INTRODUCTION TO ASTRONOMY. liii 



The rotation of the earth on its axis in twenty-four hours, from west to 

 east, occasions, as we have already seen, an apparent motion of the sun 

 and stars in the contrary direction, and the sun appears to go round the 

 earth in the space of twenty-four hours, passing over fifteen degrees, or a 

 twenty-fourth part of the earth's circumference every hour : therefore, when 

 it is twelve o'clock in London, it is one o'clock in any place situated fifteen 

 degrees to the east of London, as the sun must have passed the meridian of 

 that place an hour before he reaches that of London. For the same reason 

 it is eleven o'clock to any place situated fifteen degrees to the west of 

 London, as the sun will not come to that meridian till an hour later. If, 

 then, the captain of a vessel at sea could know precisely what was the hour 

 at London, he could, by looking at his watch, and comparing it with the 

 hour of the spot in which he was, ascertain the longitude. For this pur- 

 pose he must be furnished with two watches the one daily regulated by 

 the sun, and the other unaltered. The former would indicate the hour 

 of the place in which he was situated, and the latter the hour of London ; 

 and by comparing them together, he would be able to calculate his longi- 

 tude : this mode of finding the longitude is universally adopted. Watches 

 of a superior construction, called chronometers, or time-keepers, are used 

 for this purpose ; but the best watches are liable to imperfections, and 

 should the time-keeper go too fast or too slow, there would be no means 

 of ascertaining the error : implicit reliance cannot consequently be placed 

 upon them. 



Recourse is therefore had to the eclipses of Jupiter's satellites. A table 

 is made of the precise time at which the several moons are eclipsed to a 

 spectator at London. When they appear eclipsed to a spectator in any 

 other spot, he may, by consulting the table, know what is the hour at 

 London; for the eclipse is visible at the same moment from whatever 

 place on the earth it is seen. He has then only to look at the watch 

 which points out the hour of the place in which he is, and by observing 

 the difference of time there, and at London, he may immediately determine 

 his longitude. 



Let us suppose that a certain moon of Jupiter is always eclipsed at six 

 o'clock in the evening at London, and that a man at sea consults his watch, 

 and finds that it is ten o'clock at night where he is situated, at the moment 

 the eclipse takes place : he would be sixty degrees east of London; for the 

 sun, which travels (apparently) fifteen degrees an hour, must have passed 

 his meridian four hours before it reaches that of London ; for this reason, 

 the hour is always later than in London when the place is east longitude, 

 and earlier when it is west longitude. Thus the longitude can be ascer- 

 tained whenever the eclipses of Jupiter's moons are visible. 



The latitude shows on what meridian you are situated, and the longi- 

 tude on what part of that meridian ; therefore, when you can ascertain 

 both these, you discover the very spot in which you are situated. But it 

 is not only the secondary planets which produce eclipses, for the primary 

 planets near the sun eclipse him to those at a greater distance, when they 

 come in conjunction in the nodes of their orbits; but as the primary 

 planets are much longer in performing their course round the sun than 

 the satellites in going round their primary planets, these eclipses very 

 seldom occur. 



Mercury and Venus have however passed in a right line between the 

 earth and the sun, but being at so great a distance, their shadows did not 

 extend so far as the earth. No darkness was therefore produced on any 

 part of our globe ; but the planet appeared like a small black spot, 



