Every now and then part of the material may break away from the cloud, and under the influence of gravity, it accelerates towards the sun as a comet. These comets, travelling in parabolic orbits, are known as long period comets and by definition have orbital periods greater than 200 years, though the actual periods are generally a few thousand years or more. Occasionally the orbits of comets travelling in the same plane as the planets may be perturbed by the gravitational effects of the major planets, mainly Jupiter, into elliptical orbits. These comets have shorter periods, by definition less than 200 years, and since their orbits are known fairly precisely, their returns can be predicted with some degree of accuracy. Table 12 lists comets predicted to appear during 2009, which are forecast to become brighter than about magnitude 12. It is extracted from the list of comets for 2009 from the BAA Comet Section website by Jonathan Shanklin. The table does not, of course, include any new comets which might possibly be discovered during the year. A guide to observing comets was given in MNASSA February 2007 pages 29 -34.
In the cold depths of space, comets are no more than chunks of frozen gases, ices and dust. However, in the vicinity of the sun the constituents of the nucleus vaporise and the gases and dust form a coma around the nucleus. Under the influence of the solar wind the gas and dust in the coma is swept away to form the tail, which always points away from the sun.
* From May 12 1811 a comet was visible from Cape Town for a considerable period. When Capetonians subsequently experienced an earthquake on June 7 of the same year, some felt that the town was doomed.
*The appearance of a comet indicated to the inhabitants of Morocco the coming of a year of prosperity and happiness.
* The !Kung of the Upper Omuramba, consider comets a favourable omen, saying that they are "the stars of the great Captains".
* Generally speaking comets were considered to be portents of disaster. A Ndebele attack followed in the wake of the comet of 1816 and the comet observed from Shaka's Kraal in October 1824 was perceived as a warning. This warning proved correct as they were soon thereafter attacked by an enemy force.
The Director of the Comet and Meteor Section welcomes all observations of comets, but to be of scientific value the observer should concentrate on the following:
* Estimates of the total visual magnitude of the comet, preferably made over the entire apparition to allow construction of a light curve.
* Estimates of the diameter of the coma
* Estimates of the degree of condensation of the comet
* Estimates of the length and position angle of the tail
* Detailed visual descriptions, sketches and photographs of the comet
In making the above observations it is essential that the observer uses the standard procedures developed and used by observers world-wide. Detailed notes on observing techniques and visibility of comets may be obtained from the Director at the address below. Novice observers should note that comets are notoriously unpredictable and that the predicted brightness in Table 12 is given as a guide only. The magnitude given is the total magnitude of the coma and the brightness is spread out across the whole diameter of the comet. For this reason the comet will appear much fainter than a star of the same magnitude. As a guide, a comet of magnitude 10-11 would appear about as bright as a star of magnitude 12-13.
Comets are brightest when near the Sun and are often visible only in twilight. Every ten years or so a bright comet with a prominent tail makes its appearance. Comets typically have two tails, a dust tail, which is easiest to see, and an ion tail, only visible in some very bright comets. When visible, the ion tail is oriented at a slight angle to the dust tail and is bluish in colour.
Comets move at about the same speed as planets and do not shoot across the night sky as is the popular belief. Instead, they rise and set along with the stars. Only close observation reveals their slow motion relative to the starry background.
* The Sotho people knew comets as naledi tsha mesela, "stars with tails". The Tsonga call comets shimusana or nyeketi ya musana, "stars of dust".
* A dramatic painting found in a rock shelter in the Free State, depicted a group of people dancing with objects streaking above them and breaking into two pieces.
* Two other possible fireball images in the area depicted objects with circular heads followed by long straight tails.
* "The Great Southern Comet" of 1887 was well known in East Africa. The Kamba called it "the star of the famine" because of the subsequent great famine.
Comet & Meteor Section
Details on how to observe either comets or meteors are available from the Director of the Comet and Meteor Section, T P Cooper, PO Box 15336, Farrarmere, 1518. e-mail [firstname.lastname@example.org]
Quick facts about comets
Long-period & high-inclination short-period comets Source region: Oort Cloud, [10.sup.3] - 105 AU distant
Low-inclination short-period comets Source region: mainly Kuiper Belt, 30 - [10.sup.3] AU distant
Short-period comets (orbital period P < 200) average number of apparitions per year: 17 typical discovery rate per year for new comets: 6 average semi major axis: 5.8 AU
Long-period comets (orbital period P > 200 yr) typical discovery rate per year for new comets: 6 semi major axes: [10.sup.2] - [10.sup.5] AU
Number of known periodic comets seen at more than one apparition (as at September 16 2008): 205 Total number of observed comets: > 2400
nucleus diameter: 1.0 - 40 km (Halley = 16 x 8 x 7 km) mass: [10.sup.14] - [10.sup.19]g (Halley = [10.sup.17] - [10.sup.18]g) mass loss per apparition: ~1% of total mass
coma radius: [10.sup.4] - [10.sup.5] km
hydrogen cloud: radius: [10.sup.7] km
ion tail (type I) length [10.sup.6] - [10.sup.8] km direction: anti solar
dust tail (type II) length: [10.sup.6] - [10.sup.7] km particle size: 0.1 - 100 microns direction: initially anti solar, becoming curved as dust particles follow independent orbits.
Table 12. Comets reaching perihelion during 2009 Comet Designation Date N Mag Christens P/2003 K2 Jan 09 09.00 Lulin C/2007 N3 Jan 11 06.00 Kushida 144P Jan 27 11.00 Kopff 22P May 25 09.00 Wild 116P Jul 19 11.00 Schaumass 24P Aug 10 11.00 Howell 88P Aug 12 09.00 Siding Spring C/2007 Q3 Oct 19 10.00 Key: Date: Perihelion date 2009; N Mag: Possible maximum brightness the comet could reach during its apparition.
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|Publication:||Sky Guide Africa South|
|Date:||Jan 1, 2009|
|Next Article:||Meteors or 'shooting stars'.|