This week’s Cosmic Quiz winner is Greg McMurray, who asked:
How fast is the Earth moving relative to everything else?
Let’s break it down.
The first motion of Earth that we’re all familiar with is rotation – the movement of the Earth that gives us day and night. Here in Salt Lake City at about 40 degrees north latitude, Earth’s daily rotation about its axis carries us along at about 760 miles per hour.
Then there’s the next familiar motion of our planet, Earth’s orbit around the Sun each year. Our distance to the Sun is roughly 93,000,000 miles, which makes the circumference of the circle (yes, I know our orbit is slightly elliptical but for the purposes of this exercise let’s keep it simple) divided by 365.25 days work out to an average speed of 67,000 mph. That’s like covering the distance between Los Angeles to New York City in 3 minutes.
Next, we have the motion of our solar system, which includes us, through the Milky Way galaxy. We’re about 27,000 light years from the center of the galaxy, and complete one “orbit” of the galactic center in about 220 million years. That works out to a speed of about 500,000 miles per hour. How does L.A. to New York in 20 seconds sound?
Our Milky Way galaxy is itself moving relative to other galaxies. We’re actually moving toward the Andromeda Galaxy, our nearest neighboring spiral galaxy, at 200,000 mph. In a few billion years Andromeda and the Milky Way will merge, and that will be spectacular, but that’s a subject for blogging about at another time.
Finally, our “Local Group” of galaxies (made up of our Milky Way galaxy, the Andromeda galaxy and a few nearby smaller galaxies) is moving towards the “Great Attractor” supercluster of galaxies hundreds of millions of light years from us at better than a million miles per hour.
But there’s one more bit of brain-bending universe-in-motion information to consider. None of the speeds described above can hold a candle to the speed at which space is itself expanding as a result of the Big Bang which began the universe 13.7 billion years ago.
Imagine you’re watching a NASCAR race. Cars are roaring around the track at 200 mph, jostling for position, some cars gaining on the others and some cars pulling away from the others.
Now imagine that the racetrack itself is expanding in all directions 100 times faster than the fastest car on the track.
Yes, it still matters that car #1 is moving towards or away from nearby car #2, but those motions pale compared to how fast the track itself is getting bigger, and overall the distances between race cars is increasing as the track itself expands.
The most distant galaxies visible from Earth are more than 10 billion light years from us. We can measure their speed away from us by studying the red-shifting of their light. It turns out that these galaxies are receding from us at something like one-fourth the speed of light – not because that’s how fast they’re moving through space, but because space itself is expanding and carrying these galaxies with it away from us.
So when your toddler is tearing around the house, seemingly incapable of holding still even for a moment, you can take comfort in the knowledge that even if that adorable child should briefly and blessedly take a break, you’re still screaming through the universe at astonishing speeds, and the concept of “holding still for a minute!” really is just a parental fantasy.