Page:Astronomy for Everybody.djvu/119

to be placed there, an ordinary man would weigh two tons, and be crushed by his own weight.

When the sun is carefully examined with a telescope, one or more seemingly dark spots will generally, though not always, be seen on its surface. These are, of course, carried around by the rotation of the sun, and it is by means of them that the time of rotation is most easily determined. If a spot appears at the centre of the disk it will, in six days, be carried to the western edge, and there disappear. At the end of about two weeks it will reappear at the eastern edge unless it has, in the meantime, died away, which is frequently the case.

The spots have a wide range in size. Some are very minute points, barely visible in a good telescope, while on rare occasions one is large enough to be seen with the naked eye through a dark glass. They frequently appear in groups, and a group may sometimes be made out with the naked eye as a minute patch when the individual spots cannot be seen.

When the air is steady, and a good-sized spot is carefully examined with a telescope, it will be seen to be composed of a dark central region or nucleus, surrounded by a shaded border. If all the conditions are favourable, this border will appear striated, like the edge of a thatched roof. The appearance is represented in the cut, which also shows the mottling of the photosphere.

The spots are of the most varied and irregular forms, frequently broken up in many ways. The shaded border.