Jupiter, the king of the planets comes to opposition, opposite the Sun in the sky, on 5 January to present close to the very best possible observing opportunity the gas giant ever can.

Damian Peach imaged Jupiter on 1 December in good seeing conditions. Oval BA is well seen at lower left with its strong orange colouring. See larger image.
 
Lying among the bright stars of Gemini, embedded in the winter Milky Way, Jupiter rises at sunset and will culminate due south at midnight, just about as high as it ever gets above the horizon, a whopping 63 degrees from London. This gives observers almost a 12-hour observing window in which even small telescopes will be able to show the major belts and zones and the multitude of Gallilean satellite phenomena which makes the planet so special to observe.

Jupiter is by far the largest planet in the Solar System and undoubtably the most rewarding planet to observe, image and study for the amateur astronomer. Despite its huge distance from us, its mean distance being more than five times further from the Sun than we are, for most of the time it's the largest planet to view in a telescope, beaten out only by Venus when near inferior conjunction. Jupiter can get as large as 50 arcseconds in apparent equatorial diameter (it almost achieved this in September 2010 when it reached 49.9 arcseconds at a near-perihelic opposition) and varies between this and a minimum size of 43 arcseconds, with the mean being 46 arcseconds.

Jupiter's mean sidereal period or one journey around the Sun is 11.9 years. The last opposition in December 2012 was very favourable with the gas giant peaking at 48.5 arcseconds and shining at magnitude -2.8, a mere tenth of a magnitude short of its maximum possible opposition brightness. When you add to this Jupiter's very favourable northerly declination at the moment (for Northern Hemisphere observers) helping it to culminate high in the sky, almost at the zenith from states in the far south of the USA, and well away from the added turbulence close to the horizon, then now is a great time to observe Jupiter.

Jupiter's appearance

The face that Jupiter presents to us is not a solid surface like Mars or Mercury but the cloud tops of its incredibly dynamic atmosphere with constantly changing features and detail within a familiar structure of long-lasting dark belts and bright zones that generations of astronomers have kept under constant surveillance. Jupiter's disc can be resolved even in binoculars and even a 50-mm class 'scope will show the two major equatorial belts and the polar regions. Employing a magnification of 150x will be sufficient to show lots of detail through the eyepiece; high quality refractors in the 125-mm (five-inch) class and above and reflectors and SCTs in the 150mm- 200-mm (six to eight-inch) class will reward the keen Jovian observer with a lifetime of enjoyment.

 
Images show Jupiter as a very colourful planet but visually don't expect to see such vivid hues, although the larger the aperture of your telescope, assuming quality and clean optics and good quality eyepieces, the brighter Jupiter will be with more contrast, making cloud features easier to see and their colour easier to perceive. Colour only really becomes obvious in apertures of 250-mm (10-inch) or larger.

Colour filters will help the view through all telescopes but will really benefit smaller apertures. Red or orange filters will help enhance the bluer areas of Jupiter and help penetrate deeper into the atmosphere, while blue filters enhance the redder or darker areas. To enhance the contrast between the belts and zones try a Wratten No. 80A or No. 82A (light blue) and dark features such as festoons will benefit from a Wratten No. 12 (yellow) or No. 21 (orange). Other useful filters are No. 25 (red) and No. 38A (dark blue).

Great Red Spot

Jupiter's longest lasting feature is the famous Great Red Spot (GRS), a huge oval shaped, seemingly perpetual anticyclonic storm that has been observed since the 17th century. It varies in size and intensity and currently its most intense brick-red colour, which makes it easy to see in a 50-mm (two-inch) 'scope, is more strongly orange now. The GRS drifts constantly in longitude through the southern South Equatorial Belt (SEB) and can currently be found at System II longitude of 204 degrees. This drift amounts to as much as 12 degrees per year and is of course independent of the exceedingly fast rotation of the planet itself; there are two distinct rotation zones on Jupiter; the equatorial region, which encompasses the area from the southern half of the North Equatorial Belt (NEB) to the northern half of the SEB. Astronomers cal this System I and has a defined mean rotation period of 9 hours 50 minutes and 30 seconds.

Damian Peach imaged Jupiter on 9 December in fair to good seeing. The Great Red Spot looks superbly coloured near the egde. Also visible is the NNTZ little red spot in the northern hemisphere and the subtly red coloured 'new red spot', formerly White spot Z, in the North Tropical Zone. See larger image.
 
System II is the rest of the planet and has a mean rotation period of 9 hours 55 minutes and 40.6 seconds. The GRS is best seen when it transits the Jovian central meridian (CM), an imaginary line drawn through the centre of Jupiter's disc from pole to pole. Jupiter's fast rotation as much as 90,000 km per hour at the equator causes the globe to bulge outwards, giving the planet a very noticeable flattened appearance shaped like an oblate spheroid. This helps to define the CM easier and the system II longitude of the CM can be worked out in advance for any time, helping to pin-point a time when the GRS will be visible. Given any reasonable amount of clear periods this month it should be easy to observe it as Jupiter has a long observing window and the planet's fast rotation will quickly bring the GRS and any other noticeable feature into view.

