Today the biggest planet of them all is at opposition to the sun and closest and brightest for the year. You’ve no doubt noticed Jupiter rising in the northeastern sky during late evening twilight – its penetrating pale yellow light catches the eye before any other star in the sky. Outside of Venus, which will depart the evening sky in just one week, Jupiter is the brightest planet in the heavens.
Once a year, Jupiter and Earth draw closest to one another as they line up on the same side of the sun. 391 million miles (629 million km) separate the two worlds, which may sound like a lot, but that’s a good deal closer than when they’re on opposite sides of the sun. Try 584 million miles (940 million km) this July 24 when the planet is in solar conjunction.
Being close also means being big and bright. Jupiter appears 47 arc seconds across (60 seconds = 1 arc minute = 1/30 the moon’s diameter) and beams at magnitude -2.7 or three times brighter than Sirius, the brightest star.
Even binoculars will make it look “fatter” than a star, plus you can see two or three and sometimes up to four bright moons. Known as the Galilean moons because they were first seen by Galileo, the satellites shuffle back and forth around the planet, changing positions from night to night. The closest-in moon Io takes only 1.8 days to circle Jupiter; the furthest of the four, Callisto, completes a revolution in 16.7 days.
To find out what Jupiter’s satellites are doing any time of day or night, check out Sky and Telescope’s Jupiter Moons utility. It also lists times when the moons pass in front of or are eclipsed by the planet.
Largest of the eight planets, Jupiter is completely covered by clouds of ammonia ice and has lots of weather just like Earth, making it one of the most rewarding objects to follow in a telescope.
Low magnification (40- 50x) will easily show two gray “tire tracks” across the planet’s disk. These are the North and South Equatorial Belts which flank Jupiter’s equator. Higher power, steady air and visual concentration will reveal thinner stripes like the North and South Temperate Belts.
Dark belts are separated by lighter zones and the whole works is streamed into stripes by narrow, high-speed winds called jets that border the zones and belts. Winds rip along at up to 400 mph (640 km/hr). Because Jupiter makes a complete spin on its axis at the amazing rate of just 9.9 hours, you can watch new features rotate into view by revisiting the planet in your telescope several times during the night.
With Jupiter well placed for observing two hours after sunset until two hours before sunrise, it’s possible to observe the planet through more than one full rotation over the long winter night for northern hemisphere skywatchers.
Jupiter’s weather is as changeable as Earth’s. Belts narrow, widen, split in two or even disappear altogether for a couple years before reforming. The familiar Great Red Spot (GRS), a hurricane-like storm more than twice Earth’s diameter that’s raged for centuries, changes color from pale tan to brick red. This year it’s redder than in the previous few years and somewhat easier to see. I say somewhat because the GRS has been shrinking over the years.
Drawings from the 1800s show it as a big red hot dog easily twice the size it is now. Small telescope users should use magnifications of at least 100x and plan their hunt when the air is least turbulent (steady seeing). The best time to view the GRS is when it’s lined up on the planet’s meridian and squarely faces Earth. To find out those times for your location, click HERE.
Despite the inevitable cold weather winter brings, January is the best time to go outside and enjoy a peek at Jupiter. For northern hemisphere observers, the planet is now at its highest point in the sky, far above haze and much of the turbulent air present at lower altitudes. If you’re using a telescope and taking it outdoors from inside your warm house, remember to let it cool down for at least a half-hour before you observe the planet otherwise heat from the tube and optics will distort and soften the image.
Have at it by Jove!