Scripta clarissimi mathematici M. Ioannis Regiomontani, de torqueto, astrolabio armillari, regula magna ptolemaica, baculo[que] astronomico, & observationibus cometarum, aucta necessariis, Ioannis Schoneri ... additionibus. Item observationes motuum solis, ac stellarum tam fixarum [qua ...] erraticarum. Item libellus M. Georgii Purbachii de quadrato geometrico ...

of the desired elevation in that quadrant upon which it leans, provided that it hangs freely. For if the suspended semicircle, together with its plumb line, has lain upon the circle of latitude and for that reason has been deprived of free and suitable suspension, the ends of the ruler of longitude must be alternated; this indeed happens when the face of the semicircle looks toward the East, but the Sun occupies the western quadrant of the ecliptic, or vice versa when this is in the West and that is situated in the East. Remember also to exercise this alternation of the rulers in subsequent precepts whenever it is necessary. For other stars, however, arrange the ruler of longitude just as we have now ordered for the Sun. Place the head of the ruler of latitude above the latitude of the proposed star, if it has any, for if it lacks one, adapt the ruler of latitude itself to the center line. Then roll the region of the signs together with the crest toward the star, so that its body is seen through both holes of the sights. For then the plumb line of the suspended semicircle will indicate the desired altitude of the star, just as we related concerning the Sun. From this it is clearly gathered that as often as the true place of any star is taken through the preceding problems, the elevation of it above the horizon also offers itself to be recognized of its own accord, and that indeed because of the coherence of the suspended semicircle with the ruler of latitude.

SEVENTH PROBLEM.

To inquire into the diurnal arc of the Sun.

Place the ruler of longitude at the place of the Sun in the ecliptic signifer, and move the region of the signs with the face of the circle of latitude turned toward the East until the plumb line of the suspended semicircle adheres to the center line, which clearly separates the two quadrants. Then, finally, consider the mark of the equinoctial circuit to which the place of the Sun, accepted in the equinoctial signifer, lies next, which you will call the Eastern mark. The meridian mark, however, shall be called that point which is weighed highest on the equinoctial circuit, from where, clearly, the numbering takes its beginning. Therefore, the portion of the equinoctial circuit contained between such two marks will represent the semi-diurnal arc, which, when doubled, will form the entire diurnal arc.