Hi INTRODUCTION TO ASTRONOMY. 



from the opposite extremity in the direction CB ; the shadow of the earth 

 cannot, therefore, extend beyond B. As the sun is larger than the earth, 

 the shadow of the latter is conical : it gradually diminishes, and is much 

 smaller than the earth where the moon passes through it, and yet we find 

 the moon to be not only totally eclipsed, but some length of time in 

 darkness. The moon, shining only by reflected light, disappears abso- 

 lutely when the light of the sun is intercepted from her. She does not, 

 therefore, become invisible, like the sun, only from particular spots on the 

 earth's surface, but absolutely and universally, until the light of the sun 

 begins to shine upon her again. The length of an eclipse depends on the 

 respective distances and magnitudes of the sun, earth, and moon. The 

 diameter of the moon is about -^ T of that of the earth, or the whole bulk 

 of the earth about 49 times that of the moon. 



When the moon eclipses the sun to us, the earth is eclipsed to the moon ; 

 for if the moon intercepts the sun's rays, and casts a shadow on us, we 

 must necessarily disappear to the moon, but only partially a black spot 

 will appear to pass over the earth, as in figure 17. 



In the distant planets, few days elapse without an eclipse taking place ; 

 for among the number of satellites, one or other of them is continually 

 passing into the shadow of the planet, or between the planet and the sun. 

 Astronomers are so well acquainted with the motion of the planets and 

 their satellites, that they have calculated not only the eclipses of our 

 moon, but those of Jupiter, with such perfect accuracy, that it has afforded 

 a means of ascertaining the longitude. When, as on land, we know where 

 \ve are situated, there is no difficulty in ascertaining the latitude or longi 

 tude of the place by referring to a map ; but the question is to find out 

 our situation when we do not know where we are : for instance, at sea, 

 interrupted in our course by storms, a map would afford no assistance in 

 discovering where we were. The latitude may be found by taking the 

 altitude of the pole : that is to say, observing the number of degrees that 

 it is elevated above the horizon, for the pole appears more elevated as we 

 approach it, and less as we recede from it. It is true that the pole is not 

 visible to us ; but the north pole points constantly towards one particular 

 part of the heavens, near which a star is situated, called the Polar Star. 

 The altitude of the polar star is therefore nearly the same number of degrees 

 as that of the pole; and, as this star is visible in clear nights from every part 

 of the northern hemisphere, it furnishes an easy mode of ascertaining the 

 latitude in all that half of the world. The latitude may be more accu- 

 rately determined by other observations, which may be made on the sun 

 or any of the fixed stars ; the situation, therefore, of a vessel at sea, with 

 regard to north and south, is easily ascertained. The difficulty is respecting 

 east and west that is to say, its longitude. As there are no eastern poles 

 from which we can reckon our distance, some particular spot must be 

 fixed upon for that purpose. The English reckon from the meridian of 

 Greenwich, where the Royal Observatory is situated ; in French maps, 

 the longitude is reckoned from Paris. 



