Planet Jupiter. Credit: NASA, ESA, A. Simon-Miller (GSFC),
I. de Pater, M. Wong (UC Berkeley)It may sound like a very large political movement of some kind, or a counter-insurgency in some part of the world, but the Jupiter Opposition I'm speaking of is an astronomical event: the time when Earth passes between the Sun and Jupiter, making the Earth-Jupiter distance its smallest and placing Jupiter at the point in the sky exactly opposite from the Sun—hence the name, "opposition."
Opposition is the best time to see a planet like Jupiter: it's at its largest visual size, it's nowhere near the glare of the Sun, and it's in the sky all night long. Score on three counts—how many things in life work out so that the best of all worlds occur at the same time?
The Opposition of Jupiter occurs about once each year, the timing governed mostly by Earth's relatively speedier orbit around the Sun. Earth takes one year to revolve, while Jupiter takes closer to 12 years. So, opposition actually happens every year plus one month; in the time Earth has taken to come back around to the previous point of opposition, Jupiter has moved along one twelfth of its orbit, so it takes Earth an extra month to "catch up."
This year, the date of the Opposition of Jupiter is September 21, at which time the planet will be only 3.95 Astronomical Units away—about 591 million kilometers (369 million miles). Sounds faraway—and it is—but Jupiter's size helps make up for this. With a diameter of 142,984 kilometers (over 11 times that of Earth), even at this distance there's a lot to see, even through a small telescope.