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.
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 firstname.lastname@example.org.