Читать книгу The Children's Book of Stars - Mitton Geraldine Edith - Страница 7

What must the sun appear to Mercury, who is so much nearer to him than we are? To understand that we should have to imagine our sun increased to eight or nine times his apparent size, and pouring out far greater heat and light than anything that we have here, even in the tropics. It was at first supposed that Mercury must have an extra thick covering of clouds to protect him from this tremendous glare; but recent observations tend to prove that, far from this, he is singularly free from cloud. As this is so, no life as we know it could possibly exist on Mercury.

His year – the time he takes to go round the sun and come back to the same place again – is eighty-eight days, or about one-quarter of ours. As his orbit is much more like an ellipse than a circle, it follows that he is much nearer to the sun at one time than at another – in fact, when he is nearest, the size of the sun must seem three and a half times greater than when he is furthest away from it! Even at the best Mercury is very difficult to observe, and what we can learn about him is not much; but, as we have heard, his axis is supposed to be upright. If so his seasons cannot depend on the bend toward or away from the sun, but must be influenced solely by the changes in his distance from the sun, which are much greater than in our own ease. There is some reason to believe, too, that Mercury's day and year are the same length. This means that as the planet circles round the sun he turns once. If this is so the sun will shine on one half of the planet, producing an accumulated heat terrific to think of; while the other side is plunged in blackness. The side which faces the sun must be heated to a pitch inconceivable to us during the nearer half of the orbit – a pitch at which every substance must be at boiling-point, and which no life as we know it could possibly endure. Seen from our point of view, Mercury goes through all the phases of the moon, as he shines by the reflected light of the sun; but this point we shall consider more particularly in regard to Venus, as Venus is nearer to us and easier to study. For a long time astronomers had a fancy that there might be another planet even nearer to the sun than Mercury, perhaps hidden from us by the great glare of the sun. They even named this imaginary planet Vulcan, and some thought they had seen it, but it is tolerably certain that Vulcan existed only in imagination. Mercury is the nearest planet to the sun, and also the smallest, of course excepting the asteroids. It is about three thousand miles in diameter, and as our moon is two thousand miles, it is not so much bigger than that. So far as we are concerned, it is improbable we shall ever know very much more about this little planet.

But next we come to Venus, our beautiful bright neighbour, who approaches nearer to us than any other heavenly body except the moon. Alas! when she is nearest, she like Mercury, turns her dark side toward us, coming in between us and the sun, so that we cannot observe her at all.

Everyone must have noticed Venus, however carelessly they have looked at the sky; but it is likely that far more people have seen her as an evening than a morning star, for most people are in bed when the sun rises, and it is only before sunrise or after sunset we can see Venus well. She is at her best from our point of view when she seems to us to be furthest from the sun, for then we can study her best, and at these times she appears like a half or three-quarter moon, as we only see a part of the side from which the sunlight is reflected. She shines like a little silver lamp, excelling every other planet, even Jupiter, the largest of all. If we look at her even with the naked eye, we can see that she is elongated or drawn out, but her brilliance prevents us from seeing her shape exactly; to do this we must use a telescope.