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Total speed of Earths movement

  1. Sep 1, 2010 #1
    I've been trying to work out for a while now the total speed that something on Earth's surface travels through space at any given time. I'm not sure about my maths though, so i'm wondering if anyone can help me.

    These speeds are rough estimates based on various articles and explanations that i've read:
    Earth rotates at roughly 1000 mph at the equator
    Earth orbits the sun at 66,000 mph
    The solar system orbits the galaxy at 483,000 mph
    The galaxy is moving through space at 1,300,000 mph

    I'm probably about to make a very elementary mathmatical mistake here, but does that mean that, taking into account all those various speeds, something on the surface of Earth is travelling at 1,850,000 mph through space? I'm really not sure if that's right or not, so I thought i'd ask.

    Also, I wasn't able to find out, but does the solar system itself rotate at all during its orbit of the galaxy? From what i've been able to find out it's at something like 60 degrees to the galactic plane, but does it flip/rotate in any direction at all?

  2. jcsd
  3. Sep 1, 2010 #2


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    It is meaningless to talk about an object's "movement through space", you can only talk about movement with respect to a reference frame. Thus.
    With respect to the Center of the Earth
    With respect to the Sun
    With respect to the center of the Galaxy
    With respect to the Hubble Flow.
    Since all these various motions at different times are in different directions from each other, you cannot just add them together to get an object's total velocity with respect to the Hubble flow.
  4. Sep 1, 2010 #3
    Thanks for the answer Janus, I was sure my answer was too simplistic but I wasn't sure how to check so I thought i'd better ask. I think I understand it a little better now.
  5. Sep 1, 2010 #4
    Last edited by a moderator: Sep 1, 2010
  6. Sep 1, 2010 #5


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    Yes, exactly:
    That's the point: Since it is moving with respect to just about everything in the universe, which one of those things do you measure its speed against? You have a near infinite number of choices of speeds.

    However, not one of those is relative to space itself:
    Location is a real concept, but only objects can have locations. "space itself" is not an object and can't be detected, so it can't be assigned a speed. What you can do is pick an arbitrary reference frame with no object in it and calculate a speed with respect to it. But that frame can be anywhere and isn't "space itself".

    Einstein's theory of relativity describes and evidence supporting it proves that there is no meaningful "space itself" reference frame.
  7. Sep 1, 2010 #6
    I think i'm getting this now.

    Velocity is only a viable measure of distance covered over time by an object when you have another object to compare the first moving object against.

    Since space doesn't have a speed, we can't compare anything within it to that hypothetical speed, and therefore we cannot say that the speed of space is the absolute speed of all things within it. Even if we could, there are still things within space that move at different velocities, and compared to them, Earth will be moving at a different speed than it will be to space.

    Is that along the right lines? Please correct me if I am mistaken.
  8. Sep 1, 2010 #7


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    Giant, you should find out what Janus means when he partially, in a sense, agrees with you here. The units "mph" throw me off but the speed and direction of the galaxy is know, with respect to the Hubble flow, or more practically speaking with respect to the Cosmic Microwave Background. (same thing basically)

    The CMB provides the closest thing we have to an absolute frame of reference, so you should learn about it.

    The CMB is light from ancient matter when the universe had not formed stars/galaxies and was mostly just hot gas. Expansion has "cooled" the light so it is now just faint microwave radiation. The temperature in each direction can be measured very accurately. To better than one part in 10 thousand.

    The CMB light is almost the same temperature across the whole sky, but there is a Doppler "hot spot" in the direction of the constellation of Leo. That is because the solar system (and with it the Earth) is moving in that direction and causing a Doppler effect. Slightly shorter wavelengths or hotter light is observed in that direction.

    From this we deduce that the solar system is moving at speed of 380 km/s with respect to the ancient light, the CMB.

    The speed of the solar system orbiting the center of our galaxy can be deduced by other observations, this allows us to figure out the speed/direction the center of the galaxy is moving relative to CMB. This is what you were talking about. "1 300 000 mph". The usual way to describe the galaxy motion relative CMB is something like 600 km/s in the direction of (roughly) centaurus in the southern hemisphere. I worked it out one time and the direction is actually that of a smaller southern constellation called Crater (the winecup or drinking bowl). I don't recall the exact speed, something like 600 km/s.

    You don't just add speeds because usually they are in different directions and there is some cancelation.

    The galaxy is moving fast, but our orbital speed is partly backwards, so it cancels and the net solar system motion is only 380 km/s. Any clearer?
  9. Sep 2, 2010 #8
    Let's see, there are roughly 10 to the 80th fundamental particles in the universe.

    So from this you get "a near infinite number of choices of speeds".... how, exactly?

    Recall that it takes lots and lots of fundamental particles to make up just one star.

    That is precisely the subject, the motion of SOMETHING moving THROUGH SPACE.

    Voyager is MOVING THROUGH SPACE, and has long since exited our solar system. Let us say it is on a relatively direct track for Alpha Centauri and is getting closer at some 39,000 miles per hour.

    Voyager is a REAL object, and it has a REAL location, in space, and a REAL speed, relative to earth, the sun, and Alpha Centauri.


