Familiar Talks on Astronomy; With Chapters on Geography and Navigaton (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1899 edition. Excerpt: ...which it is for so short a distance. This method of measuring a base-line is not much used now; it is not accurate enough. But how did Roemer discover the velocity of light? By the eclipses of Jupiter's satellites. The times of these occultations are predicted (as I have told you) and inserted in the astronomical almanacs. Now, Roemer observed that these eclipses happened 16 min. 26 sec. later when Jupiter was in conjunction than when it was in opposition. (See Fig. 8). Jupiter, you see, is about 185 millions of miles farther off when in conjunction than when in opposition. What idea naturally occurred to Roemer? Why, that light did not travel instantaneously; but that it took 16 min. 26 sec. to traverse the diameter of the earth's orbit. This distance is about 185,000,000 miles; divide it by 16 min. 26 sec, or 986 seconds, and you have the velocity of light per second. Very simple, is it not? And I again call your attention to the manner in which astronomers go on building up the science, step by step, like the " House that Jack built" Jansen invents the telescope; with it Galileo discovers the satellites of Jupiter; and by the eclipses of these satellites, Roemer discovers the velocity of light. The velocity of light can be, and has been, calculated by other methods, which it is not necessary in these talks to explain.1 I have said that the planets belonging to our system were Mercury, Venus, the Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Now, I am quite prepared to hear you say you have frequently seen in the papers that " a new planet had been discovered." This I must tell you about. 1 See End. Brit. Article, Light. The distance between Mars and Jupiter is very great. If you will look back to the series, given...