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Do planets lose energy?

  1. Aug 11, 2005 #1
    Why do planets rotate around the sun? due to the gravitational field of the sun, which is obvious, but why do they not gradually crash into the sun, due to their motion and the loss of energy after a while :confused: ?
     
  2. jcsd
  3. Aug 11, 2005 #2
    What's resisting earth's motion in a way that doesn't have a neglible effect?
     
  4. Aug 11, 2005 #3

    rbj

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    check out what "leap seconds" are all about. the need for an occasional leap second, here or there, is due exactly to loss of kinetic energy in the rotation of the earth.

    http://tf.nist.gov/pubs/bulletin/leapsecond.htm

    there is kinda an odd reason (having to due with geology and the speed of the Earth's crust) for why they haven't added leap seconds in the past few years. but the rotation of the earth, as a whole, is still slowing down very minutely.

    http://www.ucolick.org/~sla/leapsecs/dutc.html
     
  5. Aug 11, 2005 #4

    pervect

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    Slowing down the rotation of Earth isn't going to cause it to crash into the sun, though.

    A careful answer would involve figuring out all the effects that might brake the Earth's orbital motion (there are some), plus the effects that would move the Earth into a higher orbit (there are some of those, too).

    A non-careful answer would say that these effects are all so small that we can expect the Sun to turn into a red giant first, at which point the question becomes moot.
     
  6. Aug 11, 2005 #5

    BobG

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    The loss of kinetic energy in the rotation of the Earth is due to the combined Earth-Moon system. The Earth and Moon will eventually become tidally locked with each other (the Moon already presents a virtually identical face towards the Earth). The Earth's rotation rate gradually slows down and the distance between the Earth and Moon gradually increases (the combined angular momentum has to stay constant). The varying rotation rate is why leap seconds are required.

    The reason they haven't added leap seconds is buried deep in the article (it's not very well organized, even though it pulled together some good material). Primarily, it's because it's becoming more important for systems that have to communicate with each other to be using the same time and, unfortunately, leap seconds have to be added in (or subtracted) manually at irregular times. You'd think it wouldn't be that difficult to add in the leap second at 0000 Universal Time on the designated day, but ........... if a computer can't do it automatically, then it must not be a very good thing.

    GPS particularly hated leap seconds, since they had around 24 satellites that had to have the same time to be effective for navigation (a 1 second difference only results in a 186,000 mile error, so I'm not sure what the big deal is :rolleyes: ). Plus, they can only add in so many leap seconds before their counter overflows - they already received quite a bit of flack from GPS users when their week counter overflowed back in the fall of 1999 and weren't too excited about facing that again. In the fall of 1999, some users suddenly had their clocks reset to 1980 and their receivers were suddenly obsolete and useless. (This is really due to the fact that anyone can design a receiver to receive navigation and timing signals from GPS and some receiver designers failed to plan ahead - the good designers, Magellen, Garmin, etc., adhered to the 'official' GPS standards and had no problem.)

    Bottom line is that computer and electronics folks hate time that doesn't update at a constant rate, forever and ever. They hate having to roll over form 99 to 00, 1024 to 0000, and especially hate having to figure out how to account for a second that can be added every 18 months, or maybe every 12 months. For computer programmers, reality is a b....
     
  7. Aug 11, 2005 #6

    rbj

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    that is absolutely most correct. i only tried to imply that there the matter in space that slows down the rotation will also slow down the revolution about the sun. i don't think any human beings will be left (it will take a while) if it slows to the point of crashing into the sun.

    sorry for being non-careful.

    no. it's clear that they are measuring a net loss of angular momentum.

    yes. and the variation is generally a negative trend.

    they've never been subtracted. only added. there haven't been any added since the 90s because of some geological phenomenon that is, for the time being, causing the rotation of the earth's crust to have accelerated slightly.

    they have to decide what the designated days are and i don't think they have any days slated for a leap second yet. they don't want to subtract any (because of the recent crust acceleration) if they have to add them back later (because of the general deceleration).

    the reason is that in the SI system, the second is pretty well fixed, normally there are 86400 seconds per day, but the days are getting longer and longer (even if it's getting closer to fall :rolleyes: ).
     
  8. Aug 12, 2005 #7

    pervect

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    Here are as many of the relevant interactions as I can remember or think of - there's probably more.

    Tidal interactions between the Earth and moon won't slow down the Earth's orbital speed or speed it up, they are basically irrelevant.

    There are two tidal interactions between the Earth and sun, both of which will increase it's orbital velocity very slightly (spin-orbit coupling). The first is due to the spin of the Earth - the second due to the spin of the sun. These are similar to the spin-orbit coupling that's raising the moon's orbit.

    There will be tidal interactions between the Earth and other planets, but I don't think they'll do much one way or the other.

    There is probably a small amount of friction with the interplanetary medium which will probably tend to slow the Earth's orbital speed down.

    There's probably some effect from the solar wind, but I don't know what (if anything) it does to the Earth's orbital motion.

    There is an effect whose name I just can't remember, that's mostly important for small particles, that slows the Earth down because, in a heliocentric coordinate system, the earth emits higher frequency light with more energy and momentum in the direction of its orbit, and lower frequency light with less energy and momentum in the opposite direction.

    And, probably least important of all, I believe that the Earth-sun system, like any binary system, emits a minuscle amount of gravitational radiation (which causes the orbit to decay, ever-so-slightly).

    Probably none of these effects will cause any great change in the Earth's orbit by the time the Sun progresses to the red giant phase, at which point it shouldn't matter much.

    There is also some chance that a major orbital collision could alter the Earth's orbit (it would take a very large impactor to do this). Apparently chaotic dynamics of some of the planets make really long term predictions of the Earht's orbit unreliable, though from what I've read, long term simulations show that the chaotic effects usually don't matter too much.
     
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