Hunting For Comets

Press, Volume CVI, Issue 31153, 1 September 1966, Page 12

Hunting For Comets

The recent discovery of a faint comet by Allan J. Thomas, at the Mount John Observatory, has prompted several correspondents to inquire as to how comets are located. At the present time there are four comets visible in the sky. All are faint objects and all have been found by photography. Two were discovered by accident in the course of other work, and two were found as the result of photographic searches around the predicted positions of periodic comets due to return to the Sun’s vicinity. Of the comets now visible the one which has been visible longest is known as Comet Van Biesbroeck, or 1965 d. Originally it was discovered in 1954. When its orbit had been worked out a return in 1965-66 was predicted. It was found by Elizabeth Roemer at the United States Naval Observatory’s station in Arizona in May, 1965, close to its predicted position. It was then an extremely faint stellar-like object Although it has brightened over recent months it still is faint and has a visual magnitude of 14. It is unlikely to get any brighter. During September it will be difficult to see, even in large instruments, because it will be crossing the Milky Way in the constellation Sagittarius. First of Year The first comet found this year was another periodic' object, Comet Neujmin I, or 1966 a. This was first photographed at the Boyden Observatory, in South Africa, on May 16. This comet comes closest to the sun next December. However it will get no closer than the distance of the planet Mars and because of this will not become bright. At present the comet is of magnitude 15 and during September will be in the southern constellation of Lupus, the Wolf.

The second comet discovered in 1966 was Comet Kilston, or 1966 b. This was detected on a plate taken at the Lick Observatory. This month this comet will move very slowly through Ophiuchus, close to where that constellation borders on Hercules and Serpens. It is tenth magnitude and has been seen with a twoinch telescope. In appearance it is fuzzy and without a tail. The comet independently discovered by Thomas at Mount John is the third found this year and will be known as 1966 c. It was discovered four days earlier by Barbon, in Europe, but his discovery was unknown in New Zealand until Thomas’s find had been reported. Thomas is responsible for taking the plates required for a photographic atlas of the southern sky. For this purpose he makes exposures each of 90 minutes on selected areas in the sky when they are crossing the meridian. Eventually he will obtain 145 plates which will cover the entire southern sky from declination north 20 degrees to the south celestial pole. Declination is merely an astronomer’s term for latitude in the sky. After developing his plates, Thomas compares them with various charts of the region concerned to see whether they show anything unusual. This was how his keen eyesight picked up the tiny fuzzy image that turned out to be comet 1966 c. Visually this object is of the eleventh magnitude and is hazy in appearance without any trace of a tail. It is somewhat brighter photographically and on the original plates shows a small central nucleus, surrounded by a coma from which there extends a very small tail. The comet is in the constellation Cetus and was found close to a fifth magnitude star, known as 20 Ceti. The comet is moving very slowly south-east. Systematic Searches

In the account of the foregoing discoveries we have two of the methods by which comets are found. First there is the photographic recovery of known periodic comets on plates exposed on their predicted positions. Second, there is the accidental discovery of comets on plates taken as part of some other programme. Quite often observers concerned with these programmes come to regard comets as nuisances because he is diverted from his main objective by having to take additional plates solely to obtain positions and motions of the comets discovered.

There are, however, a number of astronomers, mainly amateurs, who search for comets in a systematic manner. Generally their equipment is simple, comprising a fairly small telescope with a low magnification. This gives a wide field of view. Quite often the instrument is nothing more expensive than mounted binoculars, with a magnification of 25 times and thus giving a field of view covering three degrees. These people select certain areas of the sky and very carefully sweep over these, whenever the moon is absent from the sky. Since comets are generally brighter when close to the sun preference is given to the sky in the west and southwest after sunset, working up from the horizon. In the morning hours they search the region in the east and south-east before dawn. Amateurs’ Successes A person who sets out systematically to search for comets will eventually find new ones, provided he keeps at this work. One of the best known amateurs was W. F. Denning, in England, who discovered 5 comets. His discovery rate was one comet for every 119 hours of patient sweeping. That was before the days when photography resulted in most discoveries. A more recent experience has been that of another English amateur, G. E. D. Alcock. In 697 hours of searching he has located three new comets. In 1959 this observer had an exciting and unusual experience. In previous years he had spent 600 hours sweeping for comets. Then in August, 1959, he discovered two new comets within five days. But it is unlikely that he or anybody else, will surpass the record of Pons, who made 37 independent discoveries of comets.

Including comets recorded as far back as the Cinese Annals some 1700 comets are known. Normally an average of four or five are visible each year. Some years, of course, are much richer In discoveries than others. Occasionally a year will pass with only one or two comets being seen. It is quite a task to keep track of all these objects. As a result there are several methods by which they are named. Naming Comets

When a new comet is discovered it is given the name of the person who found it. Thus the second comet found this year is called Comet Kilston. Quite often several observers discover a comet independently, within a few hours or days of each other. In these cases the comet is given the names of all the discoverers stopping at the third name taken in the order of their cabled advices to the central information bureau of the International Astronomical Union. Thus the recently discovered comet could become known as Comet Bar-bon-Thomas, but this remains for a decision to be given. Sometimes a comet may be observed at several returns before it is recognised as being the one comet making several returns. Occasionally such objects become known by the name of the computer who proved that the various objects are in reality the one and same comet. The best known example of such naming is Halley's Comet. Titles Also Given In addition to the above names a Comet is given a title, comprising the year in which it was discovered followed by a letter. Each year these letters start again with “a” and run in the alphabetical order according to the dates oil which advice of the discoveries reached the Central Bureau. Thus Comet Kilston is also known as 1966 b. These designations are only provisional. Some years later after discovery each comet is given a definite designation. By this time the orbits have been computed. These definititive titles are assigned according to the chronological order of the perihelion passages of the various comets. Thus these titles comprise first the year of perihelion passage, followed by a roman numeral indicating the order of the comets passing perihelion in that year. Thus the comet referred to earlier as being found at the Boyden Observatory is known as Comet Neujim (1) 1913 111 indicating that it was the third comet in 1913 to P 385 through perihelion. The bracketed figure (1) after the name of the discoverer merely indicates that this was

the first of several discoveries by Neujim. On its present return it is also provisionally termed 1966 a. being the first comet found this year.

Prior to the discovery of comet 1966 c by Thomas the last comet discovered by a New Zealand observer and not found earlier elsewhere was Comet Jones, 1946 h. This was accidentally located by Albert Jones during the course of observations of variable stars and was detected visually. It is probable that with the photographic programmes in operation at Mount John there will not be another interval of 20 years before the next New Zealand discovery.