ASTRONOMY. 



thus, if the diameter of the moon is to 

 that of the shadow as four to five, then 

 the eclipse is said to be fifteen digits. 

 The duration of a lunar eclipse is vari- 

 ous, it sometimes lasts two or three hours. 

 The eclipses of the sun are owing to a 

 different cause than those of the moon. 

 They are occasioned by the moon's com- 

 ing directly between us and the sun, and 

 therefore obstructing our view of it. 

 When the moon happens to be in conjunc- 

 tion with the sun, or between the sun and 

 the earth, viz. at the time of the new 

 moons, the shadow of the moon falls up- 

 on the surface of the earth; hence, pro- 

 perly speaking, such eclipses should be 

 called eclipses of the earth. But the 

 whole disc of the earth cannot be involv- 

 ed in the shadow of the moon, because 

 the moon is much smaller than the earth, 

 and the shadow of the moon is conical. 

 Thus, in Plate III. fig. 1, the rays of the 

 sun, S, being intercepted by the moon, L, 

 form the conical shadow C D G, which, 

 falling upon the surface of the earth, en- 

 tirely deprives that portion of it upon 

 which it falls of the sun's light, and of 

 course the inhabitants of that part of the 

 earth will have a total eclipse of the sun. 

 Beyond the dense conical shadow, C D G, 

 there is a diverging half shadow, or pe- 

 numbra, C D E F, which is occasioned by 

 the moon's intercepting only a pi' rt of the 

 sun's rays from those places wliiuii fall 

 within this penumbral cone, and are out 

 of the dense shadow. Thus, from the 

 part of the earth Z the portion Y Y B of 

 the sun only can be seen ; consequently, 

 the inhabitants of that part will have a 

 partial eclipse. As the moon is not al- 

 ways at the same distance from the earth, 

 it sometimes happens that the conical 

 dense shadow does not reach the earth, 

 as in fig. 2, and only the penumbral sha- 

 dow falls upon it, the eclipse consequently 

 is partial to every part of the earth. 

 Those who are at the centre of the 

 penumbra will lose sight of the centre 

 of the sun by the interposition of the 

 moon's body, which, subtending a small- 

 er angle than the sun, will not entirely 

 cover its surface, so that there will be a 

 ring of light all round. The eclipse is 

 then said to be annular. The satellites, 

 or moons, are often eclipsed by the pla- 

 nets to which they belong. The eclipses 

 of Jupiter's moons, as we have already 

 observed, are very useful in ascertaining 

 the longitude. When any of the planet- 

 ary bodies disappear, by another coming 

 before it, it is called an occupation. The 

 occultations of the fixed stars by the moon 



are of great importance also in determin- 

 ing the longitudes of places. 



OF THE TIDES. 



The ebbing and flowing of the sea was 

 first shown by Kepler to be owing to the 

 moon's attraction, and Newton demon- 

 strated it upon the principles of gravita- 

 tion. The attraction of the moon cannot 

 alter the shape of the solid of the globe ; 

 but it has a considerable effect upon the 

 fluid part, which it causes to assume a 

 spheroidal figure, the longest axis being 

 in the direction of the moon. It is there- 

 fore the highest tide at that place per- 

 pendicularly under the moon, or where 

 the moon crosses the meridian. The sun 

 also has some action upon the waters, 

 though its attraction, on account of its dis- 

 tance, is not so strong as that of the moon. 

 When the action of the sun and moon 

 conspire together, the tide rises higher, 

 and produces what are called springtides. 

 On the contrary, when they counteract 

 each other, they produce neap tides. The 

 ocean, it is well known, covers more than 

 one half of the globe ; and this large body 

 of water is found to be in continual mo- 

 tion, ebbing and flowing alternately, with- 

 out the least intermission. What con- 

 nection these motions have with the 

 moon we shall see as we proceed ; but at 

 present it will be sufficient to observe, 

 that they always follow a certain general 

 rule. For instance, if the tide be now at 

 high-water mark in any port or harbour 

 which lies open to the ocean, it will pre- 

 sently subside, and flow regularly back 

 for about six hours, when it will be found 

 at low-water mark. After this, it will again 

 gradually advance for six hours, and then 

 return back in the same time to its former 

 situation ; rising and falling, alternately, 

 twice a day, or in the space of about 

 twenty-four hours. And by observing 

 the tides continually at the same place, 

 they will always be found to follow the 

 same rule ; the time of high water upon 

 the day of every new moon being nearly 

 at the same hour, and three-quarters of 

 an hour later every succeeding day. Let 

 M (fig. 3.) represent the moon, O the 

 centre of the earth, and A, B, C, &c. 

 different points upon its surface, and let 

 us suppose the earth to be entirely co- 

 vered by the ocean. Then, because it is 

 the property of a fluid for its parts to yield, 

 and obey any force impressed upon them, 

 it is clear that the moon M acting upon 

 the surface of the sea at the points A, B, 

 C, &c. will elevate the waters in those 

 parts, and draw them towards her, by her 



