October 2, 2011
Statistically speaking, October is the clearest month of the year in Central Ohio, making it a great time to dust off that old telescope in your garage and get in a little stargazing. An added bonus is the presence of the two most spectacular telescopic planets, Jupiter and Saturn.
Jupiter is the bright “star” in the east after about 10 p.m. As the brightest object in the nighttime sky after the moon and Venus, Jupiter is a hard object to miss as it slips slowly among the stars of Pisces and Aries in that direction. Binoculars show its four brightest moons in a line around the planet. A small telescope reveals its brown, zebra-stripe cloud bands.
Wait until the end of October, and Saturn will peek over the eastern horizon just before dawn. Because of its extreme distance from us, Saturn is considerably fainter than Jupiter, and in fact is slightly outshone by the bright star Spica to its right. Sorry, binoculars won’t help you here. Saturn is an enormous planet with rings spanning more than a quarter of a million miles. But at a billion miles away, Saturn requires at least a small telescope to reveal its rings.
Over the course of the night, Jupiter moves from horizon to horizon, rising in the east, moving higher and farther south as it gets later, and setting toward the west as it gets later still. Saturn is doing the same thing over the course of the daylight hours. Those motions are, of course, an illusion. As Earth turns on its axis from west to east, the stars, sun, moon, and planets seem to move from east to west.
Observe the planets over the course of days or weeks, and you’ll notice another motion. From night to night, Jupiter will move a bit farther west against the background of stars. Eventually, Jupiter will move so far west that we’ll see it only briefly as it sets just after the sun. Over the same period, the prognosis is better for Saturn. As it moves west, it will rise a little earlier each day until it is finally visible in the evening hours.
That second motion, the one against the starry background, gave stargazers fits for thousands of years. As it turns out, the solution was simple, even if it violated common sense. The planets must be in orbit around the sun. They move slowly from east to west because that’s the direction of their orbits.
For a while, no one could figure out why they stay in orbit. They are careening around the sun, and there’s no obvious reason for them not to spin off into space. If you don’t believe me, try spinning a ball on a string around your hand. Let go, and the ball will zip away from your hand, knocking over a lamp and making you curse the day you ever read this column.
Some invisible cosmic string must be holding each planet in its place, and it’s hard to understand what the string is. It works mysteriously at a distance with no visible means of support.
Scientists came to call that force “gravity,” whatever the heck that was. The planets don’t zoom into the inky depths of space because the sun’s gravity is inexorably pulling it inward. They don’t drop into the sun because the velocity of their motion exactly counteracts the force of gravity.
Scientists still aren’t quite sure what gravity is. It has been called a universal force, a fundamental property of matter, and a bending of spacetime. Whatever. Everything is attracting everything else. Thanks to Isaac Newton and Albert Einstein, that attractiveness can be calculated with such precision that scientists are able to do remarkable things like keeping spacecraft in orbit.
The International Space Station is in orbit around Earth, but it has not escaped Earth’s gravity. It is still falling toward Earth. Its speed (at a whopping 17,500 miles per hour) exactly counteracts the gravity that wants to turn it into a spectacular, flaming fireball. When the astronauts want to return to terra firma, they simply slow themselves down a little. Gravity takes care of the rest.
The astronauts also have not escaped Earth’s gravity. They are instead in a constant state of “free fall,” which is akin to riding down the big hill on a roller coaster during every moment of their trip aboard the ISS.
Many astronauts celebrate their first trip into orbit by losing their lunch. Yes, the ISS is equipped with airsickness bags — or, in this case, spacesickness bags — and plenty of them. They can thank gravity for that.
Tom Burns is the director of Ohio Wesleyan University¹s Perkins Observatory. He can be reached at firstname.lastname@example.org.