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HISTORY OF PHYSICAL ASTRONOMY.

travelling in his orbit, during the period included between the two observa-tions, while on the other hand the position of the star is invariable, it ismanifest that the excess of the second of these intervals over the first,when converted into space by supposing every 15° equivalent to an hour,will indicate the arc of right ascension described by the sun during theintermediate period. The half of this arc then gives the distance of thesun from the solstice at either observation, and since the difference of theright ascensions of the sun and star is known by the clock, the distanceof the star from the solstice is also at once ascertained.

It is obvious that the practice of observing a star in connexion withthe sun according to the above method, is attended with a twofold advan-tage. In the first place it serves to determine the right ascension of thesun independently of the solar tables, by affording an accurate measure-ment of the arc described by that body, during the interval of time thatelapses between the two observations. Secondly, it leads at the same timeto a knowledge of the absolute right ascension of the star. According toevery method hitherto practised by astronomers, the ascertainment of theabsolute right ascension of the sun was effected by noting the time of hispassing through the equinox, and calculating his subsequent motion bythe aid of the solar tables. The places of the stars with respect to theequinox were then determined by measuring the distance between the sunand a bright star, employing for this purpose the moon or Venus, as theintermediate object of observation. The method of Flamsteed had theadvantage of assigning an easy and direct process by means of which theright ascensions of the sun and star might be simultaneously determined.Moreover, since the altitude of the sun is supposed, according to thismethod, to be the same at each of the two extreme observations, the finalresults are not affected by any uncertainty in the values of parallax or re-fraction. They are also, independent of a knowledge of the latitude ofthe place of observation, an element which entered essentially into allthe ancient methods for determining the place of a celestial body withrespect to the equinox.

The absolute right ascension of any one star being accurately deter-mined, the right ascensions of all the others may be readily found by acomparison with the fundamental star. In the year 1690 Flamsteeddetermined the absolute right ascensions of no less than forty stars bycomparing them directly with the sun in the mode above described, andemployed the results in establishing a series of points of reference to aidhim in his future observations. By his labours on this occasion his namehas become imperishably associated with one of the fundamental methodsof astronomical science.

Having established the elements of the solar theory and other pointsof primary importance, Flamsteed was now enabled to derive from hisobservations the absolute positions of the celestial bodies. He thereforeproceeded to compute the observed positions of the moon and planets,with the view of making the results subservient to the rectification of thetheory of their movements. He also commenced the formation of a cata-logue of the fixed stars upon a more extensive scale than any which hadbeen hitherto executed. It must be acknowledged that in the prosecutionof these arduous labours, he received very little assistance from the Go-vernment. They had not hitherto furnished the observatory with a singleinstrument, nor did they allow him even the smallest sum for the occa-sional repair of such instruments as he was actually using. When' he