The sun is the centre of our planetary system, and holds all the planets in their proper orbits by the force of his attraction. We have seen in our glance at the asteroid Ceres, that the attraction of gravity depends upon the distance and the weight or _mass_ of the body exercising it. If our earth should suddenly grow lighter it could not hold the moon in its present orbit; if its attractive power over the moon ceased, the moon would go tumbling off into space. The sun exercises immense attractive force by virtue of its enormous mass. If all the planets were put into one pan of a pair of scales, the sun in the other would outweigh them seven hundred times over. Three hundred and fifty thousand such globes as we live upon would be needed to balance the sun. This gives us some idea of the mass or weight of the sun. What is its size? Our world is nearly eight thousand miles in diameter at the equator; over two hundred and forty thousand miles distant from the earth the moon revolves around it; but if a circle as large as the sun could be drawn, the earth might be placed in the centre, the moon rolled in her proper path around it, and beyond this orbit to the line representing the circumference of the sun would be a distance of nearly two hundred thousand miles more. This vast orb shines with no borrowed ray. It is a great fountain of burning light which shines now as it shone long before the building of our world began. No light is so powerful and intense as sunlight. A flash of lightning is scarcely seen across a sunny sky; the light of lamps is not visible in strong sunshine; no invented light can approach sunshine in brilliancy. Twenty millions of the brightest stars would not shed upon the earth the radiance poured out by the sun. According to the nebular hypothesis of Laplace, the sun not only provides the planets with light and heat, but is the source and parent from which they sprung into space. Thus the most distant planets, as Neptune and Uranus, were the ones first thrown off, and the earth, as only in the third remove from its source, is one of the latest of the sun children. We know that in the far-off ages, when the earth was being built into a habitable globe, the sun shone just as it does to-day. We know this from the eyes of fossil animals, which show that they were formed to receive sunlight just as eyes do to-day. Also the petrified rain-marks, of which we spoke in a former chapter, tell the same story; for without the sun there would have been no rain, nor would imprints have dried or baked so rapidly.