Jimmy Westlake's Celestial News column appears monthly in the Steamboat Today.
Find more columns by Westlake here.
After the sun goes down Jan. 21, step outside and marvel at the 10-day-old waxing gibbous moon and the dazzling planet Jupiter sitting side by side. The two will be less than 1 degree apart for most of the early evening. In fact, when the duo are at their closest at about 8:30 p.m., you barely would be able to squeeze another moon between them.
Of course, the apparent closeness of Jupiter and the moon is just a line-of-sight effect. The moon only is 250,000 miles away from Earth, while Jupiter is 1,600 times farther off in the distance, 414 million miles away. The moon passes Jupiter in the sky once each month, but a conjunction this close is very unusual.
The close encounter of Jupiter and the moon Jan. 21 provides a perfect opportunity to observe the moon’s orbital motion in progress. Catch the two early in the evening and then step out and look again at one-hour intervals. The moon moves its own diameter eastward each hour in its orbit around the Earth, so you literally can watch as it slowly orbits past the giant planet.
Aim your binoculars toward the pair to get a better view and you might also glimpse two of Jupiter’s own four giant moons, Ganymede and Callisto. The two moons will look like little stars, one on either side of the planet. With a small telescope, you also should be able to catch sight of Jupiter’s remaining two giant moons, Io and Europa. They will appear together, much closer to Jupiter and almost lost in its glare. Of these four giant moons, only Europa is smaller than our moon. Ganymede is the largest moon in the solar system — its girth is greater than that of the planet Mercury!
About 5 degrees off to the moon’s lower left on Jan. 21 will be the bright orange-giant star Aldebaran, the eye of Taurus the Bull. The Pleiades star cluster will be twinkling about 9 degrees to the moon’s upper right.
Folks living in parts of central South America will get to see the moon temporarily eclipse Jupiter, but for all of North America, the event will be only a near miss.
Professor Jimmy Westlake teaches astronomy and physics at Colorado Mountain College’s Alpine Campus. Check out Westlake’s astrophotography website at www.jwestlake.com.