* 46, 47] 



Ptolemy 



star appears to the observer at B nearer to the zenith z 

 than it actually is, i.e. the light appears to come from s' 

 instead of from s ; it is shewn further that this effect must 

 be greater for bodies near the horizon than for those near 

 the zenith, the light from the former travelling through 

 a greater extent of atmosphere; and these results are 

 shewn to account for certain observed deviations in the 

 daily paths of the stars, by which they appear unduly 

 raised up when near the horizon. Refraction also explains 

 the well-known flattened appearance of the sun or moon 

 when rising or setting, the lower edge being raised by 



FIG. 32. Refraction by the atmosphere. 



refraction more than the upper, so that a contraction of 

 the vertical diameter results, the horizontal contraction 

 being much less.* 



47. The Almagest is avowedly based largely on the work 

 of earlier astronomers, and in particular on that of Hippar- 

 chus, for whom Ptolemy continually expresses the greatest 

 admiration and respect. Many of its contents have there- 

 fore already been dealt with by anticipation, and need not 

 be discussed again in detail. The book plays, however, 

 such an important part in astronomical history, that it 

 may be worth while to give a short outline of its contents, 



* The better known apparent enlargement of the sun or moon 

 when rising or setting has nothing to do with refraction. It is an 

 optical illusion not very satisfactorily explained, but probably due to 

 the lesser brilliancy of the sun at the time. 



