SOUTHERN ASTRONOMICAL DELIGHTS
J U P I T E R
Part 2 : OBSERVATIONS & The GREAT RED SPOT


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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. Jupiters apparent maximum visual magnitude reaches −2.5v at opposition and this decreases marginally to −2.0v at conjunction.

Disk diameter disk subtends somewhere between 30 and 50 arcsec, depending on the apparent position of Jupiter in relation to the Sun from the observer. Minor differences in the observed size is also caused by the combination of its slight orbital eccentricity and the slow annular changes in distance from the Earth to Jupiter. Oppositions 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 easterly zodiac constellation. If opposition if it happens in the early December, Jupiter will not have any opposition in the next year. If opposition occurs very close to mid-year, two oppositions that year are possible.

Due to the fast rotation of the disk, changes in the features can be observed throughout the night. For example, the equatorial regions, being roughly between the north and south equatorial belts (NEB and SEB), will move more rapidly than those in the more temperate or polar regions.

Usually the equatorial regions are associated with so-called System I, while the remainder – both north and south temperate / polar regions are known as System II. System I measured 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. This is equivalent to some 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 period known as System III, which was discovered by radio astronomers in the 1950s. The true rate of this rotation still remains uncertain, yet, the recent period of 09h 55m 29.7s is sometimes quoted.

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. Longitude of the central meridian of Jupiter is easily calculated via suitable tables as given in most yearly ephemeris. Even the smallest of telescopes will reveal some detail.


Observing the Atmosphere of Jupiter


As the Jovian atmosphere is rapidly and forever changing, so any series of new observations can add to 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 orbiting spacecraft do provide adequate coverage of the planet, the surveillance is not actually continuous — therefore amateur observation or CCD imaging still remains important. This is especially valuable when the seemingly regular Jovian atmospheric outbreaks of activity occurs. This alerts some planetary astronomers to more detailed observation, using either, orbital satellites or ground-based observatories.

Jupiter is divided into various belts and zones, which are mostly permanent areas along differing latitudes. There are three major latitudes, being usually broken into the singular Equatorial Zone, North and South Tropical Zones, North and South Temperate Zones, and the North and South Poles. Nearest to the equator are four main belts that can be considered permanent, but over time, may significantly vary in strength of colour, width and apparent brightness. Brightest and widest is the white or fawn equatorial zone, that sometimes displays a solitary central equatorial band. This zone is surrounded each side by much browner North Equatorial Belt (NEB), and the South Equatorial Belt (SEB) – the latter holding the Great Red Spot.

Figure. 2. Jupiter’s Atmospheric Nomenclature

Over time the boundary between these equatorial belts are not straight but are ragged and turbulent. Above either the two equatorial belts and equatorial band, incidentally half the area of the whole oval disk of Jupiter, are the boundaries above the tropical zone. These form the next two brown belts — North Temperate Belt (NTB) and the usually less prominent South Temperate Belt (STB). Sometimes these individual belts can split into two distinct separate belts, labelled as the North and North North Temperate Belts (NNB) – and similarly applied to the South. Ending the belts are the Polar Regions. These are often less active and paler than the regions closer to the equator, though the can be similar or differ greatly in colour and size. Frequently, and currently, the South Polar region has a lower latitude than the south, and is usually more obvious in the telescope. Sometimes small spots or streaks can be found there. In all, these given divisions are only approximate, as they can vary significantly over the years. Frequently new belts or spots are seen, where regions show breakouts of activity.


The GREAT RED SPOT (GRS)


Jupiter main visible atmospheric feature is the enduring Great Red Spot (GRS) that was probably first observed by the German observer, Heinrich Schwabe on 5th September 1831, who thought it as a hollow or a depression. For some time, it has been often quoted that the first to see the Great Red Spot was Robert Hooke in 1664, or even or Giovanni Cassini in 1666, but this has been considered as another different and unrelated spot. Since Schabe, the GRS has been seen to display various shades of orange or red, or even salmon red or as plain coloured yellowy buff white, while at times even has become very obvious, faded dramatically or has temporarily disappeared. It is found lying along the same boundary on the South Equatorial Belt (SEB) and the South Tropical Zone (STZ) at about 22°S latitude. Earlier observers thought that the rotation of Jupiter could be determined using it, but it was later shown to slowly but irregularly drifts in either direction in longitude, extending over years or decades.

