Earth
Brace yourself, because this question is a lot more complex than it sounds.  We’re asking what the Earth’s speed is, but speed is always measured in relation to something and we usually use the Earth itself as the reference point.  When we say a car is traveling at 120 km/h, we mean that it travels 120 km across the Earth in one hour and ignore the fact that the Earth itself is moving.  In ordinary everyday situations, we simply assume that the Earth is our reference point for almost everything.  We live on it, we move around on it, we anchor our houses in it, it is the centre of our world.  But one of the most important things astronomers discovered hundreds of years ago was that Earth is not at the centre of the Solar System, but instead moves in an almost circular path around the Sun.  
So we can rephrase the question as: “How fast does the Earth move around the Sun?”  Well, we can figure this out with simple arithmetic.  We know how far the Earth is from the Sun, and we know that it takes one year to make a complete circuit.  We can calculate the distance traveled by the Earth in a year (If we draw a circle representing the Earth’s orbit, then the radius of that circle is the distance from Earth to Sun.  If you’ve been to school, then you know that the circumference of a circle is 2πR).  Speed equals distance divided by time, so by dividing the calculated circumference (in kilometers) by the number of hours in a year, we get a speed around the Sun of about 107,000 km/h.
More recently, astronomers learned another interesting fact.  Despite the fact that it really looks as if we’re in the middle of the galaxy, the Solar System is actually nearer to the edge, and orbiting around the galactic core.  By measuring the speed that other galaxies are moving towards or away from us, we can get a reasonable idea of our own orbital speed, which is about 230 km/s (or 828,000 km/h.  Fast!).  But we can’t just add this to Earth’s speed around the Sun, because we’re moving in circles – all we can say is that Earth’s speed around the galaxy is somewhere between 721,000 km/h and 935,000 km/h depending on the day of the year.
But we’re not done yet, because the Milky Way galaxy is not the centre of the Universe!  Instead, it is just one member of a cluster of galaxies, known as the Local Group.  Relative to the centre of this group, the Milky Way travels at about 40 km/s (144,000 km/h).  
The Local Group cluster is part of a larger structure made of all the neighbouring clusters:  The Local Supercluster.  Relative to this, the local Group moves at about 600km/s (2.16 million km/h!).
So what is the Local Supercluster’s speed then?  Well… that’s a problem.  We haven’t yet seen what larger structure it might be part of, which we could use as a reference point.  We can of course pick any old frame of reference, but it’ll be totally arbitrary.  That speed could be anything from zero to almost infinity and it would be right for some frame of reference.  The idea of an absolute speed, not relative to anything, is actually a meaningless concept – there’s no such thing.  So… sorry!  I can’t answer the question as phrased!
 

 
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About Allen Versfeld

Allen is an amateur astronomer, an IT professional, a podcaster, a father of five beautiful kids and a barely competent chess player. He is also the director of the Astrophotography Section of the Astronomical Society of South Africa, where he coordinates and promotes the activities of people who are far better photographers than him.


Comments

How fast is the Earth travelling through space? — 12 Comments

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  2. Brian Cox said the earth was rotating at 750mphand travelling at the speed of light,186,000 mps.Was he right?.

    • Are you sure that’s what he actually said? First up, you can’t really measure the rotational speed of the Earth in mph because it depends on which part of the Earth you’re measuring. At the equator, it’s about 1000 mph, but it gets slower the further you get from the equator until at the poles it’s 0 mph – stand on the north pole and you’re turning at one revolution per 24 hours, but your straight line speed is zero. Stick some lego people on different parts of a turntable and spin it to see how this works (I mean, sure, the Earth is a ball not a disc, but the principle works exactly the same)

      As for moving at the speed of light… yeah, I suppose you can make a physics based argument for that, involving special relativity and the fact that all speeds are measured relative to something else, but it’s not a very useful thing to say in practical terms. If we look at our solar system, or even our own galaxy, we’re definitely not traveling anything near the speed of light, compared to that. If we were, then our view of those parts of space that can be seen with our eyes and backyard telescopes would be VERY different to what we actually see.

  3. Thank you for this informative blog piece. Tis very interesting, I was wondering, is there an available calculator online that you know of that calculates the speed we are hurtling at from the town / city / area where we are located on the Earth at?

    • None that I know of
      But if all you want is to know how fast you’re spinning around the Earth’s axis, then it’s easy to work out with any calculator that has scientific functions (like the Calculator App in Windows and other operating systems). Start with the speed at the equator: 1000 mph (or 1600 km/h, if you’re metric like me). Then look up your latitude, whether from a GPS receiver, google maps, or some other resource. My latitude, for example, is about -25°. Find the cosine of your latitude by typing it into the calculator and hitting the “COS” button (in my case: about 0.91). Multiply that by the equator speed (1000 mph) and there’s your answer! Sitting at home as I type this, I’m whizzing around the centre of the Earth at about 910mph!

  4. Hi, surely when asking how fast the earth is moving through space, the most relevant reference point to use is the point where the big bang started.
    I believe the latest theory is the expansion of the universe is accelerating. Is there a estimated speed of expansion at this point in time?

    • That’s good thinking, and makes a lot of sense, but unfortunately it doesn’t work. The problem is that there is no single point in space where the Big Bang happened – the entire universe, along with space and time themselves, were formed in the Big Bang. This means that every point in space was all squeezed up together, and it means that the Big Bang happened everywhere.
      As for the acceleration of the universe’s expansion, that’s more than a theory – it’s an observed fact that physicists are still struggling to explain. It makes no sense, yet as telescopes get bigger and more powerful, it becomes harder and harder to ignore. There’s obviously some sort of force pushing the universe apart, which we call “Dark Energy”, and physicists have a number of ideas as to what it might actually be, but so far nobody’s managed to come up with a convincing proof for any of them. It will probably be a while before anybody’s confident enough in those ideas to start writing them into textbooks!
      As for how fast it’s accelerating… that’s a tough one. I have not been able to find any precise figures (although I’m hoping some actual physicists who’re working in this area will comment and fill us in), but I can say that acceleration only began about five billion years ago. Before then, the expansion was slowing down (as you’d expect, thanks to gravity). Presumably, the force exerted by dark energy has been increasing over time until it was strong enough to overcome gravity and cause expansion to speed up.

  5. Pingback: Where is the centre of the Universe? - Urban Astronomer

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