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OBSERVING GUIDE: JUPITER and its GREAT RED SPOT
by: Clay Sherrod
Physical Characteristics fo the Giant Planet
Detailed observations of Jupiter's Great Red Spot (GRS) in 1975, nearly five decades ago, at the Arkansas Sky Observatory revealed a crimson-red and large oval, the shape and color normally associated with this wonderful cyclonic feature first noted by Cassini nearly 400 years ago in 1660. It is believed that this is the famous spot through which Cassini actually first determined the reasonably accurate rotational rate of the gas giant. However, it was not long after observations in 1975 when this famous celestial bookmark began to show signs of fading both in intensity and color; within only a few years the spot had all but disappeared from view, noted ONLY though is absence in an indentation in the South Equatorial Belt (SEB) that is known as the "Red Spot Hollow" (RSH). [ P.C. Sherrod, Project Publication 003, Midsouth Astronomical Research Society, 1975).
Thus, for the next thirty years during which time the GRS was all but invisible, the exact rotational rate and drifting in both latitude and longitude was measurable through where it was "supposed to be" rather than where we actually "saw it to be." Such remarkable fading of the GRS is not at all unusual and, in time, the spot will eventually return to its characteristic intensity and color.
The GRS, like most features on both north and south latitudes from the North Equatorial Belt and South Equatorial belts, respectively, moves in a slightly slower rotational rate - 9 hours 55 minutes 40 seconds - than the Jovian equator itself - 9 hours 50 minutes 30 seconds. Since the GRS is actually positioned latitudinally in System II, but intrudes northward into the SEB which is in System I (see latest drawing, below) you can image how it "stirs the waters" so to speak as it attempts to retard the motion of clouds, storms and anomalies located in the faster equatorial stream.
The faster-moving System I is confined to a narrow band centered on the Equatorial Belt that spans a total of about 10,000 to 15,000 miles north-to-south, all moving about 200 miles per hour FASTER than the clouds and features seen north and south of it. In the second figure below more of the planet is exhibited from the following morning's observations which reveal the "opposite side" of Jupiter, nearly 180 degrees from the GRS shown in the more narrow scale view in the first drawing. NOtice the remarkable intensity of the North Equatorial belt (both components).
Since reappearing in morning twilight in fall of 2001, the GRS has shown a slight intensity change that has continued until today. A sample drawing of this region is shown below. It was made on the morning of September 3, from 10:53 Universal Time to 11:20 UT, with the steadiness of the air rated at "8" on a scale of 1-10 with "10" best. It was drawn using the 0.31m (12") Meade Schmidt-Cass at the Observatory at 625x using no filters. NOTE that drawings of Jovian features MUST be made very quickly (albeit very accurately as well!) since the rotational rate of the visible features results in very rapid movement across the center of the planet (Central Meridian - "CM"). This drawing was made within minutes of the CM crossing, which allows very accurate determination of Jovian longitude.
INDEX TO NOMENCLATURE:
- Zones (brighter areas)
NNNTZ - North-North-North Temperate Zone
NNTZ - North-North Temperate Zone
NTZ - North Temperate Zone
NTrZ - North Tropical Zone
NEZ - North Equatorial Zone
SEZ - South Equatorial Zone
STrZ - South Tropical Zone
STZ - South Temperate Zone
-Belts (dark "stripes") -
NNNTB - North North North Temperate Belt
NNTB - North-North Temperate Belt
NTB - North Temperate Belt
EB - Equatorial Belt
SEBn - South Equatorial Belt, north
SEBs - South Equatorial Belt, south
STB - South Temperate Belt
SSTB - South-South Temperate Belt
1. Red Spot Hollow: you can clearly see the GRS hollow indenting the SEBs, and thus creating the Great Spot Hollow. In smaller telescopes, the GRS will likely ONLY be noted by observing this hollow at least early in the 2001 apparition, since the spot itself is very vague and lacks much contrast or color at this point. Note from the drawing how the SEBs preceding (westward) the GRS appears to be disturbed by the intruding motion of the GRS itself; indeed, a small "festoon" can be seen just west of the GRS intruding into the STrZ as though "splintered" or pushed southward from the SEBs.
