THE PLANET-SATURN

Taranaki Daily News, 23 March 1935, Page 13 (Supplement)

THE PLANET-SATURN

A MOON GOES BACKWARD

(By the

Rev. B. Dudley,

F.R.A.S.)

A favourite object for owners of telescopes is the ringed world at present in our morning skies. Nor is this to be wondered at; for it is certainly a superb sight, viewed under favourable conditions. Seen with the naked eye the planet is dull enough, having the appearance of a yellowish star of the first magnitude. No one looking at .it as an ordinary star among the celestial host would imagine the splendid figure it presents in, say, a 9-inch aperture telescope. Thus seen for the first time it invariably inspires--. exclamations of wonder and delight.lts,. magnificent ring system offers a spectacle not approached by any other planet. One observer fairly described its general characteristic by saying “it looks like the broad rim of a straw hat taken from the hat and placed collar like over a sphere somewhat smaller than the rim itself.” Sometimes, as during 1907, 1921 and again shortly (January 1936), the rings are seen edge-on, when they appear aS a thin line of light. Like thin sheets of silver they lie in concentric circles in the plane of the planet’s equator. They were among the first fruits of discovery in Galileo’s telescope invented in 1609 or 1610. Although he did not realise the significance of what he saw, their outstretched expanse presented itself as a pair of “wings” or “handles” to the planet. It was Huygens who in 1655 discovered their true nature or, perhaps it should be said, established it, since previously it had been suspected. The rings are not solid, but consist of swarms of particles closely compacted together, though not so closely as to render them perfectly opaque throughout; for stars can sometimes be seen through parts of them, a proof of transparency. The entire ring system is of the order of 40,000 milles in width; but only 10 or 15 miles in thickness. Within this thin structure the particles, -which are only a few miles or less in diameter, lie strewn. Professor J. E. Keeler in 1895 directly demonstrated this meteoritic character of Saturn’s rings. He made use of the fact that the motion of a source of light in the line of sight can be measured by means of the spectroscope. The particles of the inner edge of the ring system, he found, revolve around the planet in about five homes, while those of the outer edge require about 13.7 hours. An upper limit to the possible total mass of the ring system lias been determined by a study of its feeble gravitational effects upon the motions of the satellites. And it has been shown that the weight of the whole cannot be more than one twenty seven thousandth part of that of the planet, or about one two hundred and eightieth part of that of the earth. In all probability it is much less. The innermost part, known as the crepe ring, is so tenuous that the brighter of Saturn’s moons have often been seen through it. It is not unlikely that the particles of which the rings consist come into collision at times. In this circumstance, especially when two colliding masses are. moving at high speed relative to each other, much heat would be generated and immediately radiated away. Thus energy would be lost at the expense of the orbital energy of one or both, and the orbit of at least one of them reduced in size. In this way the rings must suffer a gradual diminution in size. Possibly the crepe ring is made up of masses whose orbits have been reduced by precisely such collisions. Dr. Moulton thinks that, seeing Saturn has existed for more than, a thousand million. years, such reduction of the orbits of the separate particles has gone on very slowly and that it. may 1 safely be inferred that collisions, have been anything but numerous and. that the rings are very tenuous. The planet itself is not of great density, being not more than an eight part as dense as the earth. Most of its mass, too, is concentrated near the centre. It is not,, therefore, a matter for surprise that its equatorial regions are greatly bulged, due to the axial rotation of the giant world. This bulging is not less than 10 per cent. In other words, while the equatorial diameter of the planet (discounting the rings) is 75,100 miles, the polar diameter is 7900 miles less. If we measure the diameter of the entire Saturn system, including the wide lateral expanse of its rings, then the vastness of the scheme is greatly enhanced, being 172,000 miles. Such markings on Saturn’s surface as could 'assist in determining the planet’s rotation period are of infrequent occurrence. There are no features resembling the great red spot on Jupiter, but at times bright spots have- been evident. A white spot appeared iri : lB76 and lasted several weeks, and this gave a rotation period of 10 hours 14 minutes. Another such spot in 1903 gave a period 24 minutes longer. The accepted estimate is given as of the orde-r of 10 hours. But, as in the , case of Jupiter, the rotation is faster at the equator and slows down as the polar regions are approached. Evidently, then, the planet is mostly in a fluid and probably in a gaseous state. Quite recently another remarkable white spot appeared on the surface of this planet’s equator. This spot had a diameter of about one-tenth that of the planet itself, or 7000 miles. It has just been made the basis of a fresh determination of Saturn’s rotation period. But the results confirm previous determinations made ever since 1876. Saturn is second in size only to Jupiter, the largest of the planets. It is about 820 times as large as the earth, and is 9.5 times as far away from the sun. It takes 29.5 years to make one revolution about the solar centre.

Like Jupiter, Saturn • has nine satellites, the largest and first known of these being discovered in 1655 by Huyghens. Since then eight others have been found, the latest addition having been detected by W. H. Pickering in 1899. There may be others as yet unknown; for these bodies are but feebly illuminated by the sun, owing to the planets’ great distances from that luminary, namely, 886 million miles. Pickering’s discovery created a great deal of interest, since it presented the first known instance of a satellite revolving in a direction opposite to that of the rotation of the planet which controls its motion. Since that, time, however, two Jovian moons behaving'in a similar manner have been discovered. On the first of April Saturn will rise in the east at about 3.30. a.m. It is entering the constellation pf .Aquarius, the Water Bearer. As none of the stars in this group are of very great'brilliance, Saturn stands out conspicuously upon it, and should be detected by its steady and rather sallow hue. By .the end of April it will be fairly high in the heavens at the hour named. Not until August does it begin, to show to full advantage in our evening skies. On the first of that month it will rise at about 7.30 p.m.