciably affected by the moons presence"
???

BobG
Homework Helper
hey,
can somebody plz help me examine the truth of the statement

"the moon moves in a near-circular orbit around earth. because the earth is so much more massive, it own motion is not appreciably affected by the moons presence"
???

Gravity affects both objects equally (equal and opposite reaction). The result is that both objects orbit their combined center of mass.

Mass of the Moon: 7.349 x 10^22 kg
Mass of the Earth: 5.9736 x 10^24 kg
Average distance between the two: 384,400 km

Find the combined center of mass and you'll find it lies inside the Earth. I guess "appreciably" is a subjective term, since the combined center of mass is still quite a ways from the Earth's center of mass.

DaveC426913
Gold Member
It's about 400 miles from centre is it not?

Astronuc
Staff Emeritus

the barycenter of the earth-moon system is 4670 km from the center of the earth. For comparison, the earth's radius is approximately 6380 km. The barycenter is about 1000 miles below the earth's surface on a line between the center of the moon and center of earth.

Perhaps one is thinking of the barycenter of the sun-earth system.
If the Earth were the only satellite of the Sun and the Earth were moving in an elliptical orbit about the center of mass of the Sun-Earth system then the Sun would also be moving in an elliptical orbit about that same center of mass. The scales of the orbits would be in proportion to the ratio of the masses. The Sun is about 333 thousand times as massive as the Earth. Thus if the average radius of the Earth's orbit is 93 million miles (150 million km) then the radius of the Sun's counter orbit is about 280 miles (450 km). The center of mass of the Sun-Earth system is well within the body of the Sun.
http://www.applet-magic.com/centermass.htm - This also mentions the barycenter of the earth-moon system.

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hey thanks guys this info helped alot.
tony that animation was pretty good thanks