Sky Lights

By Bob Berman|Friday, September 01, 2000

While sprawled in a lawn chair, munching a Labor Day hot dog under the late-summer sun, you may feel perfectly at rest. It's an illusion, of course. Earth is spinning, orbiting, and swooping at hundreds of miles per second, carrying you on a complicated pathway through deep space. The motions range from the familiar progression around the sun, which touches off the start of autumn on September 22, to arcane movements that betray the hidden construction of the universe.

The primary step in this celestial dance is your daily circle on the spinning Earth, and your speed has everything to do with latitude— zero at the poles and a maximum of 1,040 miles per hour, or 0.3 miles per second, at the equator. If you're not afraid of the math, you can find your exact speed by multiplying the cosine of your latitude by 1,040. This works out to nearly 800 miles per hour in New York City or Indianapolis (latitude 40 degrees north) but just 450 mph in Fairbanks (65 degrees north).

The next complicating factor is Earth's yearly revolution around the sun, at an average speed of 18.5 miles per second. To get a feel for this movement, face the midday sun and picture yourself orbiting to the right. When watching a sunrise, our planet's rotation carries you forward— that is, toward the sun— but the much faster orbital motion is hurling you upward, around the sun and toward a spot high in the south.

The sun itself takes a 250-million-year carousel ride around the nucleus of our galaxy, the Milky Way, dragging Earth along for the ride. Visualizing this swift motion, some 140 miles per second, is particularly easy this month. Along with most of the stars of the night sky, we rush straight up at nightfall, toward the cross-shaped constellation Cygnus and its brightest star, Deneb, nearly overhead in early evening. The galaxy's densely packed nucleus hovers lowish in the south, marked by a brightening in the misty glow of the Milky Way.

The Milky Way, part of a small gathering of galaxies called the Local Group, is moving too. We're falling toward the center of the group at about 25 miles per second. And in 1969, scientists noticed a strange distortion in the ubiquitous cosmic microwave background, radiation thought to be left over from the Big Bang. That distortion implies that the whole galactic neighborhood is moving about 370 miles per second toward the constellation Hydra. This unexpected and still unexplained motion might result from the pull of vast superclusters or walls of galaxies— the biggest known structures in the universe.

Our celestial velocity could also reveal what holds the superclusters together. Cosmologists believe that more than 90 percent of the universe consists of dark matter— invisible subatomic particles that make themselves known only by their gravity. These particles, if they exist, do not generally associate with ordinary atoms, but they occasionally collide anyway, producing some secondary effects.

So as we circle the Milky Way's center, we should plow through swarms of dark matter. In June, our planet's orbit around the sun carries us in the same direction as the galactic rotation; in January, we move against the flow. We should therefore slam through more of the evanescent particles in summer than in winter. This past February, a Chinese-Italian research collaboration claimed to see just such an effect (see last month's Works in Progress column). Interestingly, September brings not only the time of the equinox, when day and night are equal in duration. It also gives us the time of another equality, when Earth's motion about the sun neither adds to, nor subtracts from, our orbital velocity around the galaxy. Something to ponder as you savor the dwindling lazy days of summer.

For more information on the motion of Earth and the Local Group, see answerbook/motion.html.

Find answers to your most pressing astronomy questions and browse an archive of answers to questions posed by other curious astrophiles: docs/ask_astro/ask_an_astronomer.html.

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