146 OUTLINES OF NATURAL PHILOSOPHY. 



is 63.862, the shadow could not reach the earth, 

 and the eclipse could not any where be total. 



b. When the earth is in the perihelion, the length of 

 the moon's shadow is 57.76 ; and if at the same time 

 the moon be in the perigee, or indeed nearer than 

 her mean distance, a total eclipse may happen. 



c. The moon's mean motion about the centre of 

 the earth is 33' in an hour ; and the shadow of the 

 moon, therefore, traverses the surface of the earth 

 when it falls on the surface perpendicularly, with 

 a velocity of about 380 miles in a minute. When 

 the shadow falls obliquely, its velocity appears 

 greater in the inverse ratio of the sine of the obli- 

 quity. 



Relatively to a point on the earth's surface, the sha- 

 dow may go much faster than this, as its motion 

 may be in an opposite direction to the diurnal ro- 

 tation. 



The duration of a total eclipse, in any given place, 

 cannot exceed 7' 58". LA LANDE, 1777. 



An Annular Eclipse, or one where the sun's disk 

 appears like a ring all round the moon, may last 

 12m 24 s . 



To have a partial eclipse of the sun, it is not requi- 

 site that the shadow should reach the earth ; it is 

 sufficient that the distance of the centres of the 

 sun and moon be less than the sum of their appa- 

 rent semidiameters. 



148. When 



