ASTRONOMY. 



[SOLAR ECLIPSES. 



the sun U altogether below the horizon, ma well as that 

 part of the moon which U ecliped, elevate* both objects 

 above the horizon, and an observer at A (Fig. 129) will 

 be able to MM the sun at one tide, and the eclipsed moon 

 on the other. 



I-SHI OF THB Saw. Whilst eclipses of the moon 

 follow from the intenxjsitiou of the earth, and prevent 

 tin- liu'lit of the sun from reaching to, and illuminating 

 the surface of our satellite in the usual manner ; eclipses 

 of the sun are due to the contrary cause, being the 

 shadow of the uioon falling on certain parts of the earth 

 (passing over it like the shadow of a cloud), and hiding 

 the disc of the sun at those districts. The dimensions of 

 the shadow at the surface of the earth may bo found in 

 the same manner as in the former case ; and an idea of 

 the different kinds of solar eclipses may be gathered 

 from the following diagrams. Let L be the position of 

 the uioon (Fig. 130) situated exactly in a line with the 

 Fig. 130. 



Fig. 1J3. 



son S, and the earth T, tnd at a distance LT, which 

 <x>rrespondg to the distance of the sun and raooa at the 

 time The smallest distance of the centres of the earth 

 and moon being equal to 55 '047 semi-diameters of the 

 former body, and the greatest value of the shadow O L 

 ulated in the manner already mentioned) being 59-73 

 emi-diameters of the earth, it would follow that, under 

 those circumstances, the shadow of the moon would ox- 

 tend beyond the centre of the earth, and consequently, 

 that at that region of the earth where the shadow would 

 fall (as at u 6 Fig. 131), the eclipse would be total the 



Fig. izi. 



uioon completely obscuring the sun's disc. On the con- 

 trary, if we take the greatest distance of the earth and 

 moon, which is equal to.CS 802 radii of the former body, 

 and the smallest value of the length of the shadow, which 

 U found to be 67 '76 radii, tlie summit of the cone of the 

 shadow does not reach to the earth (Fig. 132); and, when 

 thU U tlie case, the eclipse eaunot be total for any part 



Fig. 132. 



of the earth ; on all that hemisphere turned towards the 

 an, we perceive a portion, though not the whole of its 

 die. There is, however, a peculiarity to be noted ; which 

 is, that if we prolong the cone of the shadow of the moon 

 'beyond the summit < ). the base of tlie second cone formed 

 will be situated on tlie portion of the surface of the earth 

 c d, and for all this part the eiifipso-will appear < 

 i. e., the mom will appear ^rojoflM upoB the disc of 



the sun at all those parts of the earth, and the exterior 

 portion of the disc of the sun will consequently a; 

 under the form of a luminous rin;,'. When the moon is 

 placed between the sun and the earth, thu eclipse will bo 

 either total or annular for certain parts of the earth, 

 according as the distances of the sun and moon are 

 more or less great This is' also apparent from consider- 

 ing the respective dimensions of the discs of the sun and 

 moon at different periods. The apparent diameter of the 

 moon, as seen from the surface of the earth, may increase 

 to 34' 0', whilst the diameter of the sun may decrease to 

 31' 31*; and if an eclipse take place under those circum- 

 stances it must be total. A- 

 the diameter of the moon, on 

 other occasions, may decrease 

 to 29* 22', whilst that of the 

 sun can attain its maximum 

 value of 32'35*'6, it will follow- 

 that when an eclipse happens 

 under such circumstances, it 

 must be annular, and that for a 

 few moments, at such parts of 

 the earth as are exactly in a line 

 with the centres of the sun and 

 moon, the moon will be seen projected upon the sun ; 

 the exterior parts of the sun will continue, visible, 

 and shine with their usual brilliancy (Fig. 133). 

 At the parts of the earth adjoining those which are. 

 exactly in a line with the centres of the sun and 

 moon, the eclipse cannot be total or annular, as 

 only a portion of the moon can be projected on 

 the disc of the sun. This will be apparent from 

 Fig. 130, in which it is easy to perceive that at 

 all those points situated to the interior of the cone 

 C O' C', and not comprised within the real sliadow 

 B O B', a similar appear- rig. 131. 



once -will be produced as 

 in the case of the penumbra 

 of the earth'* shadow, on 

 the lunar disc, during an 

 eclipse of the moon. From 

 any such point we should 

 consequently perceive the 

 circular disc of the moon in- 

 tercepting a portion of the 

 surface of the sun (as in 

 Fig. 134) ; and tlie 

 fiearer the observer 

 is situated to the 



real cone of shadow BOB 7 , or the further he 

 is distant from the surface of the cone C O' C', 

 the greater will be the portion of the solar 

 surface covered by the moon. We have seen, 

 in the case of lunar eclipses, that the whole disc 

 of the moon can be hid by the sliadow of the 

 earth ; but the moon's shadow only covers an area whose 

 diameter is 180 English miles at the greatest ; while the 

 penumbra may reach to nearly 6,000 miles in diameter. 

 The dimensions of the lunar shadow, here mentioned 

 at the surface of the earth, are calculated in tlie same' 

 manner as that of the earth's shadow. We have already 

 seen that thu semi-diameter of the latter is equal to the 

 parallax .of the sun added to that of the moon, 

 and from which the apparent semi-diameter of 

 the sun is to be subtracted ; in the same way 

 the semi-diameter of the moon's shadow at 

 h is equal to tin: parallax of the earth 

 added to the parallax of the sun atthe distance 

 of the moon, from whieh sum we subtract the 

 apparent semi-diameter of the sun, as ssen 

 from the moon. The latter quantity is found 

 by increasing the semi-diameter as seen from the earth, 

 in tlie proportion of the distances of the moon and 

 earth from the sun at those times, or the apparent 

 semi- diameter of the sun seen from the moon, 



= apparent semi-diameter of the sun ( j as seen from tlie 

 earth X ~j.< K being the distance of the earth from the 



