We know that, roughly, a year is the time it takes the four seasons to come and go, and that this is done when the Earth travels once round the Sun; that the month is based on the time it takes the Moon to travel once round the Earth; that the week has nothing to do with the Sun, Earth, Moon or Stars, but is a pure invention, and, finally, that the day is the time it takes the Earth to turn once round on its axis. But when we want to know what time it is we mean, of course, what the hour, the minute and, sometimes, even the second of the day is, and these are the small measured parts of time we want to find out about. The time it takes the Earth to make one complete turn on its axis is divided naturally into two parts, more or less equal, depending on where we live, and these parts are daytime and nighttime. This general division of time, marked by alternate daylight and darkness, may have served every need of the cave man at first, but just as he came to have sense enough to crawl into his cave to get out of the rain so the blazing Sun must finally have driven into his awakening brain its use to him as a means of marking time. As his savage mind grew less animal and more human, ideas were formed in it either by instinct or by the first vague glimmerings of reason and he began to think. He saw that when the light of the Sun fell on the trees and the rocks, long, strange, black marks, which we call shadows, were cast by them, and he must have noticed that the shadows swung round the trees and rocks in the opposite direction to the way the Sun was traveling. To mark off with his eye and place a stone at a point somewhere near the middle of the shadow cast by the rising Sun and that cast by the setting Sun, and which would mean that the day was half done was the next great step. These things were, quite likely, the first feeble efforts of the human race to measure time, and out of which the sundial came, as well as the crude beginnings on which the science of astronomy is based. =Solar Time, or Time by the Sun.=—To make a sundial which would give correct Sun time was so hard a problem that men had to think about it for a million years before they could solve it and the chances are that then it was invented by a boy. You will find directions for making a simple sundial in Chapter III. Now let’s find out how we can know when a day begins and when it ends. On first thought this would seem to be an easy thing to do and it is if we are not particular about being exact. You will remember that a day is measured by the time it takes the Earth to turn once round on its axis and what we want to know, now, is how to tell when the Earth has made one complete turn, no more and no less.