REFRACTION. 



REFRACTION, in astronomy, or RE- 

 FRACTION of the stars, is an inflexion of 

 the rays of those luminaries, in passing 

 through our atmosphere, by which the 

 apparent altitudes of the heavenly bodies 

 are increased. This refraction arises from 

 hence, that the atmosphere is unequally 

 dense in different stages or regions ; 

 rarest of all at the top^ and densest of 

 all at the bottom ; which inequality, in 

 the same medium, makes it equivalent to 

 several unequal mediums, by which the 

 course of the ray of light is" continually 

 bent into a continued curve line. And 

 Sir Isaac Newton has shown, that a ray 

 of light, in passing from the highest and 

 rarest part of the atmosphere, down to 

 the lowest and densest, undergoes the 

 same quantity of refraction that it would 

 do in passing immediately, at the same 

 obliquity, Ciit of a vacuum into air of 

 equal density with that in the lowest part 

 of the atmosphere. 



Hence arise the phenomena of the cre- 

 pusculum or twilight ; and hence also it 

 is, that the moon is sometimes seen 

 eclipsed, when she is really below the 

 horizon, and the sun above it. 



That there is a real refraction of the 

 stars, &c. is deduced not only from phy- 

 sical considerations, and from arguments 

 a priori, but also from precise astronomi- 

 cal observations ; for there are number- 

 less observations, by which it appears 

 that the sun, moon, and stars rise much 

 sooner, and appear higher, than they 

 should do according to astronomical cal- 

 culations. Hence it is argued, that as 

 light is propagated in right lines, no rays 

 could reach the eye from a luminary be 

 low the horizon, unless they were deflect- 

 ed out of their course, at their entrance 

 into the atmosphere : and therefore it ap- 

 pears that the rays are refracted in pass- 

 ing through the atmosphere. Since the 

 stars appear higher by refraction than 

 they really are, to bring the observed or 

 apparent altitudes to the true ones, the 

 quantity of refraction must be subtracted. 

 Accordingly the ancients, as they were 

 not acquainted with this refraction, rec- 

 koned their altitudes too great. Refrac- 

 tion lengthens the day, and shortens the 

 night, by making the sun appear above 

 the horizon a little before his rising, and 

 a little after his setting. Refraction also 

 makes the moon and stars appear to rise 

 sooner, and set later than they really do. 

 The apparent diameter of the sun or moon 

 is about 32' ; the horizontal refraction is 

 about 33' ; whence the sun and moon ap- 

 pear wholly above the horizon when they 



are entirely below it. Also from observa- 

 tions it appears, lhat the retractions are 

 greater nearer the pole than at lesser la- 

 titudes, causing the sun to appear some 

 days above the horizon, when he is really 

 below it ; doubtless from the greater den- 

 sity of the atmosphere, and the greater 

 obliquity of the incidence. 



Stars in the zenith are not subject to 

 any refraction: those in the horizon have 

 the greatest of all : from the horizon the 

 refraction continually decreases to the 

 zenith. All which follows from hence, 

 that in the first case, the rays are perpen- 

 dicular to the medium ; in the second, 

 their obliquity is the greatest, and they 

 pass through the largest space of the low- 

 er and denser part of the air, and through 

 the thickest vapours; and in the third, 

 the obliquity is continually decreasing. 

 The air is condensed, and consequently 

 refraction is increased, by cold ; for which 

 reason it is greater in cold countries than 

 in hot ones. It is also greater in cold 

 weather than in hot, in the same country ; 

 and the morning refraction is greater 

 than that of the evening, because the air 

 is rarified by the heat of the sun in the 

 day, and condensed by the coldness of 

 the night. Refraction is also subject to 

 some small variation at the same time of 

 the day in the finest weather. 



The horizontal refraction, being the 

 greatest, is the cause that the sun and 

 moon appear of an oval form at their 

 rising and setting ; for the lower edge of 

 each being more refracted than the up- 

 per edge, the perpendicular diameter is 

 shortened, and the under edge appears 

 more flattened also. Again, if we take 

 with an instrument the distance of two 

 stars when they are in the same vertical 

 and near the horizon, we shall find it 

 considerably less than if we measure it 

 when they are both at. such a height as to 

 suffer little or no refraction ; because the 

 lower star is more elevated than the 

 higher. There is also another alteration 

 made by refraction in the apparent dis- 

 tance of stars : when two stars are in the 

 same parallel of declination, their appa- 

 rent distance is less than the true ; for 

 since refraction makes each of them light- 

 er in the azimuth or vertical in which 

 they appear, it must bring them into 

 parts of the vertical where they come 

 nearer to each other ; because all vertical 

 circles converge and meet in the zenith. 

 This contraction of distance, according 

 to Dr. Halley, (Philos. Trans, numb. 368) 

 is at the rate of at least one second in a 

 degree ; so that, if the distance between 



