ABERRATION.
to y Draconis, at about the same distance from the north pole of the equator:
for though this star seemed to move the same way as a nutation of the
earth’s axis would have made it, yet, it changing its declination but about
half as much as y Draconis in the same time, (as appeared upon comparing
the observations of both made upon the same days, at different seasons of
the year,) this plainly proved that the apparent motion of the stars was not
occasioned by a real nutation, since, if that had been the cause, the alteration
in both stars would have been near equal.
The great regularity of the observations left no room to doubt but that
there was some regular cause that produced this unexpected motion, which
did not depend on the uncertainty or variety of the seasons of the year.
Upon comparing the observations with each other, it was discovered that in
both the forementioned stars, the apparent difference of declination from the
maxima was always nearly proportional to the versed sine of the sun’s distance -
from the equinoctial points. This was an inducement to think that
the cause, whatever it was, had some relation to the sun’s situation with respect -
to those points. But not being able to frame any hypothesis at that
time sufficient to solve all the phæenomena, and being very desirous to search
a little farther into this matter; I began to think of erecting an instrument
for myself at Wansted, that, having it always at hand, I might with the
more ease and certainty inquire into the laws of this new motion. TThe
consideration likewise of being able by another instrument to confirm the
truth of the observations hitherto made with Mr. Molyneux’s was no small
inducement to me; but the chief of all was, the opportunity I should thereby
have of trying in what manner other stars were affected by the same cause,
whatever it was. For Mr. Molyneux’s instrument, being originally designed
for observing y Draconis, (in order, as I said before, to try whether it had
any sensible parallax,) was so contrived as to be capable of but little alteration -
in its direction, not above seven or eight minutes of a degree : and there
being few stars within half that distance from the zenith of Kew bright
enough to be well observed, he could not, with his instrument, throughly
examine how this cause affected stars differently situated with respect to the
equinoctial and solstitial points of the ecliptic.
These considerations determined me; and by the contrivance and direction
of the same ingenious person, Mr. Graham, my instrument was fixed up
August 19, 1727. As I had no convenient place where I could make use of
so long a telescope as Mr. Molyneux’s, I contented myself with one of but
