Page:Astronomy for Everybody.djvu/117

ence can be seen even without a telescope, if we look at the sun through a dark glass, or when it is setting in a dense haze. The falling off in the light is especially rapid as we approach the extreme edge of the disk, where it is little more than half as bright as at the centre. There is also a difference of colour, the light of the edge having a lurid appearance as compared with that of the centre.

All this shows that the light of the sun is absorbed by an atmosphere surrounding the sun. We readily see that, the sun being a globe, the light which we receive from the edge of its disk leaves it obliquely, while that from the centre leaves it perpendicularly. The more obliquely the light comes from the surface, the greater the thickness of the sun's atmosphere through which it must pass, and hence the greater the portion lost by the absorption of that atmosphere. The sun's atmosphere, like our own, absorbs the green and blue rays more than the red. For this reason the light has a redder tint when it comes from near the edge of the disk.

Careful observations show that the sun, like the planets, rotates on an axis passing through its centre. Using the same terms as in the case of the earth, we call the points in which the axis intersects the surface the poles of the sun, and the circle around it halfway between the poles the sun's equator. The period of rotation is about twenty-six days. As the distance around the sun is more than one hundred and ten times that