2. South Equatorial Belt: Note in the above drawing both "Systems" are rotating from right to left (east to west); however, it is important to note that System I (usually the SEB - particularly the north component - , the Equatorial Zones and the North Equatorial Belt (not shown) is moving FASTER from right to left than the GRS and the rest of System II; this is interesting in light of the disturbance following (east) of the GRS within the space between the SEBs and SEBn. Such activity is common where this differential rotation interaction occurs and many times will lead to major disturbances following the GRS and extending far south into the STrZ. The color of the SEB (both north and south components) is currently a bluish-brown and nearly equal to both intensity and color to that seen in late 2000.
3. The Great Red Spot: The GRS itself presents a somewhat "football shape", not at all uncommon. The size is an incredible 14.5 degrees in width. The actual size of any Jovian feature can be determined by the simple formula: x = (E) x 1187, where "X" equals the actual size in kilometers of the feature (in this case the GRS), "(E)" is the longitudinal expanse (14.5 degrees) and "1187" is the number of kilometers per degree of longitude seen on Jupiter. [from: A Complete Manual of Amateur Astronomy, P. Clay Sherrod, Prentice Hall, 1981). Hence, the present size of the GRS is some 17,211 kilometers east-to-west....easily large enough for the earth to nestle within with plenty of room to spare! Although this is very large by any standard (10,671 miles), the GRS has been recorded as large as 30,000 miles in width - enough to encompass THREE Earths!
At present, do NOT look for a "RED" spot.....look for a large pink oval that intrudes northward into the very conspicuous SEBs. As seen in the drawing above, the GRS is not at all uniformly dark nor colored. The west, or preceding, end of it is much lighter and without much color, while the eastern (right in the drawing) side is most definitely a ruddy pink color. An unusual aspect of this spot right now is the wonderfully defined pink "outline" that completely inscribes the perimeter of the GRS! This is NOT seen in telescopes smaller than 8" (0.2m) aperture, but is unmistakable in larger instruments.
4. North Equatorial Belt: Although not shown in the strip drawing above, the North Equatorial Belt (NEB) is the darkest feature by far on the planet, exhibiting a very intense orange-brown color; it - like the SEB - is showing TWO components that seem to be nearly perfectly parallel for the entire 360 degree expanse of Jupiter. The vivid darkness and color, as well as this duplicity, is something that cannot be missed in common telescopes of all sizes. The color, particularly in higher longitudes as shown in the second drawing above, is clearly a very reddish brown; note the white oval at 219 degrees System II that is shown in this drawing and extending into the North Tropical Zone
This drawing, done on September 5, 2001, demonstrates a very dark and large elongated oval, or "Bar" in the tiny zone between the North and South Components of the North Equatorial Belt. Because of this location, more observations will be required to determine if this feature is moving in System I or System II (see text). This appears the same intensity as a dark shadow cast from a satellite would, but it is clearly elongated and smaller in N-S width, yet is visible easily in a 5" telescope under high magnification and good seeing.
Observers who wish to determine the exact time of the transit of the Great Red Spot and other features, need only to download onto their computers the fabulous "Meridian" program by Claude Dulpessis, of Quebec. This will provide ephemeris data of EVERY planet, and for Jupiter will determine for your exact location and times the passage of the GRS across the central meridian. This freeware is available at: http://pages.infinit.net/merid/index.html . Note that there is also a CUSTOM central meridian passage tool available here at ASO under the ONLINE TOOLS tab.
For complete tips on observing Jupiter, please read completely ALL Jupiter Observer's Guides on the ASO website.
Jupiter will dominate the evening skies of winter in the northern hemisphere and will be ideally located for both north and south of the equator for nearly nine months in 2001 and 2002. Observing Jupiter will allow you to move from a "casual" observer to a contributing "avocational astronomer" through your drawings, timings of features and general patrol observations that can lead to discoveries of storm outbreaks and changes on the giant planet. For more vital information regarding Jupiter and your contributions that are valued for this planet, go to: http://www.alpo-astronomy.org/jupiter/sendobs_0708.html for information concerning the Association of Lunar and Planetary Observers (ALPO), forms for recording and drawing and much valuable information and links to YOUR observing success and satisfaction with the greatest planet of our Sun's remarkable neighborhood.
Wishing you the brightest in discovery amidst the darkest of skies.....