ECLIPSES. 65 



bounded by a circle whose plane is inclined 66° 30' to the 

 plane of the equator and is parallel to the horizon — which 

 passes on the equator through go° east (Bay of Bengal) and 

 90° west (Galapagos Islands, coast of Ecuador), and which 

 touches the polar circles north (head of the Gulf of Anadir, 

 eastern end of Siberia) and south (in the unknown Antarctic 

 Continent). For those 90° east of Greenwich the moon will 

 be just setting in the west ; for those 90° west of Greenwich 

 she will be just rising in the east. The others intermediate. 

 All within the hemisphere described will see her at the point 

 of centrality at the same instant of absolute time, which is 

 recorded as one thing at one place and another thing at 

 another. And it is only this hemisphere in question that can 

 see the eclipse at the moment when it is central. For all the 

 rest of the world the moon will at that moment be either not 

 yet risen in the east or already gone down in the west. 



It is clear from this that if the time of all the phases of an 

 eclipse be accurately calculated for the meridian of Greenwich, 

 or indeed for any other meridian, the hours and minutes of the 

 calculation can be turned into the hours and minutes of an}- 

 other meridian for which she may be above the horizon by 

 adding or subtracting the known difference of longitude, which 

 is time. And such, in fact, is the practice; since the Green- 

 wich Nautical Almanac sets forth, amongst other things, the 

 " elements " of every eclipse down to the hundredth part of a 

 second. 



Now when the centre of the moon is on the axis of the 

 shadow the moon has been wholly immersed one hour, and 

 will so remain for another hour. It follows that those living 

 105° to the eastward and to the westward of Greenwich (let 

 us still say ) can see the moon totally immersed. 



Generally, therefore, when circumstances are favorable, a 

 zone of 210° of longitude is so favored. 



If, now, we take into account the hour which the moon 

 requires to get wholly into the dark and the hour which she 

 requires to get wholly out again, it will appear that 240° of 



