PARALLAX OF THE FIXED STARS. From the an nual motion of the earth about the sun, it necessarily fol lows that the positions of the fixed stars differ from their positions seen from the sun; this difference of position is called the parallax of the fixed stars. It is evident, also, that by the motion of the earth the latitudes and longi tudes of the fixed stars must differ at different seasons, and also their right ascensions and declinations. But so vast are the distances of these bodies compared with the diameter of the earth's orbit, that the changes are quite insensible with inferior instruments; and it is even now a question with many, whether the instruments of astro nomy have as yet arrived at such a degree of perfection, as to render the parallax of any of the fixed stars sensible.
When the Copernican system was first given to the world, the want of a sensible parallax appeared a most for midable difficulty. It necessarily followed, that if this system were true, the bounds of the universe must be ex tended in a manner that appeared very revolting to the notions of that age.
The discovery of a sensible parallax, therefore, was an object of great importance to the supporters of the Coper nican system ; and the efforts of astronomers have been unceasingly employed, either in actual attempts, or in pro jects of investigation. Their exertions have been pro ductive of consequences very important to astronomy.
The above motive for the i.ivestigation of the parallax of the fixed stars, has indeed ceased since the discoveries of Sir Isaac Newton. It was no longer necessary for re moving all doubts of the truth of the Copernican system ; but it became an object to show by its existence the im proved state of the science, and afterwards to ascertain whether it should not be adopted as one of the many cor rections applied to the daily observations of the observa tory.
The first attempt to examine with exactness the posi tion of the fixed stars at different seasons of the year, ap pears to have been made by Tycho Brahe. He, it is well
known, was quite adverse to the theory of the motion of the earth, and the result of his observations of the altitudes of the pole star seems to have confirmed him in his opi nion. Kepler tells us, that the superior meridian altitude of this star always agreed to the same minute, and the same of the inferior. Kepler, however, was a strenuous advocate for the Copernican system ; and remarks, as to these observations of l'ycho Brahe, that neither the instru ments nor observations could be exact to the fifth part of a minute.
Galileo, referring to the observations of Tycho Brahe, seems to suppose him not to have exactly understood the consequences of the earth's motion ; that he imagined the constancy or change of the elevation of the Me itself would decide the question of the annual motion. This mistake of Tycho is not more remarkable than that of an other eminent astronomer about a century afterwards.
Galileo, who, by the discovery of the telescope, was able to adduce so many powerful arguments for the truth of the Copernican system, endeavoured also to render the telescope directly subservient to proving the motion of the earth. The method he proposed he imagined would far surpass the exactness of the instruments of Tycho Brahe. He conceived he could use instruments greatly exceeding his in magnitude,—instruments, in which he tells us one degree might be of the extent of even a mile. His plan was, to place on some building, on a very distant mountain, an horizontal bar, fit for just hiding or bisecting a star when viewed from a great distance through a telescne. He in tended to place himself with his telescope in the meridian of this object, and at a very great distance, so that one of the stars of Ursa Alajor, when below the pole, might be lust hid or bisected by the object so placed, as to appear in the telescope like the horizontal wire in our systems of wires.