On the parallax of a Aquilæ

Abstract

The telescope erected for this investigation resembles in its con­struction that which was formerly used for the observations of a Cygni. It has an achromatic object-glass of 10 feet focal length, and 4 inches diameter. The Astronomer Royal had first selected β Canis Minoris as a proper star to be compared with α Aquilæ ; but finding, upon trial, that it could rarely be seen in the day-time, he was induced to sub­stitute λ Pegasi. Not being quite satisfied of the stability of the in­strument, the author has only computed those observations in which each star was observed in the same day, and in the short interval of three hours; so that it was not likely any sensible change in the te­lescope should have taken place. The result of fifty-four observations between the 25th of July and the 29th of December 1817, afforded no appearance of parallax; indeed the author considering it as a hopeless task to establish its existence by observations on a star so far from the zenith, was about to abandon the subject, when his a tention was again called to it by Dr. Brinkley’s late communication. By reference to an annexed table, it appears that the greatest error in a series of ten observations, made with the transit, could not have been more than 0·"03, and consequently it is not probable that the error in fifty observations should have exceeded half that quantity. Taking, however, every circumstance into consideration, it is possible that the whole parallax of α Aquilæ may have amounted to half a second, which is about a tenth part of that assigned to this star by Dr. Brinkley. The author, however, proposes to continue the inves­tigation; and when his observations shall have been sufficiently mul­tiplied, promises to communicate the result to the Society.