HISTORY OF PHYSICAL ASTRONOMY.

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the same time not far removed from the ecliptic, they were subject to onlya very small error in the same direction from the parallax of the earth’sorbit *.

Flamsteed's next object was to ascertain the errors produced by the in-strument on the right ascensions of the stars. He remarked that sucherrors might arise either from the plane of the instrument not beingexactly in the meridian, or from the object-glass of the telescope not beingproperly adjusted in its cell. He proposed to determine by one processthe combined effect due to these two causes. This object he accomplishedfor all altitudes included between the equator and the northern tropic, bycomparing the times of the sun’s passage over the meridian, as deducedfrom observations of equal altitudes on each side of that circle, with thecorresponding times obtained directly by the aid of the instrument. In thismanner he found that at 40° of zenith distance, the transit of a celestialbody, when observed with the instrument, took place 33 seconds earlier thanthe true time. The errors for the parts of the arc, comprised betweenthe tropic and the north pole, and between the equator and the southernhorizon, were found by comparing the differences of the right ascensionsof several bright stars, derived from observations of their intermutualdistances with the sextant, with the corresponding results deduced fromobservations with the instrument itself. Having thus ascertained the errorsof the plane of the instrument at various zenith distances throughout thewhole extent of the arc, Flamsteed arranged the results in a table, whichhe continued to use from the year 1690, in determining by observationthe right ascensions of the stars f.

Flamsteed commenced his course of regular observations with themural arc on the lltli of September, 1689. But before he could deduceany useful results from them, it was necessary for him to determine thelatitude of his observatory with greater accuracy thau he had hithertodone. From observations of the least and greatest altitudes of circum-polar stars, he finally concluded its true value to be 51° 28' 34". He nextproceeded to derive from his observations, the obliquity of the ecliptic,and the other fundamental points of astronomical science. His labourson this occasion are distinguished by considerable originality and address.The most important part of them is that which relates to the determina-tion of the absolute right ascension of a celestial object. It has beenalready mentioned that he succeeded in determining the place of the sunwith respect to the solstice, by observing that body at equal altitudes nearthe equinoxes, and halving the intermediate arc. The magnitude of this arc,however, could only be ascertained by computation, and consequently theaccuracy of the result depended in a great degree upon the solar tables.Flamsteed eluded this source of error by a process which enabled him todetermine not merely the place of the sun with respect to the equinox,but also the places of the stars relative to the same point. For this purposethe interval between the times of the sun’s passage over the meridian andthat of a bright star near the equator, was noted by the aid of the pen-dulum clock, on the occasions when the sun attained the same meridionalaltitude near the two equinoxes. Now since the sun has been continually

* Historia Caelestis, vol. iii., p. 112.

t Hkt- Cad., vol. iii., p. 133. This table does not appear in the Historia Ceeleslis,but Mr. Baily found it among the manuscripts of Flamsteed, and has inserted it in his in-teresting collection of memorials respecting the astronomer.—(See Life of Flam-steed, p. 374.)