On the evenings of July 12, 13 and 14, 2019, watch for the bright waxing gibbous moon to swing by the giant planet Jupiter. Fortunately, the king planet is so bright that this world can easily withstand the lunar glare. After all, Jupiter is the fourth-brightest light in the heavens, after the sun, moon and planet Venus. Venus is a morning object now, virtually lost in the sun’s glare, so there’s no way to mistake Venus for Jupiter in the July evening sky.
Although the moon and Jupiter appear close together on the sky’s dome, these two worlds are nowhere close to one another in space. The moon, our closest celestial neighbor, is around its average distance from Earth (238,955 miles or 384,400 km) right now. Jupiter resides more than 1,700 times the moon’s distance from Earth. At present, Jupiter lies 4.42 astronomical units (AU) from Earth. One AU = one Earth-sun distance = 92,955,817 miles or 149,597,871 km. Jupiter is currently 5.29 AU from the sun.
After the moon and Jupiter first come out at dusk, the brilliant twosome will continue to move westward across the sky throughout the night. They’ll set beneath the southwest horizon in the wee hours before dawn. The moon and Jupiter (plus the nearby star Antares) all cross the sky for the same reason that the sun travels westward across the sky during the daylight hours, because Earth spins eastward under the sky. The Earth’s spin on its axis – from west-to-east – causes the sun, moon stars and planets to go full circle around our sky each day.
Even though, as Earth spins, the moon goes westward throughout the night, it is also moving eastward in front of the background stars and planets of the zodiac. The moon’s eastward motion in front of the stars is its true motion in orbit around Earth. As darkness falls on July 12, note the moon’s position relative to Jupiter and the star Antares. Then, at nightfall on July 13, note how much more closely the moon couples up with Jupiter. That’s because, as measured by the backdrop stars of the zodiac, the moon in its orbit travels about 1/2 degree (its own angular diameter) eastward per hour. That equals about 13 degrees eastward per day.
The moon appears so much larger than Jupiter in our sky because it’s so much closer to us than Jupiter. If you want to get some idea of the moon’s size relative to Jupiter, look at the king planet through the telescope sometime. Jupiter’s four major moons – Io, Europa, Ganymede and Callisto – are quite easy to view through a low-powered telescope. Two of these moons – Ganymede and Callisto – have diameters about 1 1/2 times that of Earth’s moon, whereas the other two – Io and Europa – are approximately the same size as our moon. Find out the present positions of Jupiter’s four major moons via SkyandTelescope.com.
Bottom line: Use the moon to locate the planet Jupiter on July 12, 13 and 14. After these nights, you’ll recognize Jupiter easily. It’ll be the brightest starlike object to light up the evening sky for months to come.