Navigation and Latitude

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Which shows how many miles are in one degree of Longitude in every Latitude between the Equinoctial and the Poles, and also the diversity of aspects of the Moon.

A Degree of Longitude

Now I will teach you the rule for how many nautical miles there are in one degree of Longitude, for it varies for every degree of Latitude between the Equinoctial and the Poles. First, under the Equinoctial there are 60 nautical miles in one degree East to West. And the number of miles in a degree of Longitude decreases as the degrees of Latitude increase, in a simple curve.

Longitude at each Latitude

Latitude	Longitude (nm)
0°	60
5°	60
10°	59
15°	58
20°	56
25°	54
30°	52
35°	49
40°	46
45°	42
50°	39
55°	34
60°	30
65°	25
70°	21
75°	16
80°	10
85°	5
90°	0

A Card for Distances

First is the making of the instrument. Around the curve of a half circle, evenly mark the degrees of Latitude from 0 to 90. Then evenly mark the number of miles in the line of the diameter or straight edge from 0 to 60 so that the mark for 0 miles is also the 90 degrees mark and the mark for 60 miles is also the mark for the equinoctial. Take a length of string which is as long as the diameter of the circle and fasten it to the 90 degree mark. To know how many miles there are in a degree at any Latitude, first lay the thread onto the Latitude mark on the curved edge. Then hold the string at the point it crossed the mark in degrees, and swing it to the line of the diameter and read off the number of miles. This instrument is simple to make and accurate in any conditions, so no Ship need be without it and no excuse can be given for not knowing how to find the length of a degree of Longitude at any Latitude.

Longitude affects the aspects of the Moon

Now if you have an Almanac or table of Ephemeredes, it will predict the times of the Eclipses and of the changing of Aspects of the Moon. However it should be known that this time is only true for the location where the Almanac was written, and for those places lying direction North or South of it. Now you must consider your distance East or West from where the book was printed. First you must know the distance in miles from your vessel to the printer's Longitude measures at your current Latitude. Then reckon this distance in degrees, with every degree in miles as per your current Latitude. And for every degree you are to the West, the Moon shall change 4 minutes earlier, and for every 15 degrees, an hour earlier. And this is because they shall touch your Meridian before theirs by one hour as time is reckoned locally. And for every degree you are to the East, the Moon shall change 4 minutes later, and for every 15 degrees, an hour later. So by this rule you may know at what hour and minute the Eclipses and changes of the Moon will happen, if you know what place your Almanac was made for.

Knowing the Aspects of the Moon

Only this much have I said touching the true time of the changes of the Moon, for though some people do say 'why do they not give or make rules for ever to know the hour and minute of the new, full and quarters of the Moon?' and yet they are utterly void of any knowledge in the sciences, whereby they might know the true time of the changes of the Moon; for it is a question Astrological to know the Moon's motion, and question Geometrical to know the true time of the aspects, and thirdly it is a question Cosmographical to know the true Longitude of the place one is in, and the time the Moon changes.