The sky chart at the top of this post is for late evening – close to midnight – on Thursday, October 24, as seen from mid-northern North American latitudes. The moon rises first, at late evening, followed by the dazzling planet Jupiter about one-half hour later. Because the rising times of the moon and Jupiter vary around the world, you may want to consult an almanac for your sky. Once the moon and Jupiter climb over the east-northeast horizon tonight, they’ll be out for rest of the night.
Jupiter shines in front of the constellation Gemini the Twins, in close vicinity to Gemini’s two brightest stars, Castor and Pollux. Although the moon will move out of Gemini in a few more days, Jupiter will remain within Gemini’s borders from now until early July 2014.
As seen from Earth, the moon looks much larger than Jupiter. But Jupiter is actually much larger than our moon. The moon only appears bigger, because it’s so much closer to us. Astronomers often list distances of solar system objects in astronomical units (a.u.). The astronomical unit equals the sun-Earth distance, a measure of about 150,000,000 kilometers (93,000,000 miles) or 8.3 light-minutes. This evening, the moon is about 1/370 of an astronomical unit away. Meanwhile, Jupiter resides nearly 4.9 astronomical units from Earth. That places Jupiter some 1800 times farther away than tonight’s moon.
Jupiter’s diameter is about 40 times greater than our moon’s diameter. To gauge the size of our moon relative to Jupiter, look at Jupiter though a backyard telescope sometime. Jupiter’s four major moons – called the Galilean moons – are pretty easy to see. You might miss a moon or two on occasion, because these moons routinely swing in front and in back of Jupiter. In their outward order from Jupiter, these moons are Io, Europa, Ganymede and Callisto. Io and Europa are about the same size as our moon, whereas Ganymede and Callisto have diameters of about 1.5 times that of our moon.
Watch for Jupiter and Earth’s moon to rise late tonight. The moon will follow Jupiter westward across the sky in the hours between midnight and dawn. If you’re an early morning person, look for the moon and Jupiter to shine high in Friday morning’s predawn and dawn sky.
The best time to view Jupiter’s moons with the telescope is during the wee hours before dawn, or when this world is highest in the sky for the night. It’s often difficult to get a crisp focus on sky objects near the horizon because of the greater thickness of the Earth’s atmosphere. If you’re not an early morning person, wait till December 2013 or January 2014 to see Jupiter in the early evening sky. In early January 2014, Jupiter will be at opposition – opposite the sun in Earth’s sky – so the king planet will be out all night long and more conveniently placed for evening viewing.
Bottom line: The bright object near the moon from late at night October 24, 2013 to dawn October 25 is the planet Jupiter.