Look to the east-southeast sky soon after midnight early Thursday (Jan. 19) for an interesting celestial array involving the moon, a bright planet and a bright star. The planet in question is Jupiter, usually the second brightest planet next to Venus.
Shining at a dazzling magnitude of -2.1, Jupiter is currently twice as bright as Sirius, the brightest star— in the night sky. Whenever it's above the horizon, Jupiter — the solar system's biggest planet at 88,800 miles (143,000 km) in diameter — always attracts immediate attention; to the eye it resembles a brilliant silvery white, non-twinkling "star."
Jupiter is currently 488 million miles (785 million km) from Earth. Although primarily an after-midnight sight right now, the planet is rising an average of 4 minutes earlier each night. By April 7, Jupiter will arrive at opposition and be visible all night from sunrise to sunset. [The Planets in January's Night Sky: How and When to See Them]
Jupiter is currently visible in the zodiacal constellation of Virgo. It can be found about 4-degrees to the north of Virgo's brightest star, the bluish 1st-magnitude Spica, which itself is a distance of 250 light-years from Earth.
Jupiter and Spica will soon be engaged in an unusually triple conjunction. In other words, Jupiter will pass due north of Spica three times this year, the first passage coming on Friday (Jan. 20), the second passage just over a month from now on Feb. 23 and the third and final pass on Sept. 5.
During this time frame, Jupiter will move through a retrograde loop, moving from west-to-east relative to Spica on Friday, then reversing its course and moving back to the west, passing Spica and second time in late February. Then the lordly planet will resume its normal west-to-east motion against the background stars and pass Spica for a third and final time in early September.
Right now, the moon is on the wane — it reached its full phase on Jan. 12 — with its last quarter (or half phase) coming up later in the day on Thursday. So it will still be a gibbous moon — 56-percent illuminated — that will be interacting with Jupiter and Spica early on Thursday morning.
Early on Thursday in the predawn sky, the moon will form a triangle with Spica and Jupiter. The moon will stand about 5 1/2-degrees above Spica, and only 2-degrees to the upper left of Jupiter. Remember: Your clenched fist held at arm's length measures roughly 10 degrees. So the moon and Spica will be separated by about half a fist and the distance between the moon and Jupiter will be less than half that distance.
The trio should make for an eye-catching sight between midnight and the first light of dawn. At the time of the moon’s interaction with Jupiter and Spica it will be about 248,700 miles (400,000 km) from Earth.
In addition to our own moon, there are four other moons that you might want to look at, namely the famous Galilean moons (Europa, Callisto, Ganymede and Io) which were first seen by Galileo with his crude telescope in 1610. The satellites are fun to watch as they change their position relative to Jupiter from hour to hour and night to night. They can be glimpsed through 7-power binoculars (if you hold them steady) or better yet, a small telescope.
On Thursday morning, you can see all four moons – on one side of Jupiter. Ganymede will be very close to Jupiter's disk having just completed a transit – passing in front of Jupiter – and moving off of Jupiter at 6:13 UT (1:13 a.m. Eastern Time). Next in line will be Io, followed by Europa and finally, farthest out from "Big Jupe" will be Callisto. The best time to observe Jupiter in a telescope is still around morning twilight, when the planet is highest in the south.
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Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for Natural History magazine, the Farmer's Almanac and other publications, and he is also an on-camera meteorologist for Verizon FIOS 1 News in Rye Brook, NY. Follow us @Spacedotcom, Facebook or Google+. Originally published on Space.com.