In 1600, Kepler had the opportunity to work with the Holy Roman Emperor’s royal astronomer, the Dane Tycho Brahe. Earlier, from his observatory Uraniborg on the island of Hven, Brahe had set up a scientific laboratory, with a number of employees to aid in the making of observations, tabulating data, and calculating planetary positions. His dedication to making the best possible observations to drive astronomy forward meant that his naked-eye data were very accurate, to within an arcminute or two. An arcminute is a sixtieth of a degree, just as the more familiar time minutes are sixtieths of an hour. (And, just for fun, the 60 seconds that make up a minute, got that name from originally being called second minutes, since they divided the minutes.)
In creating his system of nature, Tycho combined aspects of both Ptolemy and Copernicus’s systems. The earth lay at the center, stationary, heavy, and unmoving. Around it spun the sun, stars, moon, and planets every day, just as for Ptolemy. But, Brahe had the other planets go around the sun, which itself went around the earth. The combined motion of the sun and the planets around the sun took care of the looping motion.
Now, the planets didn’t quite go around the sun. Like Copernicus, Brahe used an artificial (“mean”) sun &emdash; the gray point here &emdash; a point near the true sun, which goes around the earth at a constant speed. Like Copernicus, the actual sun played no role in Tycho’s model, and he too used a double-epicycle, rather than an equant to account for the uneven motion of the planet, excluding retrogrades.