Why don't we orbit the center of our galaxy?

  • At the center of the milky way, there must be some giant mass that keeps the galaxy from just floating away from itself. So, obviously, we feel the immense gravitational pull of whatever this is. But, on a smaller scale we can see this with the earth and the sun. The sun keeps us from spinning into oblivion with its gravitational pull. The question I am asking is, why do we orbit the sun, but not the center of the galaxy, which has a much stronger gravitational pull?


  • LocalFluff

    LocalFluff Correct answer

    8 years ago

    The so called inverse square law makes gravity dominate locally. The Moon orbits Earth, Earth orbits the Sun, and the Sun orbits the Galactic center! Because the distance from Earth to the Moon is 1/500 the distance from Earth to the Sun, it is Earth's gravitational field which dominates the orbit of the Moon.



    And it is not a single thing which attracts mass to orbit the galaxy. It is the sum of mass of everything in it. The center is of course the galactic center of mass.


    Yes, the Earth is about 500 times nearer to the Moon (much closer to 400 actually), but the Sun is also 333,000 times more massive than the Earth making the Sun-Moon gravitational attraction over twice as strong. So it's actually the Sun's gravitational field which dominates the the orbit of the Moon.

    @DavidH Are you trying to make things difficult here ;-) So what is your answer?

    DavidH is correct. The gravitational force exerted by the Sun on the Moon is more than double that of the gravitational force exerted by the Earth on the Moon.

    @DavidHammen Makes sense. As per your answer here, is the inverse-square law even applicable in this case (Sun v. galactic center)? I would think not.

    Sun, earth, moon, solar system all orbit center of galaxy with a period of about 225 million years: http://en.wikipedia.org/wiki/Galactic_year

    If you want to see what the centre of the galaxy looks like, you can visit https://www.universeguide.com/fact/galaxy for what it looks like. The galactic centre is in the constellation of https://www.universeguide.com/constellation/sagittarius

    The Earth and the Moon orbit the Sun at the same speed and distance **on average**. This means we can *almost* ignore the effects of the Sun on the Earth/Moon system and just consider the gravity between the two local objects. If the Sun was to suddenly disappear, the Earth and Moon would continue to orbit each other, and their barycentre would travel in approximately a straight line, whereas at the moment, the barycentre travels in an approximate circle around the Sun. In both cases, the Earth and Moon orbit the barycentre of the Earth/Moon system, which is inside the Earth.

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Content dated before 7/24/2021 11:53 AM

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