Hello Jupiter! We’re Thrilled You’re Back

Jupiter shines brilliantly in the constellation Leo the Lion this month. Look for it in the eastern sky at nightfall. There's no brighter object in the evening sky right now. Credit: Bob King
Jupiter shines brilliantly in the constellation Leo the Lion on March 6th. Look for the planet in the eastern sky at nightfall. At magnitude -2.5, Jupiter is more than twice as bright as Sirius, the brightest star. Credit: Bob King

You’ve noticed it by now. That big, bright “star” in the eastern sky at nightfall?  By Jove, it must be Jupiter!  The Roman supreme god of the skies, Jupiter’s at opposition this week and closer and brighter than at any other time this year. You’ll find the planet a fist below the triangle-shaped hindquarters of Leo the Lion and just shy of two fists to the lower left of Leo’s brightest star, Regulus.

If you’re lucky enough to live near a lake, look for Jupiter’s gleaming path across the water. That luminous strip represents sunlight that streamed across 505 million miles (813 million kilometers) of outer space to reach the planet, taking all of 45 minutes to get there. After bouncing off Jupiter’s ammonia-ice clouds, the light turned around and traveled another 412 million miles (663 million kilometers) back toward Earth, arriving 37 minutes later in your eye. 917 million miles of light travel and 82 minutes of time are bound up in Jupiter’s beaming presence every time we behold it.

Earth gets between Jupiter and the sun this week with the planet on one side of our orbit and the sun on the other. We see them opposite each other in the sky and say Jupiter's "at opposition" to the sun. Diagram: Bob King, source: Stellarium
Earth gets between Jupiter and the sun this week with the planet on one side of our orbit and the sun on the other. We see them opposite each other in the sky and say the planet is “at opposition”. Diagram: Bob King, source: Stellarium

Earth, being closer to the sun, travels faster than Jupiter. Once a year, it swings between the two bodies. When all three briefly line up in a neat row, Earth and Jupiter are closest. Jupiter’s “at opposition” at this time because it’s opposite the sun in the sky, rising around sunset and remaining visible all night long. It shares this quality with the full moon, which also opposite the sun, rises at sunset and stays up all night.

A closer Jupiter means a bigger and brighter Jupiter. Amateur astronomers like big and bright when it comes to planets, the easier to see tiny details on their tiny disks. Jupiter’s the largest planet in the solar system. I could tell you that it would take 11 Earths placed side-by-side to equal its diameter, but I like this analogy better: scrape out the planet’s innards like a Halloween pumpkin and you could fit more than a thousand Earths inside. Even though Jupiter’s nearly half a billion miles away, it shows the largest disk of any planet (except Venus on occasion). That means you can see lots of interesting stuff even with a small telescope.

Jupiter with his stripes (two most prominent belts are the North and South Equatorial Belts) and the four brightest moons. Credit: John Chumack C8 SCT + QHY5iiL CCD camera filter 600 frames
In this photo taken on March 8, 2016, we see Jupiter’s two most prominent stripes (North and South Equatorial Belts) and the four brightest moons. The white regions between the belts are called “zones”.  Credit: John Chumack

The four brightest moons are the first thing you’ll notice, although don’t expect to see all of them at once. Sometimes one or two lie directly in front or behind the planet or hide in its shadow in eclipse.  Each orbits Jupiter with its own period, providing endless changing arrangements night to night. The North and South Equatorial Belts stripe Jupiter’s flattened globe. They’re clouds of ammonia ice crystals with a smattering sulfur compounds that provide a touch of red.  Look closely at Jupiter’s disk and you’ll notice it’s not spherical but slightly oval. Jupiter spins rapidly, completing one rotation in just 9.9 hours, and it’s made of gas, which flexes more than rock. Spin a giant gas ball fast enough and it will flatten out a bit. Rock holds together more tightly.

Jupiter's Great Red Spot stares back from the planet on March 5, 2016. Credit: Christoper Go
Jupiter’s Great Red Spot stares back from the planet on March 5, 2016. The Spot’s diameter is a little more than twice that of the Earth. Credit: Christoper Go

You can always find out which moon is which by visiting Sky & Telescope’s Jupiter Moons site. Feeling adventurous? Increase your telescope’s magnification and try to spot the Great Red Spot, the planet’s most iconic feature. This enormous, unblinking eye of a storm has been around since at least the 1600s. The best time to view the Spot is when it’s directly facing Earth, or as astronomers like to say, when it transits Jupiter’s central meridian. Those times are available HERE. Some years, the Spot’s pale pink, salmon or even beige, but this year, it’s distinctly red, making it easier to see.

The moon Io (right of center) casts a small, black shadow on Jupiter's Equatorial Zone. Credit: Damian Peach
The moon Io (right of center) casts a black shadow as it transits over Jupiter’s Equatorial Zone. Credit: Damian Peach

Besides watching the nightly moon-dance and looking for subtle changes in Jupiter’s cloud features, my favorite thing to observe are satellite shadow transits, when one or more of the moons pass in front of the planet and cast pinhead-sized, black shadows on the clouds below. This month we get treated to several double shadow transits, when two or moons transit Jupiter at the same time. Here are Central Daylight times to watch for shadow transits in March. Add one hour for EDT; subtract an hour for MDT and two hours for PDT:

* March 14: A double transit! Europa’s shadow crosses the planet between 8:46-11:34 p.m. Io’s shadow transits between 9:22-11:37 p.m. Both are visible simultaneously between 9:22-11:34 p.m.
* March 21-22: Another double transit! Io’s shadow passes over Jupiter from 11:15 p.m.-1:31 a.m. Europa’s shadow transits from 11:23 p.m. – 2:11 a.m. Both are visible simultaneously from 11:23 p.m.-1:31 a.m.
* March 23: Double transit #3! Ganymede’s shadow transits between 6:47-10:05 p.m. and Io’s shadow transits from 5:44-7:59 p.m. Both are visible simultaneously fro 6:47-7:59 p.m. Ganymede is Jupiter’s largest moon and will look obviously larger than Io.
* March 30-31: Io’s shadow transits from 7:38-9:53 p.m. Then from 10:47 p.m.-2:03 a.m., Ganymede ‘s shadow transits Jupiter.