A MOLTEN WORLD

Taranaki Daily News, 14 February 1931, Page 13 (Supplement)

A MOLTEN WORLD

JUGGLING WITH MOONS <

(By

Rev. B. Dudley,

F.R.A.S.)

(Written mainly for the Young.) Well to the north-east of the constellation Orion, and not far from the Heavenly Twins, Castor and Pollux, is an exceedingly bright star shining with a steady, white lustre—the planet Jupiter. It should be looked for at nine o’clock this evening at an altitude of about 40 degrees above the northern horizon. It follows the path of the sun through the heavens. At the present time it is slowly threading its way through the constellation of the Twins, or Gemini. Rising in the east with that group of stars early in the evening while daylight is still in the skies, it°advances in a westerly direction, a movement due to the rotation of the Earth from west to east. The real movement of the planet, like that of. all such bodids, is in the opposite direction, namely, from west to east. Hence it gradually passes out of one constellation into the next on its eastern side. Sirius, a very bright star, could easily be mistaken for Jupiter, but for the facts that the former is much higher in the heavens and that the white of Sirius is tinged slightly with blue. It does not “twinkle,” as do the “fixed stars,” but looks down upon us, as it were, with a clear, unmoved gaze. Even in a small telescope what is apparently a mere point of light expands into a disc. People who see it in this way for the first time are almost sure to exclaim: “Why, it’s like a moon—the full moon!” In this they are correct; it certainly does resemble a miniature moon in full. Imagine our satellite to retire a long way off, away from her place in the nightly heavens. She would in that case diminish in apparent size until she threatened to show very little or no disc at all. Imagine Jupiter, en the other hand, to draw slowly nearer the Earth; he would then present. a areatly expanded disc, ever increasing in apparent size, until he would be quite as large a body in our skies as the Moon is to us. Were he to arrive at a point actually as near to us as is the Moon, he would appear enormous indeed, occupying a large proportion of the entire skies—a most magnificent and dazzling spectacle. The true distance of Jupiter from the Earth varies as the two bodies constantly change their relation to. each ether. For sometimes both are on the same side of the sun, while at other times they are in opposite positions. Li one case, the size of the .planet appears small, in the other it appears large. Sometimes Jupiter conies as near to the Earth as 39’0 million miles; while, when he is on that side of the sun opposite th;j Earth, he is 576 million miles away. His mean distance from the Sun is 483 million miles. It should be remembered, too, that whereas the Earth is not quite 8000 miles in diameter, Jupiter is no less than 85,000 miles through. It is evident, then, that we are dealing with a veritable giant among t]ie Sun’s family. Diameter alone, however, does not sufficiently indicate the size of a world. If one of two balls has twice the diameter of the other, it will not be twice but many times its size. For it is, ot course, “twice as large in all directions,” across from side, to side, from top to bottom, from front to back, and from any given point to the exact opposite of that point. Sir Robert Ball, in his lectures to children, used to employ an excellent illustration of this. He pictured two plum puddings, one to represent the Moon and another to stand for the Earth, which has about, four times the lunar diameter. He proposed to his juvenile audience the problem; “If one schoolboy could eat the small pudding, how many boys would be required to dispose of the large one?” Almost inamong his auditors were several who, remembering that the diameter of one was four times that of the other, would cry “Four!” But the more reflective of his hearers would see that plum puddings have breadth and depth which are in the same proportion as length, with the result that the large one is far more than four times as big as the small one. The lecturer would then explain that it would take some scores of boys to demolish the larger dish. So, by analogy, he would teach them that if the Earth is four times the diameter of the Moon, it is in reality scores of times as large. The diameter of Jupiter being only a little more than ten times that of the Earth, it might seem rather much to claim that the larger world is thirteen hundred times the size of the smaller. But, not forgetting that lesson of the plum pudding, the matter will appear plain enough. The enormous proportions of this planet (the largest in the solar system') are not surprising when we reflect that it is yet in a molten conditionswollen with heat, one may say. It has not yet cooled down sufficiently to have formed a solid surface for itseif. In truth the planet is sometimes spoken of as a “semi-sun.” It shines partly by light reflected and partly by luminosity of its own; although the latter is but feeble in amount. It is densely shrouded within a cloud-laden atmosphere through which our vision can never hope to penetrate. And as it spins on i its axis, these clouds appear to us in the form of belts across the Jovian disc. These, after the fashion of clouds, are incessantly changing. The giant is not truly spherical in figure. The swift rotation just mentioned causes the globe to bulge out at the equator very considerably, so its polar diameter is less by 5000 miles than its equatorial, a fact which can be appreciated by anyone who views the plant telescopically. Jupiter rotates on its centre in less than ten hours, whereas the Earth, which is only the thirteen-hundredth part of its size, takes 24 hours to complete one turn. Even the Earth’s equatorial diameter is greater than its polar by about 27 miles. This molten or n-aseous world requires for its journey round the sun a period as long as 11.86 (or nearly 12) years. But the most interesting thing of all about Jupiter is his wonderful system of nine moons with which the aiant is, so to speak, constantly juggling. For these swing ever round him, four of them visible in a small telescope or a. good pair of binoculars. They range from 112,000 to nearly 19 million miles distant from their primary. In diameter they vary from 20 to 3009 miles, several being larger than our moon, which is 2160 miles through. They take from 11 hours to three years to complete a journey round their primary. The first of bodies was discovered in 1610 by Galileo, who invented the telescope, and the last in 1914.