ECLIPSES. 69 



two velocities which is observed. On the equator, therefore, 

 2163 — 1000 1 163 miles in an hour. In higher latitudes, 

 where the surface of the earth moves with less velocity, the 

 difference is greater. At 40° north, for example, the velocity 

 of the earth's surface is 766 miles in the hour and the shadow 

 would pass it at 1397 miles per hour — 2049 feet in a second, 

 the velocity of a cannon shot. 



Now, supposing the sun in Cancer when he is eclipsed, 

 the shadow of the moon will fall upon the earth vertically at 

 latitude 23° 30' north. And we will suppose the shadow spot 

 to have a diameter of 160 miles. Totality begins as soon as the 

 shadow spot covers the observer and ends when it leaves him. 



The earth turns eastward in 23° 30' north at 923 miles per 

 hour (nearly), so the shadow passes the observer at 1240 

 miles per hour, and by Rule of Three will be about seven 

 minutes and three-quarters in getting by. For so long the 

 eclipse is total. It is easy to see that these results are modi- 

 fied in various ways by the time of day, the season of the 

 year, by the latitude and by other causes, all of which have 

 to be taken into account in calculating the elements of a solar 

 eclipse. The above is by no means the way in which the 

 duration of a solar eclipse at a particular point is computed, 

 but it is on the whole, perhaps, the best way of getting a gen- 

 eral idea of the phenomenon. 



The Nautical Almanac gives the several elements of a solar 

 eclipse and a chart of its path. From these and his own 

 observations the computer at one station and at another can 

 work out local results for himself. 



