J U P I T E R is the fifth planet from the Sun. In the Greek pantheon of gods, Jupiter is known as Zeus and is the father over all the gods and of Man, whose translated name literally means “sky”. Likewise the Romans placed Jupiter as their chief god, Jupiter, who they firmly believed controlled the various atmospheric phenomena such as rain, storms, and thunder and lightning. He is the also the son of Saturn, and has several brothers include Neptune and Pluto.

Jupiter easily justifies its title as the ‘King of the Planets’. It is the largest planetary body in the Solar System - being second only to our Sun - and contains 2.5 times the entire combined mass of all the other planets and 317 times heavier than the Earth. Lying further from the Sun as a superior planet, Jupiter’s orbit averages some 5.2 AU (Astronomical Units) or 778 million kilometres from the Sun taking 11.86 years (11 years 10 months) to complete one orbit.

In the telescope the appearance is distinctly oval being flattened in the ratio of about 13:14. This is caused by the very rapid rotation of just under 9 hours and 51 minutes. In size, Jupiter is huge compared to the Earth, being some 142 984 kilometres across the planet’ girth and 133 708 kilometres at the poles - flattened by about 6.5%. The planetary axis is only slghtly tilted at 3o 07' to the ecliptic - the smallest of all the planets - so that the overall shape telescopically appears fairly constant.

Jupiter’s true surface is not solid thus differing radically from all the inner planets, being covered by thick dense clouds that descend to an unknown depths. The atmosphere general composition consist around 88% Hydrogen and 11% Helium, mixed in much smaller proportions with several gaseous compounds such as methane, ammonia, water and carbon monoxide. This is also intermixed with many more complex organic molecules. Jupiter to the naked-eye appears quite yellow - slightly lighter yellow than Saturn, but it is these colours in the telescope shows multitudes of reds, pinks or yellow-coloured shadings. These rich colours are formed by variety of ammonia-like compounds such as ammonium hydrosulfide. Several belts can be seen to display various shades and colours. The two most prominent are the so-called North and South Equatorial Zones each placed 9 to 10 degrees either side of the Jovian equator. These belts are produced by Jupiter’s complicated atmospheric dynamics that are quite different from what is observed by meteorologists on Earth. The planet comprises an outer atmosphere whose cloud tops are -125oC. This is surrounded by a lower thicker layer of gas around room temperature at pressures five times that of the Earth’s atmosphere. In the lower depths, this layer continues to get denser until you reach a solid core of crystalline metallic hydrogen or possibly even a liquid hydrogen and helium ‘sea’.

Jupiter also radiates more energy than it receives from the Sun, which was first discovered by radio astronomers in the 1950’s,. Energy emissions are thought to be due to Jupiter’s huge crushing gravity. Some other theories suggest the cause maybe atmospheric turbulence, like the Great Red Spot or perhaps some internal convection zones. Others think that thunderstorms could part of the cause.

The planet has been visited by several spacecraft including, Pioneer 10 and 11 (December 1973 and December 1974), Voyager 1 and 2, and the more recent Galileo. Vast amounts of data were gathered by the earlier brief encounters using various instruments onboard. However, during the last few years the Galileo spacecraft viewed the planet in much more detail, and has already gathered more data than all the other missions combined. Discoveries have include such things as the intense magnetic field, producing deadly radiation levels that would kill any human to a distance of several millions of kilometres from the planet. Intense aurorae have also been seen during each mission, included huge lightning discharges, faint rings, and verification of the complicated atmosphere dynamics. Numerous photographs have also revealed much complexity.

Jupiter is usually noted also for its four large moons named in order of distance Io, Europa, Ganymede and Callisto. First seen by Galileo in 1610, who imagined them all like a mini-solar system, these bodies telescopically are seen to change in positions over several minutes. We find both Ganymede and Callisto are both larger than the planet Mercury, and all of them are larger than our own Moon. The planet is surrounded by myriad of other tinier moons. Most are of little interest to the observer because they are hard to see even with the largest telescopes, each being only tiny pinpoints. In 1963 the number of Jovian moons was twelve, which increased to sixteen (1989), and thirty-two (2002). At last count the total number discovered has reached sixty-three in August 2005! (Saturn has the second most number of moons being currently around fifty (mid-2005).)


To the naked eye Jupiter appears as the second brightest planet in the sky preceding only the morning and evening 'star', Venus. Jupiter’ apparent maximum magnitude at opposition reaches -2.5 and decreases marginally to -2.0 at conjunction. The disk diameter subtends somewhere between 40 and 50 arc seconds, depending on the planet’s position in relation to the Sun. Minor differences in the observed size is also caused by the combination of a slight orbital eccentricity and the annular changes in Earth’s distance from the planet. Oppositions will occur once every 398.88 days, so that successive ones will be placed about thirteen months apart. Each yearly opposition will find the planet within a different zodiacal constellation.

