Jupiter, in Libra, is a dazzling object this month and on May 9th the planet is at its best for the year when opposition is reached. On this day, it will shine at mag. -2.5 and visible all night long. It rises in the east at sunset, reaches its highest point in the sky around midnight before setting in the west at sunrise. The planet is unmistakable to the naked eye and easily brighter than any night time star. Due to its declination of -16 degrees, Jupiter is currently better seen from southern rather than northern latitudes.
At opposition, Jupiter is located 4.400 AU (658.2 million kilometers or 409 million miles) from Earth. Its apparent size spans an impressive 44.8 arc seconds. Of all other planets, only Venus can exhibit a greater apparent size than Jupiter.
Popular 7x50 and 10x50 binoculars reveal a very small white or creamy coloured planetary disk without detail. Easily visible are Jupiter's four large Galilean moons (Io, Europa, Ganymede and Callisto). They continuously change position as they orbit the giant planet; sometimes all four are visible at once but often they can be temporarily obstructed as they pass in front of, or behind the planetary disk.
Theoretically, all four moons are bright enough to be seen with the naked eye, but their close proximity to Jupiter makes this task challenging. However, a few very keen eyed observers have managed to achieve this. For comparison, Io shines at mag. +5.3, Europa at mag. +5.6, Ganymede at mag. +4.9 and Callisto at mag. +6.0. Through a telescope, a wealth of Jupiter's surface details is visible. Even a small 60mm (2.4-inch) refractor at medium/high magnifications will show a number of details, including the great northern and southern equatorial belts. Keen eyed observers under good conditions may also be able to see shadow transits of the Galilean moons on the Jovian disk.
A larger 100mm (4-inch) telescope reveals numerous bright and dark spots and the Great Red Spot. This complex weather system has been observed for centuries, but now is diminishing in size. In the late 1800's, it spanned 41,038 kilometres (25,500 miles). In 1979, at the time of the twin Voyager space probes flybys, it had shrunk to 23,336 kilometres (14,500 miles). Recent measurements using from Hubble Space telescope images, put the current diameter at 17,912 kilometres (11,130 miles). This is a far cry from two hundred years ago, and probably some day in the future the great storm will disappear once and for all.
Through a 200mm (8-inch) or larger scope, Jupiter is a spectacular sight with well defined features including subtle markings, smaller belts, ovals and festoons in the cloud structure visible. Under good seeing, it's possible to push scopes close to their maximum magnification limits, which is generally accepted to be 50x for every 25mm (1-inch) of aperture.
Jupiter Opposition 2018 Data Table
|Opposition Date||May 9, 2018|
|Distance from Earth (AU)||4.400|
|Distance from Earth (Million Kilometres)||658|
|Distance from Earth (Million Miles)||409|
|RA (J2000)||15h 04m 02s|
|DEC (J2000)||-16d 00m 08s|
|Apparent Diameter (arc secs)||44.8|