    It's not so hard.
  10. Sep 2, 2010 #9
    Thanks Marcus, that's a lot clearer, I totally get what you're saying. I have some knowledge of CMB, but evidently I don't know enough about it, so i'll read up on it some more. I got the speeds that I put in the original post from this site in case you were wondering where the numbers came from:


    Does anybody have any idea as to my second question in my original post? About the solar system 'wobbling' as it orbits the galaxy? Or does it remain at a fixed angle with respect to the galactic plane for the entirety of its galactic orbit?
  11. Sep 2, 2010 #10


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    The solar system is in the galactic plane, and it wobbles wrt that plane. In fact, it crosses the galactic plane regularly. The 'unique' thing about 2012 is the earth-sun alignment happens to point near the galactic center on the winter solstice. Amazing, what are the odds of that? - about 100% per millenium.
    Last edited: Sep 2, 2010
  12. Sep 2, 2010 #11
    When you say it crosses the galactic plane regularly, can you possibly expand on that? How exactly does it cross it? Does it rotate across it or simply wobble slightly so that it crosses it? Sorry if I phrased that poorly, i'm not sure how to word the question exactly.

    I'm know that the earth-sun alignment is something that 2012 conspiracy theorists use prolifically to attempt to justify their speculations about the events of the said date. I'd just like to make it clear that I don't subscribe to any of the conspiracies put forward about 2012, whatever they may be. I'm just trying to get a good mental image of how earth, the solar system and the galaxy all move in relation to one another.
  13. Sep 2, 2010 #12
    Aside from that just being the observable universe...which one of those particles should you use as a reference point? Which one's special, and how?

    No, there is no motion through space, only motion with respect to other objects.

    It has a different speed with respect to each of those, and always a speed of zero with respect to itself. There is no absolute, "real" speed, velocity, or location, all must be measured relative to some arbitrarily set reference.
  14. Sep 2, 2010 #13


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    RenosantsMan: one cannot claim that an object is moving at all without an external reference by which to measure it. As you just did above, when referencing Alpha Centauri.

    If there is no reference, then it is indeed meaningless to talk about movement. General Relativity shows us that that there is no measurement whatsoever that you can do - even in principle - in an inertial frame of reference to indicate that you are "moving" as opposed to "stationary". The only way it is meaningful at all to talk about movement is with respect to another object.

    I defy you to post a sentence that meaningfully describes the speed at which an object is moving without reference to an external object.
  15. Sep 2, 2010 #14
    1. I was responding to another person's point when he said something about an almost INFINITE number. Not even close, as I showed.

    The context was not "a reference point." But since you and others seem so intent on reference points, choose your own. Voyager is moving with respect to any one you choose.

    2. Clever wordplay is no substitute for objectivity. What YOU call "motion with respect to other objects" others can quite accurately call "motion through space."

    Let us consider another mind experiment. You and a friend are playing catch in a living room.
    You toss a ball to him. He catches it. He throws it back to you. You catch it.

    Oops! Sorry. The ball is not moving, according to your dictum: "There is no motion through space."

    We expand the size of the living room and all the contents by 10^20th power.

    Or conversely, if you wish, reduce the size of the universe to 1/10^20th of its actual size.
    Put the Voyager satellite inside this living room, and look! There it goes. It's moving.
    (The better to see Voyager, we expanded it to the size of a ping pong ball in our mind experiment.)

    No kidding. How many times by how many different people will this be stated?
    Since our observation post is here, earth, let's use THAT, shall we?
    Any objections to that, take them off the earth.
  16. Sep 2, 2010 #15
    DAVE, there is ALWAYS an external reference. Please point out the situation where all stars and galaxies mysteriously disappear, and explain why that would happen.

    See, you're more interested in DEFYING me (your words) than in considering the point I made.

    We shoot a satellite into space. You can see the missile leaving the launch pad.

    It continues on, into what we so naively call "OUTER SPACE."

    It KEEPS MOVING, doesn't it. Keeps moving, conserving its momentum and vector.

    You cannot summarily remove all the contents of the universe and say it is NO LONGER MOVING THROUGH SPACE. That strikes me as consummately silly.

    Remove ANY GROUP OF BODIES YOU WISH. There are others, however distant, that the object is still moving RELATIVE TO!

    No more defiance now. I mean it.

    Anybody got a peanut? - The Giant in The Princess Bride
  17. Sep 2, 2010 #16


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    Please. Now you're just being argumentatitive. That's right, 10^80th is not near infinite. It is as infinite as it is possible to be in the real world (as opposed to abstract math).

    You score a point. :rolleyes:

    Yep, glad you could join us. You agree that movement is wrt a reference point.

    Please demonstrate that it is moving through space. Do not refer to any objects external to the ball (or this space you speak of) such as you, your friend, the room or the air.

    Precisely. And movement will be measured wrt to those distant objects.

    Note that you do not have to actually remove them from the universe to remove them from your frame of reference by which you measure movement. All you have to do is limit your observation of objects to a smaller sphere. If you limit your observation of reference objects to a 10 foot sphere, and there is nothing in that sphere except you and the ball you are throwing, then they only thing you can use to show that the ball is moving is yourself as a reference point. But it still requires a reference point.

    Note though that there is nothing you can do in this ten foot sphere of observation to prove that you are "moving through space". You could be stationary for all you know. That is the very nature of relativity.
    Last edited: Sep 2, 2010
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