IN STARRY SKIES

Evening Post, Volume CXVIII, Issue 89, 12 October 1934, Page 16

IN STARRY SKIES

SATURN

(By "Omega 'Centauri. >')

Satuijn is /undoubtedly, the most beauul object in tie. solar system. Strange to say most visitors who come to an observatory for'the first time are particularly ,:arixibus to see either Mars or Venus, buVneither of these planets can comparein. beauty,-or in interest, with Jupiter or Saturn. Jupiter has the gre.at advantage of having four of its moons .bright enough to be seen with very small telescopes or even with field glasses, but Saturn has the unique peculiarity of its marvellous system of rings: In apparent size Saturn's globe is usually about half that of Jupiter, ranging-from- 14 to- 20. seconds in diameter, in-comparison with Jupiter's 32 to 50. But the diameter of the system of rings is 2.3 times that of the disc. Venus, when in conjunction, shows a larger disc than either of these great planets, but at that time-its dark side is turned towards.us.- Again at the most favourable opposition, the apparont diameter of Mars may be 25.1 seconds,, but at conjunction it may fall to 3io. Saturn is veryoasyto identify in the sky, because its. apparent motion in'front .of; the stellar background isso slow. It entered .. the constellation Capriconius in February, 1932, and has been there ever since, except in May,

June, and July this year when it crossed temporarily.into Aquarius.. At present, with Gamma and Delta Capricorni it forms almost an equilateral triangle. Its slow.movement in the sky,induced the ancient astrologers t6 attribute strange and contradictory qualities to it. For one thing it was assumed .that it must be very old, and presumably correspondingly wise, so it was given charge of the first hour of; the first day of the week, which was hence called Saturday. The reign of Saturn was believed to be a golden age,• a time of happiness and simplicity. But. in Eome its festival ■was celebrated with unrestrained licence and merriment, so that we still speak of scenes of wild revelry as Saturnalia. Almost exactly opposite to this is the signification of the adjective saturnine, indicating a dull, phlegmatic temperament; The influence of Saturn was believed to: make people gloomy and morose, and the ancients chose for his sypibol the dull and heavy metal lead. It is strange how.far they strayed from reality in doing so, for, bulk for bulk, Saturn is by far the lightest of all the planets. Its'density is only 13 per cent, of that of the earth, and less than threequarters, of that, of water. It is the only planet lighter thaji water. One striking effect of this remarkably low density is seen in the. shape of the disc. The equatorial bulge and polar compression in. any planet depend on the speed of rotation and 'on the density. The greater the speed and the less the density the more obviously will the planet's shape differ, front"'a sphere. Now, in spite of their enormous size all tho major-; planets, rotate more than twice as fast as the terrestrial ones, whilst on an average they are- less than one-quarter. as dense. Saturn rotates on its axis.in 10hr:i4min, which is only nineteen minutes, greater , than the record established ■by Jupiter, whilst Jupiter is nearly twice as. dense. It thus happens'that the polar diameter of Saturn, 66,300 miles, is less than the equatorial, 74,100-; miles, by an. amount, almos'f equal"ftfthe'diameter of the earth and considerably greater than the diameter of "Venus. The volume of. Saturn' is, 765 times, whilst- -its/mass"'is', only, £•5.2 .times,, that of'the earth. We have seen how greatly impressed the ancient astronomers were by : the slowness ofSaturn's motion,-but in reality it rushes along its orbit at a speed of almost exactly.six miles per sec6ndj.which.,is not far short of one-third that of the'earth. Uranus, Neptune, .and Pluto are rather more deliberate in their movenients, but

speeds expressed in miles per second can hardly be described as slow. Saturn Is more than 9J times as far away from the' sun as we are, and owing to the eccentricity of its orbit, its actual dis tance varies by more than 100 million miles. It is always between 74? and 1027 million miles from the earth. Although it flies with an average speed of *six miles a second it takes nearly 29.i years to go once round the sun. The earth therefore catches up one complete round in 378 days, or about 12£ days more than a year. Successive oppositions or conjunctions of the planet occur therefore only, about 12$ days later each, year. This year it was in opposition on August 18. "The orbit of the. planet is inclined Only about 2i degrees to the ecliptic, but the equatorial plane- is tilted 26deg 45min to the planet's orbit, or 28 degrees 6 minutes to the plane of the ecliptic. The planet must therefore experience marked seasons. But the intensity of the solar radiation falling on its surface is less than one-nine-tieth part of that which we receive. The temperature of the cloudy surface that we see is believed to be 150 degrees below freezing point on the centigrade scale. The apparent brightness of the planet depends not only on its distance from us but on the aspect of the.rings, and it may be at one time three times as bright as at another. It. may be no brighter than Altair, or may exceed in brightness every star but Sirius. The system of rings lies exactly in the plane of the planet's equator, and since the rings remain parallel to themselves, twice in every period of revolution they are presented edgewise to-

wards the earth, arid twice they appear of maximum width. The outer ring has an exterior diameter of 171,000 miles and a width of more than 10,000 miles. Cassini's division between the two bright rings is about 3000 miles wide. The inner bright ring is 16,000 miles wide. Then there/ is another, gap of 1000 miles inside of which is the crape' ring, 11,500 miles wide. The total width of the rings and divisions is thus about 41,500,mi1e5, and this leaves only 7000 miles between the inner edgo of the-crape ring and the. surface of the planet. The astonishing thing is .that the thickness of this great sheet is probably not greater than teu miles.- If you made a model of the thinnest paper procurable, and took-the linear scale 10,000 miles to the inch, the outer ring would be 17in in diameter, and the paper would be too thick in proportion. The result is that when the rings are presented exactly edgewise they disapr pear, a fact which greatly puzzled Galileo. The constitution of the rings was long a mystery, but it is now known that they consist of immense .numbers of separate particles revolving in orbits round the planet. In 1889 Barnard found that the satellite lapetus remained visible through the duskyring, and in 1895. Keeler proved by spectroscopic observation that the outer edge of the ring moves more slowly than the inner, and that each part goes at the rate which a satellite would have to adopt in that position. The rings lie within what is known as ■Koche's limit, that is the minimum disitance from a planet at which a satellite can revolve without being torn to pieces by gravitation. The gaps between the rings have all been explained as due to the perturbations, caused by the satellites. They occur at distances where the period of revolution of a small body revolving round Saturn would be commensurable with that of one another of the satellites themselves.

Titan, the largest satellite, ..was .discovered by Huygens in 1655. four others "were found by Cassini between 1671 and IjSB4; two: more by Hersehel in 1789, one by Bond in 1848, and the ninth by Pickering in 1898. Titan is tho only one of these that is larger than our moon. Its dianieter is about 2600 miles. Bhea and lapettis have diameters of about 1100 miles, Dione and Tcthys 700, Eneeladus 500, Mimas 400, .Hyperion 300,. and Phoebe 150. lapetus is more than two-million miles from tho planet, whilst Phoebe moves in ,a retrograde manner at the immense distance of over eight million miles., ."