HOW LATITUDE IS RECKONED 327 



The declination of the sun is tabulated in the Nautical Almanac for noon 

 at Greenwich, England, for each day. It varies from day to day, so that, in 

 order to know accurately the declination of the sun at the time of taking his 

 observations, it is necessary for the observer to know how many hours before 

 or after noon at Greenwich the observation is taken. This is ordinarily ex- 

 pressed in terms of longitude east or west of Greenwich. 



But at the pole there is no longitude. In spite of this the chronometer is 

 equally necessary at the pole, in order to ascertain from the almanac the de- 

 clination of the sun. 



The best observer with a sextant and an artificial horizon, under ordinary 

 conditions, would hesitate to trust his observations, to determine the sun's 

 altitude, closer than a quarter of a nautical mile, or 15 seconds of an arc, a 

 nautical mile being equivalent to a minute of longitude or a minute of longitude 

 at the equator, or 6,086 feet, instead of the 5,280 feet making a statute mile. 

 This hesitation on the observer's part is due to the fact that in making obser- 

 vations there are three errors likely to be made. The first is that due to lack 

 of ability on the part of the observer himself. The second is the "instrumental 

 error" which can practically be eliminated by using the very highest grade 

 obtainable of instruments. 



But the most serious error of all is that due to refraction. 



ALLOWANCE FOR REFRACTION. 



To give an idea to the outsider of what refraction is, no better example can 

 be adduced than the appearance of an oar in the water. Everybody will recall 

 that it looks as if it were bent at the surface of the water. This is due to 

 refraction. In technical language it is expressed thus : "A ray of light is bent 

 from a straight line as it passes from one medium to another or in passing 

 through a medium of varying density." 



Thus is explained what happens in observing the sun, for the air. from a 

 maximum density at the surface of the earth, becomes thinner and thinner as 

 it gets higher above that surface, so that a ray of light from the sun, when it 

 strikes the earth's atmosphere, bends and keeps bending more and more as 

 it travels toward the earth. Tables have been prepared which give the amount 

 in degrees, minutes and seconds of this refraction. It changes as the barome- 

 ter and thermometer change and the tabulated refraction is mean or average 

 of a large number of observations to determine what the refraction is. 



