Dictionary of Physics or an Attempt at an Explanation of the Most Important Concepts and Technical Terms of Natural Philosophy

Bradley, when in the year 1725 at Kew near London he observed the distances of several stars from the zenith in succession, using a sector of 24 feet radius constructed by Graham, the graduated arc of which contained only a few minutes of a circle, in order to see if he could perceive any mark of an annual parallax of the Earth's orbit. See Parallax of the Earth's orbit. He continued his observations on this until the year 1728 and noticed that all fixed stars, at the time when they passed through the meridian original: "Mittagsfreis" during the day, moved daily a little further toward the south, while at the time when they culminated at night, they moved from day to day further toward the north; generally, however, all returned to their previous position after the lapse of a year, having meanwhile traversed an ellipse whose major axis, parallel to the ecliptic, amounted to 40'', while the minor axis, standing perpendicular to the ecliptic, was zero for stars in the ecliptic itself, was likewise 40'' at the pole of the ecliptic, and in intermediate positions behaved like the sine of the star's latitude; therefore, the star gamma or beta in Draco, which stands near the north pole of the ecliptic, seemed to describe a circle of 40'' in diameter. However, this movement did not follow at all the rules according to which an apparent movement resulting from the annual parallax of the Earth's orbit should have appeared.

When Bradley was assured that this movement was a general phenomenon of all fixed stars, he undertook to investigate the cause thereof. An annually recurring and general cause had to be discovered, the effect of which behaved like the sine of the star's latitude and amounted to 40'' at its greatest value.

Fortunately, Bradley noticed that these 40'' correspond exactly to the arc of the Earth's orbit that the Earth traverses in 16 minutes of time, and it occurred to him that light needs exactly this time of 16 minutes