Figure. 3. Jupiter’s Great Red Spot
Credit: NASA/JPL-Caltech/Space Science Institute

During the 20th Century, this overall drift has be seen the GRS make three complete circuits of Jupiter, while another more detailed analysis finds it moves backwards and forwards by about 4000 km. over a 90-day period. This provides evidence that suggests this localised part of the atmosphere is cyclonic and acts independently of the atmosphere seen in the belts. Close images of the GRS shows it is rotating in a counter-clockwise direction over the period of about a week. Also the spot size does vary significantly since regular observations were made during the 19th Century, whose oval or ovoid shape averages about 32,000±8,000×13,000±2,000 kilometres. Although likely a huge established anti-cyclonic atmospheric disturbance that has last several centuries, the cause of its persistence is still unknown to us. It might be something to do with dissipation internal heat deep within Jupiter itself whose subsequent convection feeds material into the spot. Colour differences between the GRS and rest of the atmosphere would then be caused be from other compounds generated from inside Jupiter itself.

The GRS is visible at least once each night when Jupiter is at opposition, however, the time it appears on the meridian does vary significantly from night to night. This is not the only change. Over the years the GRS has displayed a variety of colours, and reports by many observers have ranged from deep brick red, salmon pink, to fawn or pale yellow, and even white, greenish or greyish.

Historically, the first emergence of this famous spot can be traced back to the 1830s to 1850s. When noticed by visual astronomers, the area appeared more like a empty atmospheric hole or oval feature. Later the GRS was then seen to become more prominent between the oppositions of 1857 to 1859, to be seen as a pale dusky elliptical ring, before fading again. From about 1873 to 1881, the GRS became very conspicuous and bold in its colour, and in 1878, the spot turned significantly deep brick red. Three or four short years late, the spot faded almost completely, and became difficult to see for visual observers. Apparently from the mid– to late 19th Century the Great Red Spot appeared far more prominent and was easily visible in telescopes as small as 7.5cm. In more recent times, it also appeared quite prominent during the 1970s, but has gradually become more difficult to see in the mid-2010s.

From close visual observations, the GRS itself rotates once every six to eleven days, where the atmosphere seemingly interacts with the equatorial band. Looking at occasional small short-lived atmospheric features rotating around the spot identified this rotation, which seemingly swirls around the GRS like some giant whirlpool eddy – then to be torn apart by the stream, as flowing water goes down the proverbial sink.

Figure. 4. Jupiter and Its Great Red Spot
(WFC3/UVIS, 21 April, 2014)
Credit: NASA, ESA, A.Simon

Recently, the GRS has also diminished somewhat from its usual colour. Now it is almost half the size and clearly coloured as much more paler salmon or pink. In late-2012, the GRS has seemingly slightly shrunk in size, whose oval dimensions has been calculated as 18,560×12,400 km. and displaying the rotational velocity of about 135 km.s-1. The anticlockwise circulation of the spot seems to be accelerating, while the rate of movement in its drift continues to decrease, which has been more noticeable in the first decade of the 21st Century. By April 2014, the calculated size is an even smaller 15,720×10,400 km. or 14%, whose area is now 128.4 million km. — 52.4 million kilometer less in just eighteen months. Some alleged this is the smallest the GRS has ever been observed. (See Fig. 4.)

Observers are still uncertain of the future behaviour of the GRS, or whether it will continue to persist, again disappear from view for a time, or perhaps never return again. Others think it might disappear for a while and be then replaced by another large spot.

Presently the Great Red Spot requires 15cm. (6-inch) to glimpse, but more easily seen using 20cm. (8-inch) or above. When Jupiter is near opposition, the GRS is visible at least once each night, however, from night to night, times for its meridian crossing will vary significantly.


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. Onus of its use is placed solely with the user.

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Last Update : 16th May 2014

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