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Rh published in the Scripta of Regiomontanus, the first observation of this kind is from the 6th March 1489, and there are several from the following years; and as the book was published in 1544, Tycho Brahe has known Walther's plan, while the further development of it is due to himself. The method recommended itself to Tycho because it did not involve the accurate knowledge of time by clocks or clepsydræ, while he made this objection to the method followed by the Landgrave of observing the altitude of stars, together with their transits over the meridian or a certain azimuth. The meridian method had been used by Tycho to determine the places of twelve stars observed with the comet of 1577, and for this purpose he made use of Aquilæ as fundamental star, determining its right ascension by observing the meridian transits of it and the moon when not too far apart. He knew, therefore, by experience how undesirable it was to trust to the clocks.

In the spring of 1582 Venus was most favourably situated, and from the 26th February it was for about six weeks clearly visible in full daylight even before it passed the meridian, so that it could be observed at a sufficient height above the horizon to make errors in the adopted refractions harmless. With the sextans trigonicus two observers measured the distance between Venus and the sun, the shadow of the little cylinder at the centre of the arc falling on the movable pinnule; at the same time the