Is the moon moving further away from Earth and closer to the Sun? Why?

  • According to The NASA Moon Facts page:

    The moon is actually moving away from earth at a rate of 1.5 inches per year.

    Why is the moon moving further away from the Earth? Is this a result of the moons formation that set it in motion to spiral away from us? Or is this a resultant force of the gravity from the Sun and other large bodies?

    Since it is part of your question, but not explicitly answered in any of the posts to date: no, this has nothing to do with gravitational interaction with any other bodies in the solar system. Furthermore, moving away from the earth does not mean closer to the sun since during half of its revolution, the moon is further from the sun.

    If the moon is moving away at 1.48" (which is a fairly small, insignificant dimension) per year and, I assume, this has been fairly constant over the history of the earth, that means that 10 billion years or so ago the two were touching. How does that jive with the age of the earth and moon and how would that have affected the gravitational interaction of the two bodies?

    Well, neither the Earth nor the Moon (or the Sun) are 10 billion years old. The change in distance is also not constant over time. However, this doesn't necessarily mean that the Earth and Moon weren't touching at one point in time. If this doesn't resolve your question, I would suggest you ask it as a brand new question, as is the best practice here.

    Have you considered our own continental shift movements as part of the measurement? its my understanding that the measurement is calculated by the use of a beam bouncing from the surface. If the continent on which the beam is mounted is moving also, could this affect the measurement?

  • Undo

    Undo Correct answer

    9 years ago

    Yes, the moon is moving away from Earth at around 1.48" per year. According to the BBC:

    The Moon is kept in orbit by the gravitational force that the Earth exerts on it, but the Moon also exerts a gravitational force on our planet and this causes the movement of the Earth's oceans to form a tidal bulge.

    Due to the rotation of the Earth, this tidal bulge actually sits slightly ahead of the Moon. Some of the energy of the spinning Earth gets transferred to the tidal bulge via friction.

    This drives the bulge forward, keeping it ahead of the Moon. The tidal bulge feeds a small amount of energy into the Moon, pushing it into a higher orbit like the faster, outside lanes of a test track.

    So, tidal forces are ultimately what causes this to happen.

    Also, there is a Wikipedia article on tidal forces:

    Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite (e.g. the Moon), and the primary planet that it orbits (e.g. the Earth). The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking of first the smaller, and later the larger body. The Earth–Moon system is the best studied case.

    Thought, as the moon gets further away wouldn't the amount of force applied to and from the tide be reducing thus limiting the push of the moon away from us, effectively reducing the distance it moves away each year? Would it not reach a stable orbit where the force is too small to push it further away?

    @RhysW: The tides are slowing down the Earth's rotation as well as driving the Moon farther out. Ignoring other influences, the effect would stop when the Earth and Moon are both in locked rotation, each showing the other the same face (like Pluto and Charon)).

    @Guillochon: That's not *quite* what the article says. Currently the barycenter of the Earth-Moon system is inside Earth. Eventually it will be above Earth's surface. The Earth and Moon will still orbit around their common center (as Pluto and Charon do). It's more a matter of the current definition of the word "planet"; there's no great physical significance to it. See also this question on the Physics site (posted to the old Astronomy site before it was merged into Physics).

    Funny side note; due to pollution, earths gravity is increasing, counteracting the fact that the moon is moving away from us by a tiny amount and instead speeding up the moons orbit. This is because energy also causes gravity, and a warmer planet means more energy here.

    @frodeborli I'm pretty sure you're confused - to the best of our knowledge, energy doesn't cause gravity. If you can show me a source that says otherwise, though, I'm always open to new ideas.

    @Undo My source is Einstein. The formula E=mc^2. For any source of energy, you can calculate the gravity it causes by using E/c^2 in place of mass. Both kinetic and potential energy cause gravity.

    @frodeborli - I'm extremely skeptical. How much energy are we talking about? And how much mass would that make? I'm betting it's negligible when compared to the total mass of a planet.

    @frodeborli The Earth is actually losing mass-energy. See this BBC article which has some Cambridge physicists draw up an estimate of mass-energy in vs. mass-energy out.

    @Donald.McLean The kinetic energy of a planet in motion quite large. The energy due to greenhouse effect in comparison is neglible, but it is there. 9 x 10^13 kilojoule will amount to about one kilo.

    @called2voyage Nice. That article actually confirms my assertion; global warming contributes to a gain in mass of about 160 tonnes per year, a tiny slowdown of the loss of mass.

    @frodeborli Right a tiny slowdown of the significantly larger but also ultimately (compared to the mass of the Earth) tiny loss of mass.

    @frodeborli You're missing the whole point. It would be like contributing one penny a year to the US federal government and saying that you're making a dent in the US national debt.

    @Donald.McLean You're missing a key word in my funny side note, quote: "a tiny amount". But being tiny does not mean that my assertion is wrong.

    @frodeborli Your original assertion was "due to pollution, earths gravity is increasing". We have shown this to be false. Earth's mass is decreasing, thus its gravity is decreasing.

    @called2voyage Yes, you are right. I failed to state what I assumed to be obvious in what I assumed would be taken as a *fun fact* - that pollution is not the only thing affecting Earths' gravity. Thank you for contributing that. Nevertheless, pollution is reducing the rate of weight loss, and reducing the rate of loss of gravity. Would it be correct to say "it is increasing, relative to the gravity it would have if we did not pollute"?

    @frodeborli I believe the least easily misinterpreted way of stating it would be that it is "reducing the rate of mass loss". Using "increase" at all is misleading.

    @called2voyage Yes. I wish I could take back the comment. I found the fact *I intended to convey* itself amusing and spontaneously wrote it as a comment.

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