The Children's Book of Stars

rderly: the planets go round the sun and the satellites go round the planets, in orbits more or less regular; there seems no place for anyth

nd are of all shapes and sizes and never twice alike. Even as we watch them they grow and change, and then diminish in splendour. Some are so vast that men see them as flaming signs in the sky, and regard them with awe and wonder; some cannot be seen at all without the help of the telescope. From the very earliest ages those that were large enough to be seen without gl

ir. It is not often that one appears large and bright enough to be seen by the naked eye, and when it does it is not likely to be soon forgotten. In the year 1910 such a comet is expected, a comet which at its former appearance compelled universal attention by its brilliancy and strangeness. At the time of the Norman Conquest of England a comet believed to be the very same one was stretching its glorious tail half across the sky, and the Normans seeing it, took it as a good omen, fancying that it

N THE BAYEU

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t the time of the Norman Conquest is the same as that which has come back again at different times, and

articular warning to the human race. But when astronomers, by the aid of the telescope, found that for one comet seen by the eye there were hundreds which no mortal eye unaided could see, this idea seemed, to say the least of it, unlikely. Yet even then comets were looked upon as capricious visitors from outer

represent the planetary orbits, you would have to tilt one a little this way and another a little that way, only never so far but that a line through the centre of the hoop from one side to another could pass through the sun. The way in which the planetary orbits are tilted is slight in comparison with that of the orbit

returned to the sun about every seventy-five years. This could be proved, for if this idea were correct, of course the comet would return again in another seventy-five years, unless something unforeseen occurred. But Halley was in the prime of life: he could not hope to live to see his forecast verified. The only thing he could do was to note down exact particulars, by means of which others who lived after him might recognize his comet. And so when the time came for its return, though Halley was in his grave, numbers of astronomers were watching eagerly to see the fulfilment of his prediction. The comet did indeed appear, and since then it has been seen once again, and now we expect it to come back in the year 1910, when you and I may see it for ourselves. When the identity of the comet was fully established men began to search further back still, to compare the records of other previous brilliant comets, and found that this one had been noticed many times before, and once as I said, at the time of the Norman Con

till and faster flies the comet, for as we have seen it is a consequence of the law of gravitation that the nearer planets are to the sun the faster they move in their orbits, and the same rule applies to comets too. As the comet dashes up to the sun his pace becomes something indescribable; it has been reckoned for some comets at three hundred miles a second! But behold, as the head flies round the sun the tail is always projected outwards. The nucleus or head may be so near to the sun that the heat it receives would be sufficient to reduce molten iron to vapour; but this does not seem to affect it: only the tail expands. Sometimes it becomes two or more tails, and as it sweeps round behind the head it has to cover a much greater

any; some change utterly even as they are watched. The same comet is so different at different times that the only possible way of identi

omet was so near to Jupiter that it was actually in among his moons. The comet was violently agitated; he was pulled in fact right out of his old path, and has been going on a new one ever since; but he did not exercise the smallest effect on Jupiter, or even on the moons. And, as I said earlier in this chapter, we on the earth have been actually in the folds of a comet's tail. This astonishing fact happened in June, 1861. One evening after the sun had set a golden-yellow disc, surrounded with filmy wrappings, appeared in the sky. The sun's light, diffused throughout our atmosphere, had prevented its being seen sooner. This was apparently the comet's head. It is described as 'though a number of light, hazy clouds were floating around a miniature full moon.' From this a cone of light extended far up into the sky, a

nt. Everyone is quite familiar with this in everyday life, though perhaps they may not have noticed it. You cannot thrust a stick into the water without seeing that it looks crooked. Air being less dense than water has not quite so strong a refracting power, but still it has some. We cannot prove it in just the same way, because we are all inside the atmosphere ourselves, and there is no possibility of thrusting a stick into it from the outside! The only way we know it is by looking at something which is 'outside' already, and we find plenty of objects in the sky. As a matter of fact,

INTO THE WATER

This action may have something to do with electricity, which is yet little understood; anyway, the effect is that, instead of attracting the matter toward itself, in which case we should see the comet's tails stretching toward the sun, the sun drives it away! In the chapter on the sun we had to imagine something of the same kind to account for the corona, and the corona and the comet's tails may be really akin to each other, and could perhaps be explained in the same way. Now w

ount of a comet that w

tters. Now, that is 25,600. Only imagine a glare 25,600 times fiercer than that of the equatorial sunshine at noon day with the sun vertical. In such a heat there is no substance we know of which would not run like water, boil, and be converted into smoke or vapour. No wonder the comet gave evidence of violent excitement, coming from the cold region outside the planetary system torpid and ice-bound. Already when arrived even in our temperate regi

a large extent every time it returns to the sun, what happens eventually? Do the tails fall back again into the head when out of reach of the sun's action? Such an idea is inconceivable; but if not, then every time a comet approaches the sun he loses something, and that something is made up of the elements which

system, but there are facts which go very strongly against this, and astronomers now generally believe that comets really belong to the solar system, that their proper orbits are ellipses, and

, Cape of Good Ho

phed in 1901 at the Cape of Good Hope. It is only comparatively recently that photography has been applied to comets. When Halley's comet appeared last time such a thi