Note 291 — NO. II ALMAGEST; BOOK II. EXTRACT FROM CHAP. IX

Thus, in the latitude of Alexandria, when the first point of Gemini is three temporal hours past the meridian, the 16th degree of Libra will be on the ascendant, and the aggregate times of ascension of that degree in the said latitude are 109° 3′: by subtracting 90 from this sum, the remainder will be 19° 3′, the right ascension of the mid-heaven answering to the 18th degree of Cancer. In the latitude of Southern Britain, the 18th degree of Libra would be on the ascendant, of which degree the aggregate times of ascension in that latitude are 206° 44′, from which, if 90 be subtracted, the remainder will be 116° 44′, the right ascension of the mid-heaven answering to the 25th degree of Cancer. The converse of these operations seems too obvious to need explanation.