Due to the fast rotation the disk features can be observed throughout the night. Even the smallest of telescopes will reveal some detail. Amateurs can make observations of the movement of the various features by simply timing each feature as it moves across the central meridian of the planet’s disk. Jupiter’s longitude of the central meridian is easily calculated via a ta ble, and is often given in any yearly ephemeris. Equatorial features - those roughly between the north and south equatorial belts (NEB and SEB) - move m ore rapidly than the more temperate and polar regions of the planet. Usually the equatorial region is called System I, while the rema inder - both north and south - is System II. System I period is 09h 50m 30.003s while System II is a longer 09h 55m 40.062s, leaving difference of five (5) minutes of time per rotation equivalent to three (3.0o) degrees in longitude per rotation. As a consequence it is not unusual to see over days Jovian features between the two observed systems to slide slowly past each other. [There is also another System III, which was discovered by radio astronomers in the 1950’. The true rate of this rotation still remains uncertain, however, a recent period of 09h 55m 29.7s is sometimes quote.]

Jupiter’s Great Red Spot (GRS) moves at an independent rate, and can be found on the polar boundary of the South Equatorial Belt and the South Tropical Zone (STZ). The GRS drifts slowly in longitude, but this motion can be irregular and moving either forwards or backwards. The GRS is thought to be a huge anti-cyclonic disturbance in the Jovian atmosphere which has persisted for more than 350 years - the time at least since when it was first observed telescopically by either Robert Hooke in 1665 or Giovanni Cassini in 1666. The GRS itself rotates once every six to eleven days where the atmosphere seemingly interacts with the equatorial band - swirling around the GRS like some eddy in a flowing stream of water. This is not the only change. Over the years the GRS has displayed a varying colour. Reports have ranged from a deep brick red, salmon pink, fawn or pale yellow, to even white, greenish or grayish. When colour brick red in the mid- to late 19th Century the Great Red Spot was far more prominent and easily visible in 7.5cm. In recent years the GRS has diminished somewhat, being almost half the size and clearly much paler. It requires 15cm to glimpse, but more easily seen using 20cm or above.

As the planet atmosphere is constantly and rapidly changing any new observations can add our knowledge of the atmospheric dynamics. Perhaps in time this may explain some long term variations that maybe happening over decades or even centuries. Although spacecraft when orbiting the planet provide adequate coverage, the surveillance is not actually continuous - therefore amateur observation or CCD imaging still remains important. This is especially valuable when the seemingly regular ‘outbreak’ of activity occurs in the Jovian atmosphere - alerting planetary astronomers to more detailed observation with either orbital or ground-based observatories. However, this is not the only observations that can be made. Timing the regular Jovian satellite transits and occultations can be made by amateurs, and these can improve our understanding of their orbits and perturbations over long periods of time.



Io must be one of the most bizarre worlds in the entire Solar System. The moon appears as mainly yellow-orange in colour, whose surface is intermixed with black and white regions randomly scattered across it’s orb, Io’s surface looks more like “pizza with olives” ! These rich colours are due the element Sulphur and other Sulphur compounds. Io is the only known body, other than the Earth, to show active volcanism. Examined in detail in 1979 by the Voyager spacecraft, these volcanoes spew and splatter large amounts of material across the surface and into its surrounds. Sulphur due to its chemical nature can exhibit many phase transitions, so that the colours can be from black, red, or yellow, and in mixtures, many other colour combinations. The Galilean moon is continuously pulled and kneaded by the enormity of the Jupiter’ gravity, keeping the inner core hot. This easily explains the continuous activity such as volcanism. Io’s surface is thought to change completely every 20,000 years or so. (Real Estate buyers beware!)


Is an ice world that may have oceans below its surface, and endures the same tidal distortions of Io, but these are less dramatic. Great fissures and fractures appear on the surface that have become frozen on the surface.


Ganymede is the largest of the Jovian satellites and is both rocky and slightly icy.


Callisto is heavily cratered and contains both rock and ices. The appearance of the surface is dark.

The other satellites are of little interest to the observer because they are hard to find even with the largest telescopes. Most are only tiny pinpoints around 18th magnitude.


The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for regarding either legal or medical purposes. Although the data is accurate as possible some errors might be present. The onus of its use is place solely with the user.


Last Update : 25th September 2005

Southern Astronomical Delights © (2005)


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