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Rh would tend to make the longitude of Venus appear greater. It seems, however, that the real cause was the unlucky solar parallax of 3′ which Rothmann (like Tycho) had borrowed from the ancients, and which would act particularly injuriously on his results, as his observations were all made in winter, and at low altitudes of both the sun and Venus, and not combined, like those at Hveen, to eliminate errors as much as possible.

On the basis of the nine standard stars and twelve additional stars near the zodiac, Tycho Brahe built up his star catalogue. Of a star to be determined, the declination was measured directly by the armillæ or a meridian quadrant, and the distance from a known star was measured with a sextant. This furnished, as before, a spherical triangle, with the three sides known, from which the angle at the pole or the difference of right ascension could be computed. Generally the star was connected with two known stars, one preceding and one following it, which gave two results for the right ascension as a control. Tycho communicates twelve examples of this double determination, the results always agreeing within a minute. For stars in higher declinations the additional precaution was taken of connecting them with three stars, as in the case of the constellation of Cassiopea, in which Tycho was specially interested on account of the new star, and which he observed in 1578 and 1583. The other constellations were all observed in the years 1586 to 1591. It is needless to say that the