Jupiter the Solar System's largest planet has been well placed for observation for a few months now as it moves retrograde amongst the stars of Leo. Since the end of last year the planet was visible in early evening, appearing as a spectacular dazzling object. On February 6th, Jupiter is at its best for 2015 when the giant planet reaches opposition. On this day, it will rise in the east as the Sun sets and then set in west as the Sun re-rises on the opposite side of the sky. With a magnitude of -2.6, the "King of the planets" is unmistakable and far brighter than any nighttime star.
Jupiter stars the month in western Leo before crossing the constellation boundary into Cancer on February 4th where it remains for the rest of the month. Positioned 12 degrees southeast of Jupiter is Regulus (α Leo - mag. +1.4) the brightest star in Leo with the Gemini twin stars of Castor (α Gem - mag. +1.6) and Pollux (β Gem - mag. +1.2) located 25 degrees northwest of Jupiter. Although all three stars are bright, Jupiter far outshines them being about 35x brighter than Pollux and 40x brighter than Castor and Regulus.
The current northern declination of Jupiter slightly favours Northern Hemisphere observers, but even from southern latitudes where the planet appears lower down it's still unmistakable due to its brightness.
At opposition, Jupiter is located 4.346 AU (650.18 million kilometers or 404.01 million miles) from Earth. Although Jupiter is slightly further from Earth than during the last few oppositions its apparent size is still an impressive 45.4 arc seconds. Of all other planets only Venus - on rare occasions - displays a greater apparent size than Jupiter. At very favourable oppositions, when Jupiter is closest to Earth, it can reach magnitude -2.9 with an apparent size of over 50 arc seconds.
Popular 7x50 and 10x50 binoculars will reveal the disk of Jupiter. It appears off white or creamy coloured but no surface details are visible. Also visible are the four large Galilean moons (Io, Europa, Ganymede and Callisto). The moons continuously change position as they orbit Jupiter and sometimes all four are visible, but on other occasions some or all of the satellites will be temporarily obstructed and hidden as they pass behind or in front of Jupiter's disk.
The moons of Jupiter are theoretically all bright enough to be seen with the naked eye, although their close proximity to the searchlight beam of Jupiter makes this task virtually impossible. However, a few very keen eyed observers have claimed to spot the moons without optical aid. At opposition this year, Io shines at magnitude +5.2, Europa magnitude +5.5, Ganymede magnitude +4.8 and Callisto at magnitude +5.9.
Through a telescope a wealth of detail is visible on the planets disk. Even a small 60mm (2.4-inch) scope at medium to high magnification will begin to show some surface detail including the great Northern and Southern Equatorial belts. Keen eyed observers under good seeing conditions may even be able to notice the shadow transits of the Galilean moons. A larger 100mm (4-inch) telescope will reveal more details in the belts including bright and dark spots. Also visible is the most famous storm of all on Jupiter "The Great Red Spot". Recently observations have revealed that the Great Red Spot is shrinking in size! In the late 1800s its diameter was estimated at 41,038 kilometres (25,500 miles). At the time of the twin Voyager space probes flybys in 1979 it had shrunk to 23,336 kilometres (14,500 miles). Recent observations by the Hubble Space telescope (2009) put its diameter at only 17,912 kilometres (11,130 miles) which is a far cry from the 1800s. Eventually one day it will disappear completely.
With a 200mm (8-inch) telescope or larger, Jupiter is a spectacular sight with the brighter features well defined along with all sorts of intriguing and subtle details. Generally, if the seeing conditions are good it is possible to push a telescope close to the magnitude limit when observing Jupiter. The accepted values are 50x for every 25mm (1-inch) of aperture. So up to 200x for a 100mm (4-inch) telescope and 400x for a 200mm (8-inch) telescope.
But as always whatever telescope you are using or even just with the naked eye, Jupiter is a fantastic object to view especially around the time of opposition.
Jupiter Opposition 2015 Data Table
|Opposition Date||February 6, 2015|
|Distance from Earth (AU)||4.346|
|Distance from Earth (Million Kilometres)||650.18|
|Distance from Earth (Million Miles)||404.01|
|RA (J2000)||09h 21m 04s|
|DEC (J2000)||+16d 29m 51s|
|Apparent Diameter (arcsecs)||45.4|