THe New Oomet

Progress, Volume II, Issue 11, 2 September 1907, Page 396

The New Comet

(By Rkv Dr Kennedy lleanee, Hawkes "R vv )

The comet announced on July 28th by the French astronomer Flammarion, was observed in several places in Australia and New Zealand on the following morning, but on account of cloudy weather it was not until August 4th that it could be seen at the Meanee Observatorj^. On that morning the comet was too near the sun to get a good view of its tail ; still, two fairly good photographs of it were taken — one with the 9-inch Cooke photo. -visual refractor, the other with a 31inch Dallmeyer portrait lens. The portrait lens, working at F 3.5, photographs much more quickly, though on a smaller scale, than the 9-inch object glass, which works at F 17. The picture taken with the latter, during an exposure of about 20 minutes, showed very well the comet's head, consisting of a bright central nucleus and a surrounding hazy

coma, but the tail was not very distinct. The' photograph taken with the portrait lens, during the same time of exposure, showed the tail very distinctly, consisting of two long trains and two faint shorter streamers. A longer exposure could not be given on account of the proximity of the moon, the light of which would have fogged the plate. As the photograph of the crescent moon appeared on the same plate, it afforded an easy means of calculating the dimensions of the comet. It was found that the comet's head, including the coma, was about 225,000 miles in diameter, and'that the longest tail was about 9.000,000 miles in

length. These dimensions, though enormous, are not extraordinary for a comet. The head of Donati's comet, in 1858, was 250,000 miles in diameter ; that ol 1892 had a diameter of over 700,000 miles; while one in 1811 measured nearly 1,200 000 miles across the head The great comet of 1882 had a train 100,000,000 miles in length, longer than the distance between the earth and the sun On August sth another photograph was taken, but as clouds intervened, only a short exposure could be given. However, it was sufficient to show the exact position of the comet and the distance through which it had travelled during the previous 24 hours It was found that it had travelled over a distance of 3| million miles during that time, and was consequently moving at the rate of about 40 miles per second, or more than 200 times as fast as a rifle bullet. This again is not an extraordinary speed for a comet. Some ha\ c been known to travel at the rate of 200 miles a second, that is, 1000 times as fast as a rifle bullet, when whirling around the sun A few months ago a paragraph appeared in several papers in Europe, as well as in New Zealand, stating that Professor Matteucci, o f the Observatoty of Mount Vesuvius, had announced that a collision between the earth and a comet would take place in March, with the awful consequences that are generally supposed would attend such an occurrence. But the report was merely an astronomical hoax, invented by some journalist who was probably short of exciting news ; for the Professor

wrote to the Italian newspaper in which the statement originated, denying that he

bad made such a suggestion. However, it may be asked, is sucli a thing as a collision between the earth and a comet possible, or likely ever to happen ? As regaids the possibility of such an event, it must be admitted that it is quite possible. In fact, it is almost sine to happen some day, ii the earth lasts long enough ; for there are several comets whose orbits are close enough to the earth's orbit to bring them into collision with the earth, should the comet and the earth happen to arrive simultaneously at the points in their orbits which are in closest proximity. But such collisions are not likely to happen frequently, for it is estimated that they may occur about once in 15 million years. And even it such a collision did take place, it is by no means certain that it would be attended with any disastrous consequences to the earth. For while the volumn of

a comet is generally enormous, its mass is very small as compared with that of the earth. The tail, being composed of gas in a very rarefied state, is so flims}' that small stars can be seen through it, even when more than a million miles in diameter, without any perceptible diminution of brightness. As for the head of the comet, the central bright part called the nucleus appears to consist of a swarm of meteoric stones ; but whether these stones are many feet in diameter, or only a few inches, or only a few thousandths of an inch, it is impossible to say. If these solid particles should happen to be very large and to weigh several

tons, the bombardment experienced by the -\ earth when struck by a comet would be a i very serious matter ; but if, as seems more ( probable, they are very small, the result of i a collision with a comet would be merely a ■ beautiful display of meteors, or shooting stars. ! It appears that in the year 1861 the earth actually passed through the tail of a comet, ] without any disastrous consequences to any i one On a Sunday evening in the midsummer ( of that year a clergyman was reading his sermon in the church without any artificial light, when suddenly the sky became so overcast with a peculiar glow, that the sexton had to bring a pair of candks to enable the clergyman to get through his sermon. The glow, it appears, was caused by the tail of a comet which happened to come into collision with the earth just at that time ; and there is no record of any other disaster resulting from the collision than the expense of a pair of candles. As a rule comets visit us, like the present one, unannounced and unexpected They come to us apparently from the depths of interstellar space, approach the sun with ever-increasing speed, whirl around it La a path which has generally the form of a parabola, and disappear into the depths of space, never to return. There are a few comets, however, which move in elliptical orbits and return to the sun periodically. One of the most remarkable of these periodic comets is Halley's comet, which was the first whose return was predicted. This comet is especially interesting at the present time, as it is expected to return again in 1910. When the English astronomer Halley observed this comet in 1682, he found that its orbit corresponded with that of a comet which appeared in 1607, that is, 75 years earlier, and also with that of another comet which appeared 76 years earlier still, itz., 1531. On examining the record of still older comets he found that other great comets had appeared previously at intervals of about 75 or 76 years, one going back to 1066, the year of the Norman Conquest. Halley came to the conclusion that these were not all different comets, but only different appearances of one and the same comet ; and he ventured to predict that the comet of 1682 would return again in 76 years, that is. at the end of 1758, or in the beginning of 1759, as he calculated that the comet would be retarded by the action of Jupiter and Saturn. As 1759 drew near an astronomer called Clairvaut calculated how much the comet would be retarded in its course by the action of these planets. As the result of a most laborious investigation, he predicted that the comet would be in perihelion, that is, in its nearest position to the sun, about April 13th 1759, or perhaps a month earlier on account of disturbances from other planets. Great was the admiration of the astronomical world when the comet actually came to perihelion on March 13th of that year. At its next return, in 1835, it appeared within two days of the predicted time. And now astronomers are busy calculating the exact date of the comet's k next perihelion passage. After allowing for -y the action of Jupiter, Saturn, Venus Neptune and of the earth itself on the comet, they have come to the conclusion that the next perihelion passage of Halley's comet will be about the 6th or 23rd May, 1910 It will probably be visible, though very faint, in November 1909, appearing in the constellation of the Bull, not far from Hyades where the present comet first appeared. It will attain its greatest brilliancy about June 12th 1910, when it will be near the bright star Capella, in the constellation of Auriga. As very faint nebulosities can now be photographed which

would be utterly invisible in the most powerful telescope, there is a chance that Halley's comet may be photographed in 1908, or, as some think, perhaps even in October or November of the present year. Fig. 1 of the accompanying photographs was taken on the morning of August 4th from 4-45 to 5-5 a.m. The light of the moon interfered with the photograph to a great extent The cluster of stars to the left of the tail near the top is the group called the Hyadts in the constellation of the Bull. The brightest star near the central part of the tail is Aldebaran. Being a reddish star it does not come out so brightly in the photograph as it appears to the eye Fig. 2 was taken with a 3iin portrait lens on August 15th from 4-45 to 5-30 a.m. with an interruption of about 20 minutes on account of clouds In the interval the star images trailed while the telescope was