Page:Astronomy for Everybody.djvu/116

dividing by 107.5, we find the diameter to be about 865,000 miles. This is about one hundred and ten times the diameter of the earth. It follows that the volume or bulk of the sun is more than one million three hundred thousand times that of the earth.

The sun's importance to us arises from its being our great source of heat and light. Were these withdrawn, not only would the world be enveloped in unending night, but, in the course of a short time, in eternal frost. We all know that during a clear night the surface of the earth grows colder through the radiation into space of the heat received from the sun during the day. Without our daily supply, the loss of heat would go on until the cold around us would far exceed that which we now experience in the polar regions. Vegetation would be impossible. The oceans would freeze over, and all life on the earth would soon be extinct.

The surface of the sun, which is all we can see of it, is called the photosphere. This term is used to distinguish the visible surface from the vast invisible interior of the sun. To the naked eye, the photosphere looks entirely uniform. But through a telescope we see that the whole surface has a mottled appearance, which has been aptly compared to that of a plate of rice soup. Examination under the best conditions shows that this appearance is due to minute and very irregular grains which are scattered all over the photosphere.

When we carefully compare the brightness of different regions of the photosphere, we find that the apparent centre of the disk is brighter than the edge. The differ-