History of Longitude - Proposed Methods of Determining Time

Proposed Methods of Determining Time

The first publication of a method of determining time by observing the position of the Earth's moon was by Johannes Werner in his "In hoc opere haec continentur Nova translatio primi libri geographiae Cl. Ptolomaei", published at Nürnberg in 1514. The method was discussed in detail by Petrus Apianus in his Cosmographicus liber (Landshut 1524).

It appears that Johannes Werner inspired by Amerigo Vespucci's letter written in 1502 where he wrote: ". . . I maintain that I learned . . . by the eclipses and conjunctions of the Moon with the planets; and I have lost many nights of sleep in reconciling my calculations with the precepts of those sages who have devised the manuals and written of the movements, conjunctions, aspects, and eclipses of the two luminaries and of the wandering stars, such as the wise King Don Alfonso in his Tables, Johannes Regiomontanus in his Almanac, and Blanchinus, and the Rabbi Zacuto in his almanac, which is perpetual; and these were composed in different meridians: King Don Alfonso's book in the meridian of Toledo, and Johannes Regiomontanus's in that of Ferrara, and the other two in that of Salamanca."2 The best "clock" to use for reference, is the stars. In the roughly 27.3 solar days of a lunar orbit, the Moon moves a full 360 degrees around the sky, returning to its old position among the stars. This is 13 degrees per day, or just over 0.5 degree per hour. So, while the rotation of the Earth causes the stars and the Moon to appear to move from east to west across the night sky, the Moon, because of its own orbit around the Earth, fights back against this apparent motion, and seems to move eastward (or retrograde) by about 0.5 degree per hour. In other words, the Moon "moves" west only 11.5 degrees per hour."