Tonight – March 21, 2016 – look for the brilliant waxing gibbous moon to pair up with the dazzling planet Jupiter as soon as darkness falls. As the Earth spins eastward beneath the starry heavens tonight, it’s causing the moon and Jupiter to travel upward and westward, until the brilliant twosome reaches its high point in the sky at late evening. They’ll continue to move westward, to set in the west just before dawn.
The giant planet Jupiter has more than twice the mass of all the other solar system planets, dwarf planets, asteroids and moons combined. Jupiter’s mass is 318 times that of the Earth. Little wonder why Jupiter enjoys the King Planet designation!
Three of Jupiter’s four largest moons are larger and more massive than Earth’s moon. In their outward order from Jupiter, the four major moons of Jupiter are Io, Europa, Ganymede and Callisto. Europa is a touch smaller and less massive than our own moon.
You can actually view these moons as pinpoints of light with a modest backyard telescope or even good binoculars. Find out the present position of Jupiter’s moons on this handy chart, courtesy of skyandtelescope.com.
We’ve been asked why Jupiter’s moons move so quickly around Jupiter, although all four of Jupiter’s major moons lie farther away from Jupiter than our moon’s distance from Earth. For instance, Io – Jupiter’s closest moon – has a semi-major axis of 421,800 kilometers in contrast to the semi-major axis of our moon: 384,400 kilometers. Despite Io’s greater distance from its parent planet, Io revolves around Jupiter in 1.769 days. Meanwhile, our moon takes a whopping 27.322 days to orbit Earth.
It’s Jupiter’s great mass that causes Io and Jupiter’s moons to move so quickly around Jupiter. If the Earth were as massive as Jupiter, then our moon’s orbital period would be only 1.53 days. Or if Jupiter were as lightweight as Earth, then Io’s orbital period would be 31.55 days.
Bottom line: Use the waxing gibbous moon to find Jupiter, the king of the planets, on March 21, 2016!