Or perhaps I should say that we are making our closest approach to Jupiter. While all of the planets orbit the Sun on more or less circular paths, they do so at drastically different speeds. Jupiter, five times as far from the Sun as the Earth, takes almost 12 years to journey around the Sun. As a result, each year we “catch-up” to the slower-moving Jupiter, before leaving it in our dust just a few short months later. The point where we “pass” Jupiter, and the two planets are closest, is known as opposition.
Now is that time. Jupiter reaches opposition on April 7. The term opposition comes from the fact that a planet at opposition is directly opposite the Sun in the sky (see graphic). Consequently, Jupiter currently rises at sunset and sets at sunrise, meaning that we can see it all night long. Jupiter also appears brighter and larger during opposition than at any other time of the year making this the best time to get acquainted with the King of the Planets.
You don’t need a telescope to begin revealing some of Jupiter’s secrets. Aim any decent pair of binoculars at Jupiter (preferably rest them on something to keep your hands steady) and you should see a row of smaller stars lined up on either side of it, like a string of pearls. These are Jupiter’s four largest moons: Io, Europa, Ganymede, and Callisto. So large are these moons that they would actually be visible to the naked eye were it not for Jupiter’s bright glare. Galileo is usually given credit for discovering them with his telescope in 1610, but some historical accounts suggest that Ganymede, the largest moon in our Solar System, may have actually been spotted by the keen eyed Chinese astronomer Gan De in 365 BC.
Regardless of who saw them first, the moons are arguably more interesting than Jupiter itself. Europa seems to be home to an ocean of liquid water trapped beneath its icy surface. Could living organisms swim in Europa’s icy seas? We don’t know, but if we’re going to find other lifeforms in our solar system, an icy moon like Europa is perhaps our best chance.
Even though we begin pulling away from Jupiter on April 8, it will remain bright and visible in the evening hours for several months to come. By early August, Jupiter will disappear behind the Sun, hidden from view until we begin approaching it again next year.