Page:A Voyage in Space (1913).djvu/216

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planets were all rotating; and now we have to realize that the great Sun is rotating on his axis also. That was first found out by Galileo when he observed sunspots. Here is a picture of the Sun as it is taken at the Royal Observatory, Greenwich, every fine day in the year. The two lines crossing the picture are the spider lines of the telescope; they tell us which is North and South: but all the rest of the bright part is the Sun. You notice that the edge is darker than the middle; and that tells us that the Sun has an atmosphere, of which we shall have more to say presently. You see also the spots, and you will please note their position on the first picture, which was taken on February 18; and now you will see how they have changed when we come to February 19, and again to February 20, and following dates. They do not move straight across horizontally because the Sun's axis is tilted; but careful measurement shows that there is a real axis, which has remained so far as we can tell in the same position since it has been investigated.

The path of the sunspot can be seen better if we make a "composite photograph" of all the days, by photographing them all on to the same plate. We can then see how the spots are travelling. From such a composite photograph I have made (see Fig. 57) a drawing showing the position of the big spot only and leaving out the others. The positions for the different days are numbered accordingly. On February 23, and again on February 25 and 26, the weather at Greenwich did not allow a photograph to be taken.

Before we go on to consider how the Sun's rota-