# Is this statement about the orbit of the moon true?

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"

plz help

LURCH
I just depends on how one defines "appreciably". As the Earth and moon are gravitationaly bound to one another in what could be called a single system (Earth-Moon System), they each orbit around the center of gravity for that system. But because the difference between the two objects in that system is so great, the system looks very much like a stationary Earth with the Moon going in a circle around it.

The difference between the two masses means that the center of gravity for the total system is deep inside the Earth, about 1/4 of the way down to the core. So, while the Earth goes around that center of gravity in tiny circles that are somewhat smaller than the radius of the planet, the Moon swings around in a circle with a radius of about a quater-million miles.

If one were to model the system with a model small enough to see the whole thing at once, the movement of the Earth might not even be visible with the naked eye.

so how does the moon effects the earth

i know it effects it by tides but are there any others and how does it make earth have two tides every day

i am very confused

tony873004
Gold Member
The moon pulls hardest on the side of Earth closest to it. It pulls medium on the middle of the Earth, and it pulls the least on the side of the Earth farthest from it. This gravity gradient through the Earth's sphere pulls the Earth into an oval with the long axis orientated towards the Moon. But since the Earth spins, it is not exactly towards the Moon. It is slightly ahead of the Moon. This causes a torque that slows down the Earth's rotation and pulls the Moon into a higher orbit.

Thanks tony873004 that makes so much sense know

can u help me with this

imagine that the universe is static and not expanding. predict the observations that would be made in regards to:

a)red shift in galaxy spectra

tony873004
Gold Member
a) You might expect to see a random distribution of red/blue shifts. In an expanding universe, only galaxies that are very close, such as M31 will exhibit a blue shift (coming towards us). But in a static universe, even distant galaxies may exhibit blue shifts.

b) tough question. There may not even be one. I'd want to know the new theory that replaced the big bang and expansion to guess at this one.

ok thanks last one
it is observed that Mars is somewhat brighter when in opposition (ie. sun- earth - mars in that order) that at other times. How can this be explained using
a) The Ptolemaic model and
b) The Copernican model of the solar system.
(Assume that Ptolomy's construction describes the actual motion of the planet

tony873004