That dazzling starlike object close to the moon on January 13, 2014, 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 evening’s pairing of Jupiter and the moon will be visible from around the globe, and particularly striking for the Americas.
What motions of the moon and Jupiter cause them to come together this way in our sky? From tonight to tomorrow night, you will see the moon move 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.
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. 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.
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. It is much farther from us than the moon and so moves more slowly in front of the stars.
Because we view Jupiter from the moving platform of Earth, Jupiter appears to move backwards in its orbit for about four months every year. Really, this motion, called retrograde motion, is an illusion. Jupiter has been moving in a retrograde fashion – westward in front of the stars – for a couple of months now. It will end its retrograde motion, becoming temporarily stationary in front of the stars in early March 2014.
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.
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 two brilliant beauties – the moon and the planet Jupiter – close together for several nights, centered around January 14, 2014. Their close proximity on the sky’s dome will be visible throughout the world.