ECLIPSES. 



17. The greatest value of the apparent semi-diameter of the 

 moon being 1006", and the least value of that of the sun being 

 945", we shall have for the difference of their semi-diameters 61". 

 The greatest possible duration, therefore, of a total solar eclipse 

 will be the time necessary for the centre of the moon to gain upon 

 that of the sun 61" X 2 = 122". But since the mean synodic 

 motion of the moon is at the rate of 30" per minute, it follows 

 that the duration of a total solar eclipse can never exceed four 

 minutes. 



18. When the apparent diameter of the moon is less than that 

 of the sun, its disc will not cover that of the sun, even when 

 concentrical with it. In this case, a ring of light would be 

 apparent round the dark disc of the moon, the breadth of which 

 would be equal to the difference of the apparent semi-diameters, 

 as represented in fig. 5. When the discs are not absolutely con- 

 centrical, the distance between their centres being, however, less 

 than the difference of their apparent semi-diameters, the dark 

 disc of the moon will still be within that of the sun, and will 

 appear surrounded by a luminous annulus, but in this case the 

 ring will vary in breadth, the thinnest part being at the point 

 nearest to the moon's centre ; and when the distance between 

 the centres is reduced to exact equality with the difference 

 of the apparent semi-diameters, the ring becomes a very thin 

 crescent, the points of the horns of which unite, as represented 

 in fig. 4. 



The greatest breadth of the crescent will be in this case 

 equal to the difference of the apparent diameters of the sun and 

 moon. 



The greatest apparent semi-diameter of the sun being 16' 18", 

 and the least apparent semi-diameter of the moon being 14' 44", 

 the greatest possible breadth of the annulus when the eclipse is 

 centrical will be 



16' 18" 14' 44" = V 34" = 94", 



which is about the 20th part of the mean apparent diameter of 

 the sun. 



19. The greatest interval during which the eclipse can continue 

 annular is the time necessary for the centre of the moon to move 

 synodically over 94" X 2 = 188", and, since the mean synodic 

 motion is at the rate of 30" per minute, this interval will be about 



1 9^ 



= 6'26 minutes, or about six minutes and a quarter. 



oU 



20. Solar eclipses can only occur at or near a new moon. This 

 166 



