SOLAR ECLIPSES. 



equal to the difference of the semi-diameters, then the one disc 

 will lie within the other, just touching it, as shown in fig. 4. 

 This position of the disc is called that of internal contact, which 

 therefore takes place when 



D = it r. 



1 5. If the distance between the centres be less than the sum of 

 the semi-diameters, but greater than their difference, then a 

 partial interposition will take place, as shown in fig. 3. If the 

 interposing disc be less than the obscured disc, and the distance 

 between their centres be less than the difference between the 

 semi-diameters, then the interposing disc will lie within the 

 obscured disc, leaving a round ring of illuminated surface, not 

 intercepted, as shown in fig. 5. If, in this case, the centres of 



Fig. 5. Fig. 6. 



the two discs coincide, the surrounding ring of light will be of 

 uniform breadth. 



Such phenomena are called annular eclipses, and when the 

 centres coincide they are said to be centrical and annular. 



16. If the interposing disc be greater than the intercepted one, 

 and at the same time the distance between the centres less than 

 the difference of the semi-diameters, then the interposing disc 

 will cover completely the other, and a total eclipse will take 

 place, as shown in fig. 6. If in this case the centres of the two 

 discs coincide, the eclipse is said to be total and central. In the 

 case of the sun and moon the magnitude of eclipses are expressed 

 by what are called digits. If the diameter of the eclipsed object, 

 be it sun or moon, be supposed to be divided into twelve parts, 

 each of which is called a digit, the eclipse is said to measure as 

 many digits as there are such parts in the greatest breadth 

 of the obscured part, that is, in the difference between the sum 

 of the semi-diameter of the sun and moon and the distance 

 between their centres. To produce a total solar eclipse, it 

 is therefore necessary, 1st, that the apparent diameter of the 

 moon should be equal to or greater than that of the sun ; 

 and, 2ndly, that the apparent places of their centres should 

 approach each other within a distance not greater than the differ- 

 ence of their apparent semi-diameters. When these conditions 

 are fulfilled, and so long as they continue to be fulfilled, the 

 eclipse will be total. 



165 



