STARRY HEAVENS

Southland Times, Issue 21773, 30 July 1932, Page 11

STARRY HEAVENS

NOTES FOR AUGUST. (By R. D. Thompson, M.A., M.Sc., F.R.A.S.) Total Eclipse of the Sun. A total eclipse of the sun occurs on August 31, the belt of totality (the region from which the eclipse will be seen as total) extending from the North Pole down through the north-eastern part of Canada (including Montreal) and out to about the centre of the North Atlantic Ocean. A partial eclipse will be visible throughout the whole of North America and • the northern part of South America. The belt of totality is only about 100 miles in width, and in order to observe .the total phase of an eclipse astronomers are usually compelled to undergo the cost and labour of transporting the necessary instruments and equipment to some more or less inaccessible region. It is rare indeed to find a fully equipped observatory situated within the narrow limits of the belt of totality, but such is the good fortune attending the present eclipse. At the McGill University Observatory, in Montreal, the eclipse will be total for about 36 seconds, and although this period is short compared with the maximum duration of about two minutes obtaining at other ■ localities, no doubt full advantage will be taken of the facilities for observation afforded by the Observatory. The eclipse occurs at 3.25 p.m., Eastern Canadian time, which corresponds to about 8 a.m. on September 1, New Zealand time. The Planet Saturn. Saturn is now conspicuous in the evening sky. Rising in the late afternoon, it becomes visible soon after sunset fairly high above the horizon in a direction almost due east, and may readily be distinguished by its steady, yellowish glow, and by the absence of the “twinkling” effect usually characterizing the brighter stars. The planet at present crosses the meridian at 11.30 p.m., when it appears high overhead slightly towards the north, but by the end of the month it will reach the meridian soon after 9 p.m. Saturn is moving very slowly towards the west amongst the neighbouring stars, and will be prominent in the evening skies until well on towards the end of the year. The Dull and Gloomy Planet.

Saturn was the most distant planet known to the ancients, and on account of its apparently slow movement in the sky they attributed to Saturn the qualities still spoken of as “saturnine.” It was regarded as the dull, sluggish and gloomy planet, and for its symbol they chose the dull and heavy metal lead. The contrast with the reality is striking, however. Saturn certainly has a comparatively slow relative motion, requiring as it does a period of 30 years to make one complete circuit of the sky, whereas Jupiter requires only twelve, yet on the other hand Saturn is far from being heavy and lead-like, and is, in fact, the lightest in substance among all the planets. The surface of the planet is by no means sluggish, but is even more turbulent than that of Jupiter, and this dull and uninteresting planet is considered by many as the most beautiful object to be seen in a telescope. The Rings of Saturn.

Saturn, as far as is known, is unique among the heavenly bodies in possessing a system of rings surrounding the planet in addition to numerous satellites. The general appearance of these rings is no doubt familiar to everyone—even a small telescope shows Saturn encircled by “a flat ring, nowhere touching the planet” to quote the words of Huyghens, who in 1655 first investigated their true nature. From the time of Galileo onwards the changing appearance of Saturn had been a great puzzle to astronomers, for their small telescopes of poor defining power were inadequate to reveal the true ring system, and moreover the appearance of the rings depends upon the angle from which they are viewed. When they are “edgeways on” towards the earth, they become completely invisible, and since the revolution periods of Saturn is about 30 years, they disappear from sight every 14 or 15 years. They are at present at an unfavourabvle angle for observation, and will pass through a period of “disappearance” in about three years’ time. Since the rings apparently vanish in this way when viewed edgewise, they must’ be very thin; in fact, it has been estimated that their thickness cannot be more than 50 miles, and may actually be very much less. Cassini in 1675 first discovered that the ring surrounding Saturn was double, an oute: ring about 11,000 miles wide being separated from a slightly smaller but brighter inner ring by a narrow black division about 2000 miles in width. In 1850 the American astronomer, Bond, and Dawes, in England, independently discovered a third inner ring, called the “crepe” ring from its dusky appearance, which is so transparent that the disc of the planet may be seen through it. It has also been claimed that there is an outer dark ring as well, but this has never been definitely substantiated. The Nature of the Rings.

It is impossible to suppose that the rings consist of a continuous flat sheet of solid matter—such a sheet would inevitably break up under gravitational strain. Moreover, some thirty years ago Keeler was able to prove definitely that the inner portions of the ring were rotating about the planet at a much greater speed than the outer parts. It is now generally accepted that the rings are composed of a swarm of solid, separate particles, of which the exact size is not known. There is reason to believe that these particles must be very small indeed, probably microscopic in character, and similar in size to the minute drops of which terrestrial clouds are formed. Why Saturn should have a ring-system at all is a question which has never yet been satisfactorily answered. Jupiter, Uranus and Neptune do not possess one, although they are similar to Saturn in size, physical state, and rapid rotation. A French mathematician, Roche, has shown that no single solid body, such as a satellite, could possibly exist as close to a planet as the rings do to Saturn—it would be broken up by gravitational forces—but this does not explain why Saturn alone should exhibit these mysterious and puzzling formations. The Moons of Saturn. Saturn is rich in moons, no fewer than nine having been discovered circling about the planet beyond the rings. The largest (Titan) is considerably bigger than the earth’s moon, while the smallest (Phoebe) is only about 50 miles in diameter. The latter was discovered by W. H. Pickering in 1898, and is the outermost satellite so far found. A tenth moon was announced by Pickering shortly after his discovery of the ninth, but its existence has never yet been satisfactorily confirmed. The motion of the moon Phoebe is interesting in that it has been found to be “retrograde”—it is circling around Saturn in a direction opposite to that of all the other satellites. It . may be shown mathematically that this retrograde motion, which exists also in the case of the two outermost satellites of Jupiter and of the single satellite of Neptune, is a factor which tends to make for greater stability of the system as a whole.