420 ASTRO 
thing of them, as they are hid from his view by the land- 
wall or bank on the other fide, which is raifed higher than 
the marfii, to keep out the waters of the river. This cu¬ 
rious effedt is probably owing to the moift and denfe va¬ 
pours, juft above and riling from the furface of the water, 
being raifed higher or lifted up with the furface of the wa¬ 
ter at the time of high tide, through which the rays pals, 
and are the more refrafted. See the article Refraction. 
Of the ABERRATION of LIGHT in the STARS 
and PLANETS. 
By aberration is meant, an apparent motion of the ce- 
leftial bodies, occafiotied by the progrelTive motion of light, 
and the Earth’s annual motion in her orbit. Its effedt may 
be explained and familiarized by the motion of a line pa¬ 
rallel to itfelf, much after the manner that the compo- 
lition and refolution of forces are explained. If light have 
a progreflive motion, let the proportion of its velocity to 
that of the Earth in her orbit, be as the line BC to the 
line A C ; then, by the compolition of thefe two motions, 
the particle of light will fcem to defcribe the line B A or 
DC, inllead of its real courfe 
B C ; and will appear in the di¬ 
rection AB or CD, inftead of its 
true direction CB. So that if 
A B reprefent a tube, carried with 
a parallel motion by an obferver 
along the line AC, in the time 
thataparticleof light would move 
over the Ipace B C, the different 
places of the tube being A B, ab , 
cd, CD ; and when the eye, or 
end of the tube, is at A, let a 
particle of light enter the other 
end at B ; then when the tube is 
at a b, the particle of light will be 
at e, exadfly in the axis of the tube; and when the tube 
is at cd, the particle of light will arrive at f, (fill in the 
axis of the tube ; and laftly, when the tube arrives at C D, 
the particle of light will arrive at the eye or point C, and 
consequently will appear to come in the direction DC of 
the tube, inftead of the true direction B C. And fo on, 
one particle fucceeding another, and forming a continued 
ftream or ray of light in the apparent direction DC. So 
that the apparent angle made by the ray of light with the 
line AE, is the angle DCE, inftead of the true angle 
BCE; and the difference, BCD or A B C, is the quan¬ 
tity of the aberration. 
This difcovery we owe to the ingenuity of the late Dr. 
Bradley, Aftronomer Royal ; to which he was acciden¬ 
tally led by the relult of fome obfervations which lie had 
made with another view, namely, to determine the annual 
parallax of the fixed ftars, or that which we have feen to 
arife from the motion of the Earth in its annual orbit about 
file Sun. The annual motion of the Earth about the Sun 
had been much doubted, and warmly contefted. The 
difcoverers of that motion, among other proofs of the re¬ 
ality of it, conceived the idea of adducing an incontefta- 
ble one from the annual parallax of the fixed ftars, if the 
ftars lliould be within fuch a diftance, or if inftruments 
and obfervations could be made with fuch accuracy as to 
render that parallax fenlible ; and with this view various 
attempts were made. Before the obfervations of M. Picard 
in 1672, it was the general opinion, that the ftars did not 
change their pofition during the courfe of a year. Tycho 
Brahe and Ricciolus fancied that they had affured them- 
felves of it from their obfervations ; and from thence they 
concluded that the Earth did not move round the Sun/ 
and that there was no annual parallax in the fixed ftars. 
M. Picard, in the account of his Voyage d’lfranibourg, made 
m 1672, fays that the pole ftar, at clifterent times of the 
year, has certain variations which he had obferved for 
about ten years, and w hich amounted to about 40" a year : 
whence fome were led to conclude that thefe variations 
were the effedt of the parallax of the Earth’s orbit. But 
N O M Y. 
it was impoftible to explain it by that parallax ; becaufe 
this motion was in a manner contrary to what ought to fol¬ 
low only from the motion of the Earth in her orbit. 
In 1674 Dr. Hook publiftied an account of obfervations 
which he had made in 1669, and by which lie had found 
that the ftar y Draconis was twenty-three feconds more 
northerly in July than in October: obfervations which, 
for the prefent, feemed to favour the opinion of the Earth’s 
motion, although it be now' known that there could not be 
any truth or accuracy in them. Flamftead having alfo ob¬ 
ferved the pole ftar with his mural quadrant, in 1689 and 
the following years, found that its declination was 40" lefs- 
in July than in December; which obfervations, although 
very juft, were yet however improper for proving the an¬ 
nual parallax ; and he recommended the making of an in- 
ftrument of fifteen or twenty feet radius, to be firmly fixed 
on a ftrong foundation, for deciding a doubt which was 
otherwife not likely to be brought to a conclufion. In this 
ftate of uncertainty, Dr. Bradley, in the year 1725, formed 
the project of verifying, by a feries of new obfervations, 
tliofe which Dr. Hook had communicated to the public al- 
moft fifty years before. And it was this attempt that gave 
rife to the difcovery of the aberration ; for he found that 
all the phenomena Hitherto mentioned, and which had 
been inexplicable to other obfervers, proceeded from the 
progreflive motion of light, and the Earth’s annual motion 
in her orbit: for, if light w'ere propagated in time, the ap¬ 
parent place of a fixed object would not be the fame when 
the eye is at reft, as when it is moving in any other direc¬ 
tion but that of the line palling through the objedt and 
the eye; and, when the eye is moving in different direc¬ 
tions, the apparent place of the objedt would be different. 
He confidered This matter in the following manner. He 
imagined C A to be a ray of light, falling perpendicularly 
upon the line B D : then if the eye be at reft at A, the ob- 
jedl mull appear in the direflion AC, whe¬ 
ther light be propagated in time, or in an 
inftant. But if the eye be moving from B 
towards A, and light be propagated in 
time, with a velocity that is to the velocity 
of the eye, as AC to AB; then, light 
moving from C to A, whilft the eye moves 
from B to A, that particle of it by which 
the object will be difeerned, when the eye 
in its motion conies to A, is at C when the 
eye is at B. Joining the points B, C, he 
fuppofed the line B C to be a tube, inclined 
to the line BD in the angle DBC, and of 
fuch a diameter as to admit of but one par¬ 
ticle of light : then it was eafy to conceive, 
that the particle of light at C, by which 
tile objeift muft be feen when the eye arrives DA. B 
at A, would pafs through the tube B C, fo inclined to the 
line B D, and accompany the eye in its motion from B to- 
A ; and that it would not come to the eye, placed behind 
fuch a tube, if it had any other inclination to the line 
BD. If, inftead of fuppofing B C fo fmall a tube, we 
conceive it to be the axis of a larger; then, for the fame 
reafon, the particle of light at C cannot pafs through that 
axis, unlefs it be inclined to B D in the fame angle DBC. 
In like manner, if the eye move the contrary way, from. 
D towards A, with the fame velocity ; then the tube mull 
be inclined in the angle B DC. Although therefore the 
true or real place of an objedt, be perpendicular to the 
line in which the eye is moving, ybt the vifible place will 
not be fo; iince that muff doubtlefs be in the direction of 
the tube. But the difference between the true and appa¬ 
rent place, will be, catcris paribus, greater or lefs, accord¬ 
ing to the different proportions between the velocity of 
light and that of the eye : fo that, if we could fuppofe 
light to be propagated in an inftant, then there would be 
no difference between the real and vifible place-of an ob¬ 
ject, although the eye were in motion; for in that cafe, 
AC being infinite with refpedt to A B, the angle ACB, 
which is the difference between the true and vifible place, 
vaniihes. 
