144 ESSAY ON THE VELOCITY OF LIGHT. 
account of its minuteness, escape our investigations, notwithstanding the pre- 
cision of the means of observation employed by modern astronomy. But 
neither of these cases is presented for our consideration; it is true that the 
velocity of the earth is very small compared to that of light, but not so small 
that the apparent deviation due to the velocity of displacement of the observer 
is insensible: this deviation, for its maximum, is about 20 seconds of a degree, 
a quantity not only appreciable, but also even measurable with great precision 
by the aid of the instruments which now exist in observatories. ion 
It is in this apparent deviation of each star, a deviation which changes in diree- 
tion from one period of the year to another, so that the star appears to revolve 
annually around its real position, that consists the phenomenon of aberration 
discovered by Bradley. We will content ourselves with having given in what 
precedes a general idea of the cause of the phenomenon, and we will not en- 
deavor to show how by setting out from the causes indicated we can find the 
laws it follows during the different times of the year. We will simply state 
that the total apparent motion which results to each star takes place in an 
ellipse whose greater axis, as it is for all the stars, is seen under an angle of 
about forty seconds of a degree; this greater axis is always directed parallel 
to the ecliptic. As to the smaller axis of the ellipse, it varies from one star to 
another, and it is so much the smaller as the star is nearer the ecliptic: equal 
to the greater axis for a star situate at the pole of the ecliptic, reduced to zero 
for a star situate directly on the ecliptic. The ellipse of aberration, therefore, 
corresponds with a circle at the pole of the ecliptic, and in going from that pole 
it becomes more and more flattened as the star to which it corresponds is found 
nearer to the ecliptic, so as to be reduced to a simple straight line of a length 
equal to the greater axis for each star situate on this great circle of the sphere. 
As it often happens in the science of observation, Bradley discovered aberra- 
tion without seeking for it. Molyneux had placed at Kew, near London, an 
instrument constructed with the greatest care by the celebrated Graham, and 
destined to observe with all possible precision the passage of stars near the 
zenith. 'The object that astronomers had in view was to arrive at some precise 
data of the annual parallax of the stars. We know that the annual parallax 
of the stars is the displacement with respect to each other which the different 
stars ought to appear to us to have at different periods of the year, because we 
successively see them from different points of the orbit of the earth, so that, 
in going from some and in approaching others, we see them under different 
aspects as the earth carries us in its motion around the sun. Since the 
general adoption of the system of Copernicus, astronomers were desirous to 
prove the existence of that annual parallax which is a necessary consequence, 
and which would be regarded as an incontestable proof of the truth of this 
system. With the instrument of Molyneux were made the first observations 
which led to the discovery of aberration. The following is his account of the 
discovery, given in a letter to Halley :* 
‘*Mr. Molyneux’s apparatus was completed and fitted for observing about the end of 
November, 1725, and on the 3d day of December following the bright star in the head of 
Draco (marked y by Bayer) was for the first time observed as it passed near the zenith, and 
its situation carefully taken with the instrument. The like observations were made on the 
Sth, lith, and 12th days of the same month, and there appearing no material difference in - 
the place of the star, a further repetition of them at this season seemed needless, it being a 
part of the year wherein no sensible alteration of parallax in this star could soon be expected. 
it was chiefly, therefore, curiosity that tempted me (being then at Kew, where the instrument 
was fixed) to prepare for observing the star on December 17, when, having adjusted the 
instrument as usual, I perceived that it passed a little more southerly this day than when it 
was observed before. Not suspecting any other cause of this appearance, we first concluded 
*A letter from the Rev. Mr. James Bradley, Savilian Professor of Astronomy at Oxford, 
and F’. Rk. 8., to Dr. Edmond Halley, Astronom. Reg., &c., giving an account of a new dis- 
covered motion of the fixed stars. —Philosophical Transactions R. S., No. 406, December, 1728. 
