Sky Lights: Confused About Your Direction?

If you lack a sense of personal trajectory, astronomers can help.

By Bob Berman|Friday, September 01, 2006

Throughout most of history, humanity hasn't had a clue about where it's going. The idea that we're all being whisked through space aboard a spinning Earth was alien even to brilliant thinkers like Aristotle and Pythagoras. The first to argue that our planet is in motion was Heracleides, who in 350 B.C. maintained that Earth turns on an axis. A century later, Aristarchus of Samos earned a lot of laughs for saying that we go around the sun, not the other way round.

Not until the Renaissance did educated people accept that Earth rotates daily, spinning us all steadily eastward. If you've got a calculator handy, you can quickly determine your speed: Multiply the cosine of your latitude by 1,038. For instance, if you live in Denver, latitude 40, punch in "40," hit the cosine key, and multiply. Answer: 795 miles per hour. You are breaking the sound barrier just by sitting on your couch.

That's only the beginning of your astronomical movements. As Earth orbits the sun, it plows through space at 18.5 miles per second, or about 67,000 mph. At sunrise, the direction of Earth's travel is pretty close to straight overhead. The velocity is not constant, though, because Earth follows an elliptical path and accelerates as it moves closer to the sun. Counterintuitively, the top speed occurs in early January, the minimum in July.

Nearly a century ago, American astronomer Harlow Shapley enlarged the perspective on our motion. He discovered that we're not sitting at the center of our galaxy, the Milky Way. Instead, we're about two-thirds of the way to the edge, making a huge loop around the galactic core every 250 million years or so. Astronomers before Shapley had already determined that the sun and its planets are moving toward the constellation Hercules at about 12 miles per second. Today we know that this is just a sideways drift superimposed on our main forward speed around the Milky Way—a brisk 144 miles per second, about one-thousandth the speed of light. That motion carries us toward the bright star Deneb, which is high overhead this month. We will be where Deneb is now, 1,600 light-years away, in a million and a half years.

Still, we're not finished. Our entire galaxy rushes toward the Andromeda galaxy at 80 miles per second. Zoom out farther and our local group of galaxies is yanked toward "the Great Attractor," a mysterious concentration of mass beyond the constellation Virgo. Even big telescopes can't really spot the source of the pull.

On the largest scale, everything is flying apart, a cosmic divorce dictated by the expansion of the universe in all directions. Measuring the exact value of that expansion yields hints about whether the universe will keep growing forever or will someday halt and then contract. Current studies say the expansion rate is around 14 miles per second for each million light-years of distance. Alien astronomers in a galaxy 100 million light-years away would see us whizzing in the opposite direction at 1,400 miles per second.

Or are they moving away from us? The usual interpretation is that the space between us is increasing, so everybody is moving and yet nobody is actually moving. That's another way of saying that there is no center to the Big Bang. It happened everywhere and nowhere. Perhaps all we can say for sure is that we've come a long way, yet we're still going nowhere—fast.

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