IN STARRY SKIES

Evening Post, Volume CXI, Issue 36, 11 February 1926, Page 4

IN STARRY SKIES

SATURN

THE MOST BEAUTIFUL OF THE

PLANETS

ITS UNIQUE SYSTEM OF RINGS.

(By "Omega Centauri.")

Visitors to an observatory aro often disappointed when first they look through a telescope. They expect it to do much more than it possibly can in bringing distant worlds closer to us. In the case of the moon their expectations are realised. Its rugged surface and strongly-contrasted light ; and ■hade appeal to tnem at once. If the moon is not on view the safest object. to show them first ia the planet Saturn. Although it ia not magnified as much as great pictures in magazines have led them to expect/ thero is satisfaction and fascination in actually seeing for themselves the. marvellous system of lings by which it is surrounded. Saturn was at the far outer limit of the solar system as known to tlie ancients. His slow movement gave them ah impression of his great age. The god he represented was therefore believed to be endowed with vast experience, and was considered fittest to take charge of the first hour of the first day of the week. 'The day is still called Saturday, though it has lost its position amongst the s^ven days. ' Saturn is'over nine and a half times ■■ far from the sun as the earth is. Its mean distance is 886 million miles, but the actual distance varies by 50 million miles. At the timo of a most favourable opposition Saturn is 744 million Etles from the earth, whilst -at conjunction the distance may be' 1028 miles. Saturn's apparent diameter' ranges from twenty seconds to fourteen seconds. The path in which, it .moves is. nearly ten times the length of the earth's orbit, and its speed is less than a third of that of the earth,; being six miles per second to the earth's eighteen and a hall. Its- period is therefore ■ nearly thirty years, or, more exactly, 29.46 refers. Its synodic period is 378 days; tk»t is, the earth takes, about thirteen days more than a year to catch up one revolution, on the planet. Saturn takes aearly two and a half years to move tjbrough on zodiacal constellation. This year it eroMes from Libra into Scorpio. Owing to the earth's. motion between it^and the sun, the planet will appear to move backwards into Libra frpm 6th March to 25th July. It will be in oppo*itk>n on 14th May, and in conjunction, on 21st. November,\ It will thus be well placed fir observation in New Zealand for a few months after Easter, and will be easily found between Libra and the claws of the Scorpion. The •quatorial diameter of Saturn is 75,100 •nd the polar 67,200 miles. The compression at each pole, due to rapid rotation, is greater than the radius of the earth. The mean diameter is over nine times, and therefore the surface is 82 times and the volume 763 times that of the earth. Considering this, the mass is ■■rprisingly small, being only 95 times that of the earth. Saturn is thus astonishingly light, by far the lightest of all the planets, with a density only two - thirds that of water. In spite of its great size, gravity on its surface is • only about one-tenth greater than on Earth. To weape from the planet, however, would require a speed of 22 miles per second,' Bore than three times that required to )•*▼« the Earth. Saturn is therefore able to retain its densely clouded atsosphere, which gives it the high reflecting power of 60 or 70 per cent. Baturn resembles Jnpiter in .rotating with extreme rapidity and also in the' fact that the equatorial clouds have a shorter period than those, in the temperate zones, if there are any such areas on the planet. The equatorial period is given as 10 hours 14 minutes, and that of the temperate zone as 10 hoars 37 minutes. The inclination of the equator to the plane of the orbit is nearly 27 degrees, which, though not much more than that of the Earth, is greater than that of any other planet axeept Uranus. The belts of clouds lie ■ parallel to the Equator, and, though less etoarly defined, resemble those of Jupiter. The planet may still be almost entirely gaseous, and at a high temperature. As an abode of life Hinks considers it to be in a worse case even than Jupiter. We see nothing of any •olid nucleus, and if there is any such it must lie so fafwithin the cloud envelope that it can hardly be reached by lie rays of the sun. The splendid appurtenances of the encircling rings, and the great family of satellites he eanMiders to bej.thrown away upon an ■unhabitable world. •

