Gravity and the planets
Statistically speaking, October is the clearest month of the year in central Ohio, which makes it a great time to dust off that old telescope in your attic 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” high in the east just before morning twilight. Binoculars show its four brightest moons in a line around the planet. A small telescope reveals its brown, zebra-stripe cloud bands.
At the same time, look lower in the east and wait for spectacularly bright Venus to rise. Three separate motions are at work here. I hope you will take the opportunity to watch them happen.
The first motion: Over the course of the night, the stars and planets move together 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. That motion is, 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.
The second motion: Over the course of a year, the stars and planets rise a few minutes earlier each day. As you note that yearly motion, you are watching Earth travel around the sun. By winter, Jupiter, which is now a morning object, will have moved to the evening sky, and we’ll be able to observe it at Perkins.
The third motion: But notice also the motion of the planets in the opposite direction, from west to east. Venus, for example, is in late September and early October above the bright star Regulus in the constellation Leo, the Lion. During the early morning of Oct. 3, it will pass a hair’s breadth from Regulus. (Trust me. It’s worth getting up to see it.) After that, Venus will be below Regulus as it continues its death dive toward the horizon.
That third motion gave stargazers fits for thousands of years. Common sense told them that the stars and planets traveled around Earth. However, the planets appeared to have their own separate motions as they moved against the starry background.
As it turns out, the solution was simple, even if it violated common sense. The planets are in orbit around the sun and not the Earth. They move slowly from west to east because that’s the direction of their orbits.
The big problem is why they stay in orbit. They are careening around the sun, and there’s no good 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 these words.
Some cosmic string must be holding each planet in its place, but it’s hard to see 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.
Gravity turns out to be a universal force. Everything is attracting everything else. Thus, when the International Space Station goes into orbit around Earth, it is not “escaping 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 have not escaped Earth’s gravity. They are instead in a constant state of “free fall” toward Earth, which is akin to riding down the big hill on a roller coaster all the time. Many astronauts celebrate their first trip into orbit by losing their lunch. Yes, the ISS has barf bags, and plenty of them. You can thank gravity for that.
Tom Burns is the director of Perkins Observatory. He can be reached at email@example.com.