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Rh by one, now by another assistant (though their names are not given), and a great deal of it was written by the above-mentioned Elias Olsen, whose writing appears in it for the last time in April 1589. Probably he left Tycho's service at that time, as he is mentioned in the diary as having arrived and departed several times after that date.

In 1584 Elias Olsen was sent by Tycho on an astronomical expedition of some importance. At Hveen the inclination of the ecliptic had been found equal to 23° 31′.5, while Copernicus had found 23° 28′. Tycho correctly explained this by pointing out that Copernicus had measured the meridian altitudes of the sun at the summer and winter solstices without taking refraction into account, and for the latitude of Frauenburg in Prussia this would at the winter solstice cause an error of over 4′ in the altitude. Tycho, however, believed the solar refraction at the altitude of 12° to be equal to 9′; but, on the other hand, he assumed with Copernicus, that the solar parallax was 3′, so that one mistake is somewhat compensated by the other. He had also found that the solar theory of Copernicus often deviated considerably from the observed places of the sun, and he suspected that Copernicus had reduced his solar observations with an erroneous value of the latitude. He, therefore, gladly took an opportunity of verifying this latitude when, early in 1584, an embassy from George Frederic, Margrave of Ansbach, headed by a nobleman of the name of Levin