J U P I T E R

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 2½ 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 girth of the planet and 133 708 km. at the poles — being flattened by about 6.5%. Also the planetary axis is only slghtly tilted at 3° 07′ to the ecliptic, being the smallest of all the planets. This means that the overall shape appears telescopically to be fairly constant.

Differing radically from all the inner planets, the surface of Jupiter is not solid. It is covered by thick dense clouds that descend to an unknown depths. The general composition of the atmosphere 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 and yellow-coloured shadings. These rich colours are formed by variety of ammonia-like compounds such as ammonium hydrosulfide. Several main belts and equatorial zones 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 main belts are produced by complicated atmospheric dynamics of Jupiter that are quite different from what is observed by meteorologists on Earth. The planet comprises an outer atmosphere whose cloud tops are -125°C. This is surrounded by a lower and thicker layer of gas around room temperature at five times the atmospheric pressure on Earth. Into the lower depths, this layer continues to get denser until you reach a probable solid core of crystalline metallic hydrogen or even possibly existing as a sea of liquid hydrogen and helium.

Jupiter also radiates more energy than it receives from the Sun, which was first discovered by radio astronomers in B. Burke and K. Franklin in late 1954 from Seneca in Florida. Energy emissions are thought to be due to the the sizeable magnetic field of Jupiter and the electrical currents by non-thermal methods by way of synchrotron radiation. The Jovian magnetic field slowly wobbles by about 11° to the rotational axis, effecting the nature of the radio emissions. Other radiation suggest some of the additional causes may derive from large atmospheric turbulence in either the Great Red Spot or perhaps some internal convection zones. Others think that thunderstorms could significantly contribute to the .

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 Jupiter 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 the 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 like some 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. Jupiter is surrounded by myriads 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).)

OBSERVATIONS of JUPITER

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 visual magnitude at opposition reaches -2.5v and decreases marginally to -2.0v at conjunction. Diameter of the disk 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 its slight orbital eccentricity and the annular changes in distance from the Earth to Jupiter. 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 the next different zodiac 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 planetary disk. Central meridian longitude of Jupiter is easily calculated via suitable tables as given in most yearly ephemeris.

Equatorial features — those roughly between the north and south equatorial belts (NEB and SEB) — move more rapidly than the more temperate or polar regions. Usually the equatorial region is called System I, while the remainder — both north and south — is System II. System I period is 09h 50m 30.003s while System II is longer at 09h 55m 40.062s, leaving difference of five minutes of time per rotation equivalent to three degrees (3.0°) in longitude per rotation. Consequently it is not unusual to see over many days Jovian features between the two observed systems slowly slide past each other. There is also another System III, which was discovered by radio astronomers in the 1950s. The true rate of this rotation still remains uncertain, however, the recent period of 09h 55m 29.7s is sometimes quote.

Jupiter main atmospheric feature is the Great Red Spot (GRS) that moves at an independent rate of the rest of the atmosphere. and can be found on the polar boundary of the South Equatorial Belt (SEB) 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 360 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, which swirls around the GRS like some eddy in flowing streams of water. This is not the only change. Over the years the GRS has displayed a varying colour. Reports have ranged from deep brick red, salmon pink, fawn or pale yellow, to even white, greenish or greyish. 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 more recent years, the Great Red Spot has diminished somewhat, being almost half the size and clearly much paler. It now requires 15cm. (6-inch) to glimpse, but more easily seen using 20cm. (8-inch) or above.

As the Jovian 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 atmosphere — alerting planetary astronomers to more detailed observation with either orbital or ground-based observatories.

Disclaimer

The user applying this data for any purpose forgoes any liability against the author. None of the information should be used for 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 : 17th September 2008

Southern Astronomical Delights © (2008)

For any problems with this Website or Document please e-mail me.