Winter stargazing at its best: Jupiter 2012

Look for Jupiter high in the south just after dark. The bril­liantly yel­low point of light is the bright­est “star” in that direction.

Jupiter’s bril­liance is some­thing of a puz­zle. The planet is 450 mil­lion miles away right now. Mars, the dim orange point in the east before morn­ing twi­light, is half Jupiter’s dis­tance, yet Jupiter shines 40 times brighter. What gives?

Part of it is sheer size. Mars is a scant 4,200 miles wide. Jupiter has a diam­e­ter of almost 89,000 miles. Jupiter may be far­ther away, but it’s also a much big­ger mir­ror to reflect back the light from the sun.

Another fac­tor is how shiny the mir­ror is to start with. Astronomers call a planet’s rel­a­tive abil­ity to reflect light its albedo. Our planet gives a good account­ing of itself in the albedo depart­ment. Its giant oceans and clouds of water vapor con­tribute to its rather high albedo of .39, i.e., 39 per­cent reflectivity.

Mars is a ball of rock cov­ered over mostly with red­dish sand. Nary a drop of liq­uid water or vapor clouds adds to its reflec­tiv­ity. Thus, its albedo is a pathetic .16.

By con­trast, Jupiter is extra­or­di­nar­ily reflec­tive because it is cov­ered over with clouds. Sus­pended crys­tals of methane and ammo­nia reflect the sun’s light very effi­ciently, giv­ing Jupiter a whop­ping albedo of .51. Just over half of the light it receives from the sun bounces back into space. Even though Jupiter is nearly half a bil­lion miles from the sun, a hefty chunk of the planet’s light gets beamed back in our direction.

Plan­ets, as it turns out, come in many forms. Jupiter is so dif­fer­ent from Earth that it’s hard to think of them both as plan­ets. Earth is, of course, mostly a ball of rock and met­als. Jupiter has no solid sur­face at all.

Jupiter is mostly com­posed of hydro­gen, the sim­plest ele­ment in the uni­verse and the main con­stituent of the stars, our own star, the sun, included. Under its thin cloud layer, the hydro­gen is a densely packed liq­uid. Jupiter is less like a “gas giant,” as it is com­monly called, and more like a giant droplet of very cold hydro­gen spin­ning in space.

Another of Jupiter’s odd­i­ties is its rate of spin. Our planet Earth rotates once on its axis every day. Our days are pre­cisely one day long because of that spin. That’s pretty fast. As a result of Earth’s spin, you’re mov­ing around Earth at 680 miles per hour as you read these lines.

Jupiter is much larger than Earth. Over 1,400 Earths could fit inside Jupiter if you packed them down really well. You’d think that Jupiter would be spin­ning pretty slowly because of its bulk, but Jupiter rotates once every 10 hours. A Jupiter “day” is less than half the length of Earth’s. A sta­tion­ary object at Jupiter’s equa­tor trav­els at bet­ter than 27,000 miles per hour.

If you have a six-inch tele­scope or larger, ver­ify that fact for your­self. Jupiter is spin­ning so fast that its brown­ish clouds are stretched all the way around the planet like cos­mic head­bands. If you spot one of its oval storms (like the famous Great Red Spot), watch as it moves across the entire vis­i­ble face of Jupiter in a few hours.

Binoc­u­lars won’t show much detail on Jupiter’s sur­face. Still, check out the four tiny points of light in a line near the planet. You’re see­ing Jupiter’s four bright­est moons, and it didn’t take a tele­scope as big as a corn silo to spot them. They are revolv­ing so quickly around the planet that with a lit­tle patience, you can actu­ally watch them move.

Other Plan­ets

That incred­i­bly bright point of light you’re see­ing low in the south­west dur­ing evening twi­light is not a UFO, as some callers to Perkins have claimed. It’s Venus.

Mars is high in the south just before morn­ing twi­light. Look for it under the back end of the con­stel­la­tion Leo. Its color ranges from yel­low to orange depend­ing on your color sen­si­tiv­ity. The “red planet” is never really red.

Early ris­ers, please take note: Get up at 5 a.m. or so and see the pret­ti­est view of the night: In the south­east, Sat­urn is sit­ting next to the bright star Spica. The planet is to the left. Spica looks white and Sat­urn is vaguely yel­low­ish. The star and planet are almost iden­ti­cally bright. By late spring, they will both have migrated to the evening sky. For now, you’ll have to get up before morn­ing twi­light to see them.

Tom Burns is direc­tor of Ohio Wes­leyan University’s Perkins Obser­va­tory in Delaware. He can be reached at tlburns@owu.edu.

Posted by Tom Burns on Jan 23 2012. You can follow any responses to this entry through the RSS Feed. Comments can be made below.