The rings of Saturn wore a sore puzsle and' disappointment to Galileo, who (Uaeoyerad. them. He announced that the most distant of the planets has a triple form. His telescope was not powerful or perfect enough to disclose the trne nature of the appendage. The two attendants, to his dismay, became. eontxnoally smaUar and smaller, and finiHjr disappeared. "Has Saturn then," fca asked, his o/wn children? I Ao not know Tt*at to say in. a case so Barpaiißg, so -ualooked-f or and so novel. iS^M shortness of the time, the. unexjpßetod natme of the event, the •weakiiMßja of my understanding, and the fear >f Iwii iijj. mistaken, have greatly confounded me." It was not -nntil 1655 (bat Haggens perceived the ring form, and not until mnch later that the cause •f Otfiho't difficulty was explained. "Huwngft mneh is now known pf the ring ■ystam some mystery still remains. In 1675 Cassini discovered that the ring Hi doable, and the division between the two bright pAitte is still known by fcia name. In 1838 feaße saw the dusky tamer band, known as the crape-ring, Qkovgb it* dascovexy is usually attrib■tad to Bond and Dswe* in 1850. The ■pan of lite great nag system is over V7&JBW arifea. Toe two bright rings aad 6m ermpe-cing, from the outside , inwMinkt Iwtiili tie planet, axe known aa A, B, and C respectively. Their wadfln asac Bang A, 11,000 mites- ring B, 18^000 oGtemr, ring C, 10,500 miles. Ommam't dcraaon batween A and B is 2200 safes aeooaß. !Tse famer edge of tba atjajeniuji is leas titan 6000 miles toom the surface of Saturn.. This gnat zisa; system, more than 42,000 BxHea wife, S« w* more than 60 or 70, and ptpiimUf lew than 50 miles in thickMbj, Ibe physical eonstitation. of the rings-presents as interesting and diffientt problem. At first they were assumed to be «oli<f or liquid, but Casaini, in 1.715, suggested that ( they may be composed of swarms of meteorites. Laplace jprorod that if solid the ring would be dynamically unstable. Clerk Maxwell, in 1856, showed that the system could neither be solid nor liquid, but must consist of innumerable small bodies. This was definitely verified by observation in 1895, when Keler found speetfoscopleally that the innor edge of v«Sch ring/moves faster than the outer. The rings, then, consist of discontinu--cus particlos, but the problem of the gize of these is not yet completely solved. They are probably very small. The crape-ring is definitely transparent and the others axe so also to a slight extent. 'Stars' have been seen through all the rings and photographs have been taken in which tho discs of tho planet can bo detected through the outer one. The total mass of the ring system is believed to be very small. B. Strove found that if it had even i^ene twenty-seven thousandth of---the mass of Saturn, its disturbing effect on the motions of the satellites could bo detected. TJie brightest pact of, the

ring system is brighter than any part of the disc of the planet itself, and this fact is taken as additional evidence for the smallness of the particles of which the rings are composed. It is rather surprising that the vast ring system would be invisible from the north or south pole of the planet. Owing to collisions amongst the particles it is; probable that tho rings are gradually approaching the planet, and may finally be all'collected by : it. It is possible that other membors of, the solar system may have had similar appeiidages in tho distant past, but no one knows why Saturn has rings at the present time, and none of the other planets have any. The reason may be connected In some way with Saturn's phenomenally low density, which is about half that of each of the other giant planets, less than one-eighth that of the earth, and less than that of any known solid except lithium. Tho rings are closer, to Saturn than a satellite could be. Roche proved that if any satellite came within a certain limiting distance of its parent planet there would be a tendency for it to break into fragments. If the two bodies are of the same density this limiting distance is two and fivo-olevenths of the radius of the planet. A denser satellite could safely come a little closer. It is not believed that a satellite was really broken up to form tho rings, but rather that the disruptive forces prevented its formation. Cassini's Division t and other possible gaps in the ring system are believed to be due to the perturbing effects of tho satellites. A particle moving in an orbit in the position of Cassini's Division would have a period half that of the satellite Mimas, and would suffer such,recurring perturbations that its orbit would be permanently changed.