Eclipses. 29 



mencement of the emersion ; and the total emer- 

 sion, which is the end of the eclipse. 



There is also another circumstance to be ob- 

 served in every eclipse, namely, its greatness or 

 extent, that is, the portion of the luminary 

 eclipsed, or which is covered by the shadow. 

 To measure this, the diameters of the sun and 

 moon are supposed to be divided into twelve 

 equal parts, which are called digits; and an 

 eclipse is said to be so many digits, according to 

 the number of those parts which are obscured. 

 In total eclipses of the moon, it is often said to 

 be eclipsed more than twelve digits, though the 

 diameter of the moon is only estimated at that 

 number. The expression then implies that the 

 earth's shadow covers more than the disc of the 

 full moon, and the shadow is measured as if it 

 were a part of the luminary eclipsed. 



The satellites of Jupiter are very frequently 

 eclipsed. They revolve very rapidly round the 

 planet ; their orbit is very little inclined to that 

 of Jupiter ; and the mass of each of them very 

 small, compared with his. It happens, there- 

 fore, that at each revolution they are necessarily 

 plunged in the shadow of the planet, in other 

 words eclipsed. As it happens that these eclipses, 

 from the great distance of Jupiter, can be per- 

 ceived at different parts of the earth at the same 

 instant, they afford a certain and very common 

 method of ascertaining with exactness the differ- 



