TWENTY-SIX MOONS

Taranaki Daily News, 20 February 1932, Page 1 (Supplement)

TWENTY-SIX MOONS

NATURE’S JUGGLING BALLS

(By Rev. B. Dudley, FjR.A.S.)

-Shakespeare somewhere makes one of his characters exclaim at seeing “four moons to-night.” Many people are surprised to learn that within the bounds of the solar-system are at lehst 2-6 moons distributed amongst the various planets. Mercury, nearest to the sun, has never been suspected of having a satellite attending it. But this is not the case with respect to Venus, the object that shines so gloriously just now in our western skies at eventide.

On January 25, 167'2, Cassini saw a faint star resembling a crescent, such as Venus herself often presents in the telescope. The space between the two bodies was about equal to the diameter of the planet. A few years, later (August 28, 1-686) the same experienced observer saw a crescent-shaped light.east of the planets, at a distance from Venus less than her diameter. In 1740, during the month of October, Short, an optician, with two different telescopes saw a small star clearly defined, of feebler brightness than the planet, and exceeding close to it. On four further occasions during 1761 Montaigne, at, Limoges, saw what he took to be a stellite of Venus, its diameter appeariSfto be no more than a fourth of that boly. Again in 1764 several observers resident’ at places far apart saw what they felt sure ‘was a Venusian. moon. But, never ■’ since, though the minutest observations were made,' has any such object been detected, and all search has long since been abandoned. « M. Stroo'baut and others have fully discussed the question, and their researches show that in almost all cases the observers, however careful, mistook stars in the far distance for an attendant upon ‘Venus, the two bodies happening at the time to be practically in the same line of vision. In the other cases, in all probability, it was a minor planet that was seen. There are good reasons in one instance for the opinion that it was the planet Uranus which was responsible. After Mercury and Venus comes the Earth. When our own satellite was first discovered can only be It was seen first by the first eye. What we know of the Moon would fill volumes. Suffice it to state here that she is situated at an average distance of 237,000 miles from the Earth. As the latter is not exactly central to her orbit, her distance from us varies, so that she may be as far oft as 103,000 miles, or as near as 221,600. Her diameter is about 21'GOmiles, and takes —7 days, .7 hours and 43 minutes to complete a cycle about the Earth. As for lunar landscapes and many other features, a further article may have something to say. .. The case of Mars is quite different from that of Venus. The two satellites in constant subordination to the ruddy planet are easily visible in the modern telescope. They were first seen,, from Washington, U.S.A., by Asaph Hall in 1877. The larger of the two was given the name of Phobos,- (Fear) and the smaller Deimos (Dread), thus recalling the mythological horses that drew the chariot of Mars. Deimos revolves round the planet in 30 hours, 16 minutes, at a fiistance from it of about 15,000 miles, and in an orbit that is almost circular. Pho'bos takes-7 hours, 30 minutes to complete a round of the planet, while its distance is less than 6000 miles. An observer on Mars could see Phobos risein the west and set in the east, contrary to the other heavenly bodies which, as here, rise and set from east to west. _ . , , This strange behaviour is due to the rapid motion of the satellite. Phobos would, be seen to overtake and then pass Deimos in the Martian skies.y. The reason is made clear when we isarn that while the day on Mars, like that of the Earth, is about 24 hours long, Phobos swings round him in less than a third of that time. Thrice therefore in the course of a Martian day this little moon might be seen to travel across the sky. Imagine our satellite to rise and set and traverse the heavens twice during the same night! Deimos, on the other hand, takes nearly as long to journey round Mars as the planet requires to turn upon his axis. In both cases it will be seen that there is nothing comparable with the EarthMoon system. Nor is there anywhere else in the solar system. Jupiter’s retinue next calls for consideration. Here we are presented with a whole series'of moons. Like Saturn, Jupiter can boast of nine attendants. Four of these were among the first fruits of the invention’ of the telescope, Galileo being the . discoverer. Others were detected by Barnard, Perrine, Melotte and Nicholson respectively. The last was discovered in 1914. In distance from the great planet round which they revolve they range from 112,500 to nearly 19,000,000 miles. The difference in their sizes, however,, is not so great. One is a-bout 100 miles in diameter, another is -perhaps only 20 miles across; the bulk of them range from 2045 to 3350 miles. In the length of time required to revolve about theii primary they exhibit great variety, included between about 12 hours and 3 years. The' nine moons of Saturn were discovered between .1'655 and 189-8; Herschel 2, Cassini 4, Huygens, Boud-Las-sell and W. H. Pickering 1 each. Here also we have a wonderful system, most magnificent to behold in a laige telescope. Saturn is only second to Jupiter in size. The satellites vary in distance from the planet between 117,000 and 8,000,000 miles, while in size the smallest is possi-bly 200 miles in diameter and the largest 3000. They describe a journey round Saturn in periods varying from 22J hours to 546| days (1 year, ISIJ days). Uranus is accompanied by four moons, Ariel, 500 miles in diameter, Umbriel 400, Titania 1000 and Oberon 800, Titania well deserving its name. The nearest to Uranus is Umbriel, 167,000 miles distant, and the remotest Oberon, 365,000 miles from its primary. The planet is much smaller than Saturn; and his satellites travel round him in periods stretching between 2J and 134 days. Herschel and Lassell share the honour attaching to the discovery of these four dependants. There remain to be mentioned only 'two more planets —bieptune and the recently discovered . Pluto. W e have no knowledge of a Plutonian satellite. Neptune, however, like the earth, boasts of one moon. This is 2600 miles in diameter, and revolves around its primary in ■5 days, 21 hours, 3 minutes, at a distance of 221,500 miles. It was discovered by Lassell in 1846.