This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Backyard astronomers, take note: Jupiter and Earth are approaching their near-yearly rendezvous, and this time the two planets will be closer together than they have been since 1963. The giant planet's proximity should make for good viewing, weather permitting—Jupiter will appear especially bright in the sky for several nights around its closest approach on September 20.
The planet should be highly visible to the naked eye; with Jupiter's apparent magnitude of –2.94, only the moon will be brighter after Venus sets relatively early in the evening. Apparent magnitude is a measure of an object's luminosity as viewed from Earth; faint objects have large positive magnitudes, whereas bright objects have large negative magnitudes. When Jupiter and Earth draw near, the nearly full moon will shine with an apparent magnitude of –11.87.
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Jupiter will be visible in the direction of the constellation Pisces, appearing to the east and low in the sky at sunset, before moving toward the south as the night progresses. With a decent set of binoculars, stargazers may be able to identify Jupiter's four largest satellites, the Galilean moons Io, Europa, Ganymede and Callisto. Sky & Telescope has a handy Web application that shows where to find the moons in relation to Jupiter at any given time.
Jupiter takes nearly 12 times as long to complete one orbit around the sun as our planet does, so Earth laps its much larger planetary counterpart every 399 days. Because both Earth and Jupiter follow slightly elongated elliptical orbits, the distance between the two planets varies with each rendezvous.
Jupiter is currently drawing closer to the inner solar system, including Earth, as it nears perihelion, its closest approach to the sun, in March 2011. On September 20 of this year, Jupiter and Earth will be separated by a mere 591,499,329 kilometers, about 11 million kilometers closer than in 2009, according to orbital data from the NASA Jet Propulsion Laboratory's HORIZONS system. The last time that Jupiter was so close to Earth was October 1963, and it will not exceed the close proximity of this year's passage until September 2022.
Photo of Jupiter, with Europa visible at upper left: Joshua Bury/Flickr