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 tlburns@owu.edu.

Posted by Tom Burns on Sep 30 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.