Tonight – January 26, 2016 – the dazzling star-like object close to the moon is the giant planet Jupiter. No matter where you are on Earth, you’ll see Jupiter near the moon not just tonight, but for the next few late evenings and early mornings. After Jupiter … the moon will pass Mars, Saturn, Venus, Mercury! You can see all five of these bright worlds before dawn. It’s the first time we’ve seen them together since 2005.
The January 26 moon rises approximately 3 hours after sunset, and Jupiter follows the moon into the sky roughly an hour later. The exact rising times for the moon and Jupiter vary around the world, but you can’t miss this pair in clear skies by mid-evening tonight – that is, by about midway between your local sunset and midnight – no matter where you are on Earth.
Jupiter and the moon will be even closer together on January 27.
What motions of the moon and Jupiter cause them to come together this way in our sky? There are several motions to notice. First, the moon’s orbital motion carries it near Jupiter each month. Earth’s spin under the sky causes the moon and Jupiter to move westward in the course of a single night. And Jupiter is moving, too, in front of the stars.
Notice the moon’s orbital motion. It’s easy. Just look at the moon and Jupiter tonight (Tuesday night or Wednesday morning). Then notice them again tomorrow night (Wednesday night or Thursday morning). You’ll see that – from one evening to the next – the moon has moved closer to Jupiter on the sky’s dome. That change is due to a true motion of the moon itself, its motion through space in orbit around Earth.
Due to its orbital motion, the moon travels about 13o eastward in front of the backdrop stars every day. For reference, the moon’s diameter equals one-half degree.
Notice Earth’s spin under the sky. Meanwhile, in course of a single night, the moon and Jupiter go westward across our sky. They do so for the same reason that the sun goes westward during the day … because the Earth rotates from west-to-east on its axis. Earth’s spin causes the sun, moon, planets and stars to appear to move from east to west on a daily basis.
So watch for Jupiter this evening or tomorrow evening. They will be in the east. Then, if you’re up early tomorrow morning (Wednesday) or the following morning (Thursday), look for the moon and Jupiter in the west before sunrise.
This motion in the course of one night is due to Earth’s spin.
Jupiter is moving, too. Jupiter’s own motion in orbit around our local star, the sun, causes this world to change its position in front of the background stars as seen from Earth. In contrast to our nearby moon, Jupiter moves at a snail’s pace in front of the stars. Sometimes, it seems to move erratically, a fact that baffled the ancient stargazers.
It moves more slowly than our moon in front of the stars because it is much farther from us than the moon.
It moves erratically because we view Jupiter from the platform of a moving Earth. Our motion in orbit causes Jupiter to appear to move backwards (westward, or retrograde) in its orbit for about four months every year. Really, this backwards motion is an illusion. It happens because Earth moves faster than Jupiter in orbit. It’s like we’re in a fast car, passing a slower car on the highway. For a time, the slower car appears to move backwards in front of the distant landscape. That’s the case with Jupiter now. Jupiter has been moving in a retrograde fashion – westward in front of the stars – since January 8, 2016.
We’ll pass between the sun and Jupiter on March 8, 2016.
Jupiter will end its retrograde motion, and begin moving eastward in front of the stars again, on May 9, 2016.
Bottom line: The moon is edging toward Jupiter on January 26, 2016 and is even closer on January 27. No matter where you are on Earth, you’ll see Jupiter near the moon the next few mid-evenings to early mornings. After Jupiter … the moon will sweep past Mars, Saturn, Venus and Mercury! Use the moon to identify these planets.