Page:A Voyage in Space (1913).djvu/137

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That is the order in which the constellations in the Zodiac are arranged, and the Sun appears to visit them all in turn once a year, owing really to the fact that the Earth travels round the Sun. But the Earth also rotates on its axis, as we learnt in the first lecture: and in consequence of this rotation all these constellations cross the sky every day. Some of them cannot be seen to do so, because they are too near the Sun: they cross in the daytime w r hen we cannot see them; but the others cross at night, and if you care to notice at this time of year (early January) you can see the Twins rise in the east soon after sunset and cross the sky much in the same way that the Sun does, setting in the west long after you have gone to bed. If, then, we had a fixed telescope it could only catch sight of the Twins once during the night; but if we arrange that it can turn round by clockwork it can keep the constellation, or any part of it, steadily in view, all night if necessary. Sometimes an astronomer wishes to look steadily at the same object all night in this way: he would probably be also taking a photograph at the same time, of some very faint light such as a faint distant nebula, which would not appear on the plate unless he gave it a very long exposure. So he points the telescope at the object, sets his clockwork going, puts a photographic plate to receive the image: and then in another