FACING PAGE A Total Solar Eclipse _Frontispiece_ The Harvard College Observatory, Cambridge, Mass. 24 Isaac Newton 46 Galileo Galilei 52 The Lick Observatory, Mount Hamilton, Cal. 60 The Yerkes Observatory, Williams Bay, Wis. 100 The Moon, one day after First Quarter 150 William Herschel 234 Pierre Simon Laplace 364 ASTRONOMY CHAPTER I DIFFERENT KINDS OF MEASUREMENT 1. ACCURATE MEASUREMENT.--Accurate measurement is the foundation of exact science, and at the very beginning of his study in astronomy the student should learn something of the astronomer's kind of measurement. He should practice measuring the stars with all possible care, and should seek to attain the most accurate results of which his instruments and apparatus are capable. The ordinary affairs of life furnish abundant illustration of some of these measurements, such as finding the length of a board in inches or the weight of a load of coal in pounds and measurements of both length and weight are of importance in astronomy, but of far greater astronomical importance than these are the measurement of angles and the measurement of time. A kitchen clock or a cheap watch is usually thought of as a machine to tell the "time of day," but it may be used to time a horse or a bicycler upon a race course, and then it becomes an instrument to measure the amount of time required for covering the length of the course. Astronomers use a clock in both of these ways--to tell the time at which something happens or is done, and to measure the amount of time required for something; and in using a clock for either purpose the student should learn to take the time from it to the nearest second or better, if it has a seconds hand, or to a small fraction of a minute, by estimating the position of the minute hand between the minute marks on the dial. Estimate the fraction in tenths of a minute, not in halves or quarters. EXERCISE 1.--If several watches are available, let one person tap sharply upon a desk with a pencil and let each of the others note the time by the minute hand to the nearest tenth of a minute and record the observations as follows: 2h. 44.5m. First tap. 2h. 46.4m. 1.9m. 2h. 44.9m. Second tap. 2h. 46.7m. 1.8m. 2h. 46.6m. Third tap. 2h. 48.6m. 2.0m.