That dazzling starlike object close to the moon on January 20, 2013 is the giant planet Jupiter. You can’t miss this pair in clear skies this evening, no matter where you are on Earth. And they are still getting closer. Tomorrow’s conjunction of Jupiter and the moon will be visible from around the globe, and particularly dazzling for the Americas, where it’ll be the closest these two will appear in our sky until the year 2026.
The image at the top of this post is from EarthSky Facebook friend Janet Furlong in Culpeper, Virginia. She took this photo last night (January 19, 2013). The two brightest objects are the moon and Jupiter. View larger. Janet wrote:
Step outside and lift your head up and view the sky. Totally gorgeous … so much to see tonight.
What motions of the moon and Jupiter cause them to come together this way in our sky? Their most noticeable motion, in course of a single night, is that the moon and Jupiter go westward across the sky. They do so for the same reason that the sun goes westward during the day. It’s because the Earth rotates from west-to-east on its axis, causing the sun, moon, planets and stars to appear to move from east to west on a daily basis.
Meanwhile, from tonight to tomorrow night, you will see the moon move closer to Jupiter on the sky’s dome. That change will be caused by a true motion of the moon itself, its motion through space in orbit around Earth. Due to its orbital motion, the moon travels about 13oeastward in front of the backdrop stars every day. For reference, the moon’s diameter equals one-half degree.
Jupiter changes its position in front of the background stars, too. Yet, in contrast to the moon, it does so at a snail’s pace and sometimes erratically. Jupiter’s true motion is more difficult to make out than that of the moon because Jupiter orbits the sun, not the Earth. Because we view Jupiter from the moving platform of Earth, Jupiter appears to move backwards in its orbit for about four months. Really, this motion is an illusion, called retrograde motion. Jupiter has been moving in a retrograde fashion – westward in front of the stars – for some months now. It will end its retrograde motion, becoming temporarily stationary in front of the stars, on January 30.
Believe it or not, Jupiter’s orbital speed (13 kilometers/second) is much faster than the moon’s (1 km/second). Keep in mind, though, that far-distant Jupiter takes nearly 12 years to circle the sun in its great big orbit. On the other hand, our moon circles the Earth in its tiny orbit in less than four weeks. That’s why, as seen from Earth, the nearby moon appears to travel more swiftly than the distant planet Jupiter.
By the way, unlike the stars, the moon and Jupiter don’t shine by their own light. They shine by reflecting the light of the sun.
Bottom line: Look for these two brilliant beauties – the moon and the planet Jupiter – close together for several nights around January 21, 2013. As seen from the U.S. and Canada, this is the closest the waxing gibbous moon and the giant planet Jupiter will appear until the year 2026. Their close proximity on the sky’s dome wil be visible throughout the world, but particularly dazzling for the Americas.