For the next few months, Jupiter will be shining bright in the eastern night sky in the constellation Gemini as it makes its nearest pass to the planet Earth. On January 10th, Jupiter will be opposite the sun and then will go retrograde. Jupiter will appear to move slowly across Gemini, night after night, until returning back to prograde motion in mid April.
Retrograde motion is caused when the Earth passes another planet, making it appear to be moving backwards. Imagine if you will, two well lit cars driving on a 2 lane oval racetrack on a pitch black night. Both cars are traveling in the same direction as do all the planets. As car 1 passes car 2, car 2 appears to move backwards relative to the view out of the side window of car 1. All of the planets appear to go through retrograde motion. Each planets cycle varies depending on how long the planet takes to go around the sun. Jupiter goes retrograde about every 9 months and lasts for 4 months.
A fun fact about Jupiter is that there are 4 moons that are easily seen with minimal magnification. These 4 moons are known as the Galileon moons. This observation by Galileo was quite contentious in 1610 because God made the Earth at the center of the world. Copernicus, 60 years prior before passing away, published his heliocentric observations in De revolutionibus orbium. Ironically, this hardly caused a stir in the Catholic church. Not till Galileo made the scene. With the first telescope made in 1608 by a Dutch spectacle maker, Galileo was able to improve the design and make significant observations of Jupiter’s biggest moons. With a simple pair of binoculars, kids telescopes, or a good camera, you can witness for yourself 4 celestial objects that are clearly not geocentric. Go out and make observations every 30 to 60 minutes and notice how fast Io, Europa, Ganymede, and Callisto move around Jupiter. Have fun!
Post submitted by field instructor Keith Grenoble