ASTRONOMY. 
41S 
exhalations, that extend to the regions of the (ky. The 
rays of light from the Sun, Moon, and (tars, in palling to 
a fpeCtator upon Earth, come through this medium, and 
are fo refracted in their partage through it, that their ap¬ 
parent altitude is greater than their true altitude. 
To illultrate this, let AC, in the annexed figure, re- 
prefent the furface of the Earth, T its centre, B P a part 
of the atmofphere, HEK the fphere of the fixed fiars, 
A F the fenfible horizon, G a planet, G D a ray of light 
proceeding from the planet to D, where it enters into our 
H 
continually entering a denfer medium, and is every mo¬ 
ment bent towards T, which caufes it to defcribea curve, 
as D A, and to enter a fpeCtator’s eye at A, as if it came 
from E, a point above G. And, as an objeft always ap¬ 
pears in that line in which it enters the eye, the planet 
will appear at E, higher than its true place, and frequent¬ 
ly above the horizon A F, when its true place is below it, 
at G. 
This refraction is greateft at the horizon, and in this 
climate it is found to be about 33'. In climates nearer to 
the equator, where the air is purer, the refraction is lefs; 
and, in the colder climates, it increafes exceedingly, and 
is a happy provifion for lengthening the appearance of the 
light at thofe regions fo remote from the Sun. GalFendus 
relates, that fome Hollanders, who wintered in Nova Zem- 
bla, in latitude 75 0 , were furprifed with a fight of the 
Sun feventeen days before they expected him in the hori¬ 
zon. This difference was owing to the refraction of the 
atmofphere in that latitude. To the fame caufe, together 
with the peculiar obliquity of the Moon’s orbit to the 
ecliptic, fome of thefe very northern regions are indebted 
for an uninterrupted light from the Moon much more than 
half the month,, and fometimes almoft as long as it is ca¬ 
pable of affording any light to other parts of the Earth. 
Through this refraction we are favoured with the fight 
of the Sun about three minutes and a quarter before it 
rifes above the horizon, and alfo as much every evening 
aster it fets below it, which in one year amounts to more 
than forty hours. And it is to this property that we are 
alfo indebted for that enjoyment of light from the Sun 
when he is below the horizon, which produces the phe¬ 
nomena of the crepufculum, or morning and evening twi¬ 
light. The Sun’s rays, in falling upon the higher part of 
the atmofphere, are reflected back to our eyes, and form 
faint light, which gradually augments till it becomes 
day. It is owing to this, that the Sun illuminates the 
whole hemifphere at once : deprived of the atmofphere, 
he would have yielded no light, but when our eyes were 
directed towards him; and, even when he was in meridian, 
fplendour, the heavens would have appeared dark, and 
as full of ftars as on a fine winter’s night. The rays of 
light would have come to us in ffraight lines, the ap¬ 
pearance and difappearance of the Sun would have been 
inftantaneous; we (hould have had a fudden tranfition from 
the brighteft fun-thine to the 1110ft profound darknefs, and 
from thick darknefs to a blaze of light. Thus, by refrac¬ 
tion we are prepared gradually for the light of the Sun, 
the duration of its light is prolonged, and the fliades of 
darknefs foftcned. 
That there is a real refraction of the ftars and planets, 
&c. is deduced not only from phyfical confiderations, and 
from arguments a priori, and a Jimilitudine, but alfo from 
precife aftronomical obfervation : for there are numberlefs 
obfervations by which it appears, that the Sun, Moon, and 
ftars, rife much fooner, and appear higher, than they fliould 
do according to aftronomical calculations. Hence it is ar¬ 
gued, that, as light is propagated in right lines, no rays 
could reach the eye from a luminary below the horizon, 
unlefs they were deflected out of their courfe, at their en¬ 
trance into the atmofphere; and therefore it is certain, 
that the rays are refraCted in paffing through the atmof¬ 
phere. Hence the ftars appear higher by refraCtion than 
they really are ; fo that, to bring the obferved or apparent 
altitudes to the true ones, the quantity of refraCtion mud: 
be fubtraCted. And hence, the ancients, as they were not 
acquainted with this refraction, reckoned their altitudes 
too great, fo that it is no wonder they fometimes commit¬ 
ted confiderable errors. Hence alfo, refraction lengthens 
the day, and fhortens the night, by making the Sun appear 
above the horizon a little before his riling, and a little af¬ 
ter his felting. Refraction alfo makes the Moon and ftars 
appear to rife fooner, and fet 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 appear wholly above the horizon when they are 
entirely below it. Alfo from obfervations it appears, that 
the refraCtions are greater nearer the pole than at fmaller 
latitudes, caufing the Sun to appear fome days above the 
horizon, when he is really below it; and the Moon to be 
feen eclipfed, when (lie is really below the horizon, and 
the Sun above it; doubtlefs from the greater denfity of 
the atmofphere, and the greater obliquity of the incidence. 
Stars in the zenith are not fubject to any refraCtion: 
thofe in the horizon have the greateft of all: from the ho¬ 
rizon, the refraCtion continually decreafes to the zenith. 
All which follows from hence, that, in the firft cafe, the 
rays are perpendicular to the medium ; in the fecond, their 
obliquity is the greateft, and they pafs through the larged: 
fpace of the lower and denfer part of the air, and through 
the thickeft vapours; and, in the third, the obliquity is 
continually decreafing. The air is condenfed, and confe- 
quently refraCtion is increafed, by cold ; for which reafon 
it is greater in cold countries than in hot ones. It is alfo 
greater in cold weather than in hot, in the fame country; 
and the morning refraCtion is greater than that of the even¬ 
ing, becaufe the air is rarefied by the heat of the Sun in 
the day, and condenfed by the coldnefs of the night. Re¬ 
fraCtion is alfo fubjeCt to fome fmall variation at the fame 
time of the day in the fined weather. 
At the fame altitudes, the Sun, Moon, and ftars, all un¬ 
dergo the fame refraCtion : for at equal altitudes the inci¬ 
dent rays have the fame inclinations; and the fines of the 
refraCted angles are as the fines of the angles of inclina¬ 
tion, &c. Indeed Tycho Brahe, who firft deduced the re¬ 
fraCtions of the Sun, Moon, and ftars, from obfervation, 
and whofe Table of the refraCtion of the ftars is not much 
different from thofe of Flamftead and Newton, except near 
the horizon, makes the folar refraCtions about 4' greater 
than thofe of the fixed ftars ; and the lunar refraCtions alfo 
fometimes greater than thofe of the ftars, and fometimes 
lefs. 
