Jupiter, Mars and Earth

Jupiter is back to our evening sky — at last. Of course, it never really went anywhere. Diehard stargazers have been viewing it in the early morning sky for months.

Every day, Jupiter rises a few minutes earlier, and it has finally worked its way to the evening sky. Look for it low in the east around 9 p.m. The brilliantly yellow point of light is the brightest “star” in that direction.

Jupiter’s brilliance is something of a puzzle. The planet is 370 million miles away right now. Mars, visible as a dim, red point in the east just 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 — 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 480 million miles from the sun, a hefty chunk of the planet’s light gets beamed back in our direction.

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 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 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. More than 1,000 Earths could fit inside Jupiter. 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.

Come and see Jupiter at Perkins! We do our public stargazing programs every Friday night. We’re already sold out on Oct. 14, but tickets are available for most upcoming programs. Call 740–363-1257 for ticket information.

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