I-IISTOEY OF PHYSICAL ASTRONOMY.

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over that employed by the ancient astronomers. It has been alreadymentioned that Hipparchus determined the places of the stars with respectto the equinox, by using the moon as an intermediate object of observationbetween the sun and the stars. This method, however, was very imper-fect, on account of the rapid motion of the moon and the uncertainty thatprevailed respecting the exact value of her parallax. Some of TychoBrahe ’s contemporaries, among others the Landgrave of Hesse Cassel,sought to determine the absolute place of a star by means of its azimuthand altitude , combined with the time of observation. A serious objectionto this method consisted in the difficulty of measuring time with suf-ficient accuracy. Notwithstanding all the efforts of Tycho Brahe andthe Landgrave , they were unable to procure clocks that could be relied uponfor such a purpose. Tycho has not failed to enumerate the various sourcesof their irregularity. They were perpetually liable to be affected bychanges of temperature and currents of air, even although the precautionwas used of keeping them in heated apartments. Again, it was foundimpossible to construct the teeth of wheels with such regularity as toensure a perfectly uniform motion; and hence, even although the dailytimes of restitution of such clocks should appear equal, by comparing themeither with the sun or the stars, they would fail to indicate the interme-diate intervals of time with the same regularity. Finally, the heavy bodywhich occasioned the motion did not act uniformly during its descent, forwhen it was in the lower part of its course, the weight of the cord by whichit was attached tended in some degree to increase its efficac}', howeverslender the cord might be; and the smallest irregularity could not beoverlooked, since an error of four seconds in time was equivalent to oneof a minute in space *.

While pondering on the difficulties of this subject, the happy ideaoccurred to Tycho Brahe of substituting Venus for the moon, as the inter-mediate object of observation between the sun and the stars. This planethas a much slower motion than the moon, and at the same time has asmaller parallax. It is therefore obviously better adapted than that bodyfor ascertaining an element of fundamental importance, such as the abso-lute right ascension of a star. Before sunset he made simultaneous obser-vations on the sun and the planet, and when the sun disappeared he madesimilar observations on the planet and the star. In this manner he deter-mined the absolute right ascensions of several of the most brilliant stars,and by means of them he easily obtained the right ascensions of all theothersf. It is worthy of remark that Waltherus had already employedVenus for a similar purpose, but this circumstance seems to have totallyescaped the notice of Tycho .

Allusion has been made to the mode of determining the position of astar by means of its altitude and azimuth combined with the time of obser-vation. This method seems to have been practised by the Landgrave ofHesse Cassel, on the occasion of his observations of the new star whichappeared in the constellation of Cassiopeia, in the year 1572. Tycho , inhis work on the same star, has given the observations of the prince rela-tive to its apparent place in the heavens, extending from Dec. 3, 1572,till March 14, 1573 J. These appear to be the earliest observations inwhich the element of time was employed in determining the right ascen-sion of a star.

* Progymnasmata, p. 149. t Ibid., p. 491. J Ibid., p. 152.