Up to date Jovian weather

Oval BA is strongly orange coloured at present, with a white centre. It is moving fast (-14.3 system II degrees a month) in the South Temperate Zone (STZ). The North North Temperate Belt (NNTB) contains a red spot (LRS) at system II longitude 156 degrees, with a prominent red barge just to its south. Lurking in the North Tropical Zone (NTrZ), close to the North Equatorial Belt (NEB), is a new but faint red spot (ROZ) which used to be White Spot or Oval Z. There are some prominent white ovals too in the South South Tropical Belt (SSTB).

The planet has its 'normal' appearance overall, with both equatorial belts prominent, although the NEB has narrowed. Currently it shows a prominent array of dark bluish projections with festoons into the Equatorial Zone. Check out the latest BAA Jupiter section report and the ALPO Japan Jupiter section for up to date images.

Galilean moons

The four Galilean moons of Jupiter, Io, Europa, Ganymede and Callisto are another outstanding observation advantage that Jupiter has over the other planets, with their constant dance around their parent being a joy to follow and providing great observing opportunities. Binoculars are sufficient to see all four moons but perhaps a 150-mm is a good size instrument to comfortably observe most of the satellite phenomena.

The satellites move from east to west across the face of the planet, and west to east behind it. They all orbit Jupiter in the same plane as Jupiter's equator they will be in seen in transit, together with their shadows, across the face of Jupiter and seen to disappear and reappear behind Jupiter's limb (occultation). They also move in and out of Jupiter's shadow (eclipse). After conjunction with the Sun and before opposition, Jupiter's shadow falls to the west and eclipse precedes occultation and shadow-transit precedes transit. After opposition the order changes so this month through to conjunction in June, occultation precedes eclipse and transits are before shadow-transits. All such events are predicted ahead of time and timings can be obtained in the monthly issues of Astronomy Now and the Astronomical Almanac and the Handbooks of the British Astronomical Association and The Royal Astronomical Society of Canada.

Eclipses and reappearances are easy to see, the moon winking in and out of view, as are occultations. The dark shadows of the moons are readily observable in small to moderate 'scopes but the moons themselves can be tougher and generally require larger apertures (at least a 300-mm or 12-inch) and good seeing conditions. Transits produce spectacular images.

Transits this opposition

Things get especially interesting right at opposition opposition when a moon can transit its own shadow; on this coming Saturday/Sunday night (4/5 January), just before opposition, Io's shadow precedes Io itself onto the Jovian disc by one minute at 3.12am UT. Just after opposition, on the night of Monday/Tuesday (5/6 January), roles are reversed, Io preceding its shadow by one minute at 21.39 UT. These events are visible across Europe and North America.

On 6 January, Io and its shadow transit at the same time that a faint star, Tycho 1895 1727 is occulted by Jupiter. Note that Io is practically on top of its shadow close to opposition, when Earth is directly between Jupiter and the Sun. The shadow switches sides before and after opposition as our angle changes. AN graphic by Greg Smye-Rumsby.
 
Every now and again a beginner will casually glance at Jupiter through binoculars or a small scope and only see two or even one moon! These occurrences are not that uncommon as occasionally two moons will be in transit or one in transit and one hidden behind Jupiter of lost in its shadow. There are a few events this month and these are detailed below in the Jupiter's visibility sections.

Jupiter's visibility

Jupiter is resident among the stars of Gemini, moving slowly retrograde (east to west) and lying about three degrees west of Wasat (mag. +3.5, delta) on opposition night. It's a blazing magnitude -2.7 beacon in the night sky, outshining everything but the Moon and Venus, which is on its last legs as an evening star for the first week of the year. Through a telescope it shows a distinctly flattened disc (due to its fast rotation period) spanning 46.8 arcseconds at the equator.

From the UK (London) Jupiter rises at 15.55h UT and is above 20 degrees altitude in an astronomically dark sky from 18.30h until 05.45h, setting at 08.20h. At midnight it lies due south an altitude of around 63 degrees. Over a full rotation of the planet's cloud features can potentially be observed. By the end of January, Jupiter can be observed in a dark sky from 18.45h (when its already over 40 degrees up), transits or culminates at 22.09h and dips below the 20 degree altitude barrier at around 4am. By this time it has faded marginally to mag.-2.64 and shrunk to 45.6 arcseconds in apparent diameter.

From North America in New York the gas giant rises at 16.33h EST and climbs to 20 degrees altitude two hours later. It remains above 20 degrees until 05.30h, having culminated at 72 degrees at midnight. Further south in Miami, Jupiter rises at 17.36h, hitting the crucial 20 degree mark by 19.15h and staying above it until 05.40h. Observers here will see the great planet culminate almost at the zenith at 00.30h and like elsewhere, the whole of Jupiter's observing window can be enjoyed in astronomically dark skies. Observing circumstances are not too shabby either on the west coast, with Los Angeles seeing the gas giant culminate just short of 80 degrees and above 20 degrees altitude from 18.30h to 05.15h PST.

In Australia and New Zealand Jupiter is reasonably placed but with Gemini lying well north of the celestial equator, it's not nearly as favourable. From Sydney, Jupiter rises at 20.05h EDT, climbing to 20 degrees by 22.15h and sinking back to the same by 4am. The gas giant culminates at 33 degrees at 1.05h. In Wellington, Jupiter rises at 20.50h DST, climbs to 20 degrees up by 23.30pm and culminates 25 degrees above the northern horizon at 01.30am. The gas giant's relatively short observing window closes at 03.35h.