What is opposition? It’s when an outer planet is 180 degrees away from the Sun; it rises when the Sun sets and is visible the whole night long. Dynamically speaking, opposition happens when the Earth, on the fast inside track around the Sun, passes the outer planet. Opposition is also the time when the outer planet is closest to Earth (on this lap around the Sun anyway) and thus brighter. Tonight Jupiter is -2.78 magnitude – about the brightest it ever gets.
Jupiter reaches opposition about every 13 months; it takes Earth the extra month to catch up as Jupiter slowly moves along in its 12 year orbit. The slower moving Saturn reaches opposition every 54 weeks, but speedy Mars takes 26 months. It only moves slightly slower than the Earth and our Earth takes a long time to catch Mars again after passing it. The time it takes a planet, Earth and the Sun to reach the same relative positions again, such as one opposition to the next, is called the synodic period.
Jupiter is justly famous for the large number of satellites orbiting it – some 64 are known at last count. Most are just large rocks and invisible to allbut the very largest telescopes, but four, the Galilean moons, are easily visible in binoculars. They range in size from Europa, just a bit smaller than our own Moon to Ganymede which is larger than the planet Mercury! Io and Callisto are intermediate in size. All are quite bright and would be seen by the naked eye if they were not hidden in Jupiter’s glare.
On October 28, at 9:00PM three of the four are visible in binoculars. Io happens to be behind Jupiter, invisible at the moment and will pop out about 11:00PM although it probably won’t be spotted by your binoculars until about midnight. The moons move noticeably hour by hour shuffling their relative positions about in a complicated dance.
Historically the Galilean moons are of some importance; they were the first objects (1610) ever seen that were definitely not circling the Earth and were thus a point in favor of the then new heliocentric theory. (Venus provided the actual proof a little later). In addition, the moons provided the raw data for the first decent estimate of the speed of light.
See you next time!
Mr. Usher is President of the Everglades Astronomical Society which meets every second Tuesday at 7:00PM at the Norris Center, Cambier Park, Naples.