Fourth Meeting: 16th August, 1909.
Professor C. W. Egerton, President, in the chair.
Professor H. W. Segar, M.A., delivered a lecture on “Comets.”
The lecturer dealt in the first instance with the general features of comets. The special features of the more famous comets were then described. Halley's Comet, in particular, was treated at some length, in consideration of its approaching reappearance. The great fame of this comet results firstly from the fact that it was the first comet to be discovered as periodic. Halley, in making this discovery in 1705, made one of the earliest practical applications of the knowledge first given to the world in 1686 in Newton's “Principia.” A second feature that makes Halley's Comet famous is that, of all the comets that have reasonably small periods—say, less than a hundred years—it is the only one that ranks amongst the great comets which are conspicuous to the naked eye. The varied attractions of the planets make the period of Halley's Comet vary from about seventy-four to seventy-nine years. Calculations made for its next appearance by prominent astronomers differ by several weeks in their results as to the time of perihelion; but it seems reasonable to believe that the comet will be nearest the sun not earlier than about the 12th April or later than about the 10th May next. The prospects of the approaching visit are bright for the Southern Hemisphere, in spite of the fact that appears to be a necessary deduction from the nature of a comet's tail—namely, that the mass of a comet is less after each visit to the neighbourhood of the sun. The circumstances of the coming visit are very similar to those found to exist in 1066, when the comet was a great object even in the Northern Hemisphere. When nearest the sun, and for some time after, when the comet will be at its best, it will be a morning star, and must be seen before sunrise. Owing to the comet lying near the ecliptic, and to the fact that in these latitudes at that time of year and before sunrise the ecliptic cuts the horizon at a large angle, and lies high in the heavens, the comet, rising as much as an hour and a half before the sun, will be well up in the sky before the sun rises, and will be seen for some time without much interference from twilight, with the tail stretching upwards from the horizon. In the northern latitudes, however, the reverse will be the case: the ecliptic will lie low in the heavens, and the sun will rise very soon after the comet.
There will be a total eclipse of the sun on the 8th May, 1910. It may be possible to see Halley's Comet during this eclipse. Such a view of the comet would be exceptionally good, for the sun's light would be cut off before meeting the atmosphere, and the trouble from twilight, which is an atmospheric effect, would be eliminated. The line of totality of the eclipse, however, only touches land at its north-eastern extremity in Tasmania. Further, it seems possible that the comet may have set below the horizon before totality is reached. We shall want to know more certainly where the comet is in its course before we can decide. The earliest photographic observations of the comet, which are sure to be made within the next few weeks, will give us the necessary information. If the event should turn out favourably, there should be many a pilgrimage made to Tasmania on the occasion.
In concluding his lecture, Professor Segar said that this was the tercentenary of the first making of a telescope by Galileo, and its first application to astronomy. It was worth considering whether the time had not arrived when a national observatory should be established in this country. New Zealand stood easily third in population and wealth among the Australasian States. New South Wales had established a fine observatory, and Victoria had set up a great telescope before they had attained the position in which New Zealand stood to-day. We inherited the scientific knowledge discovered by the genius of our ancestors; it should be both our privilege and duty to do all possible to further extend its boundaries.