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 2010, which are forecast to become brighter than about magnitude 12. It is extracted from the list of comets for 2010 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.
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 bV observers world-wide. Detailed notes on observinq techniques and visibility of comets may be obtained from the Director at the address below. Novice a 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.
Quick facts about comets
Long-period & high-inclination short-period comets Source region: Oort Cloud, [10.sup.3]-[10.sup.5] 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
Physical parameters: 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.
African lore ...
* Some of the tribes living around Mount Kenya, namely the Chuka and Embu, attributed the appearance of Halley's comet in 1910 to the fact that King Edward VII had died.
* Generally speaking, in Xhosa culture, a comet Intshobololo is considered to be bad luck and people associate them with famine and the death of livestock. When a comet comes close to the Sun and develops a long tail, people say it is spraying diseases on the Earth.
* One of the Zulu terms for a comet is inkanyezi enomsile. Curiously, the root -sila can signify both the tail of the comet and the bad fortune it brings.
* 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 Xhosa dubbed the 1910 return of Halley's comet Uzatshoba, or "bushy tail".
* 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.
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 14740, Bredell 1623. e-mail [email@example.com]
Table 12. Comets reaching perihelion during 2010 Comet Designation Date Mag Wild 81P Feb 22.7 09.0 McNaught C/2009 K5 Apr 30.6 10.0 Tempel 10P Jul 04.9 10.0 Encke 2P Aug 06.5 04.0 Hartley 103P Oct 28.3 05.0 Key: Date: Perihelion date 2010; Mag: Possible maximum brightness the comet could reach during its apparition.