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in accuracy - and he began to achieve this around 1585. One of Tycho's first major projects, which he began as early as 1576, was the accurate determination of Hven's latitude and he did this by two different methods. One was based on averaging the maximum and minimum altitudes of circumpolar stars, and the other on observations of the noon Sun. He found that the results from the solar method consistently were lower than those based on the stars, and by early 1584 he identified the reason: atmospheric refraction. Tycho's recognition of the serious effects of this phenomenon on astronomical observations was a major step forward.

However, Tycho's quantitative calculations of the effect of refraction were erroneous due to his corrections for solar parallax, which he took to have the excessive value found in Ptolemy. In order to get theory to match observation, Tycho's corrections for refraction had to account for the inaccurate solar parallax as well as for the phenomenon itself. Since solar parallax was not relevant to the fixed stars, the light from which also underwent refraction, Tycho was led

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