Navigation with a Globe

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In which is shown how to sail by the Globe, and to know how much the water is higher than the level between any two ships on the sea, which grows by the roundness of the earth.

The World is a Globe

Now it is convenient to speak of why and when to sail by the Globe. For generally seamen plot their course as though the earth were a platform, for they do not consider that the earth is a Globe, and that the Meridians grow narrower towards the Poles. It is impossible to draw both the face of the earth and of the sea truly on a flat surface. If you describe the lands true, then the seas shall not be true, for as you go towards northern parts, your Meridians grow closer together, so to make your lines according to the art of Hydrography, the Sea must be broader to the North parts than it is. Now if you would describe the Sea truly, with lines, courses, distances, havens and dangers, then likewise your land should be broader to the north than it is. For your better understanding, I will list the circumference of the earth under sundry parallels or circles. First, under the Equinoctial (where the earth is of greatest girth), by going directly East or West, by a straight line over Sea and Land, the two Poles being on your Horizons, you will travel 21,600 nautical miles to return to the place you departed from. Under the North Tropic Circle, the North Pole being raised 23°30', going directly East and West it is 19,810 miles in circumference. At our circle of Seagate where the North Pole is raised 37° it is 17,250 miles, and under the Arctic Circle where the North Pole is raised 66°30' it is only 8,610 miles. By this you see that the distance East to West is much lesser to the North than it is to southwards, wherefore when you shall have any occasion to attempt any voyage to the north, it is best to sail by a Globe; for so shall you better see the distances and size of the lands, and in like manner your lines and courses.

The Use of a Globe

To sail by a Globe, first keep a perfect account and reckoning of the ship's way (according to your accustomed manner), noting by what course or point your ship has made her way, then you must resort to your Globe. Consider next what place and parallel you are in, which you may do by the Sun by day, and the Stars by night. Now tilt your Globe until the elevation of your Latitude is highest, and rotate it until the place of your Zenith is centred. Your location is then level and flat, and courses and bearings may be set as if on a chart. This requires a Compass Rose, which is to be constructed from your East West Latitude line, with the exact quarter of a circle inscribed around your location from this line Polewards, and divided into the 8 points of your compass, doing so likewise on the other sides. And thus you must do as often as possible, for the oftener you observer this customer, the better and more perfect shall your course be.

But for those that travel to southern parts, and yet not so far south that the weather again turns chill and the sun's declination great, nothing is better than their charts. And again, for those proceeding into the pitiless Southern Ocean, a Globe is of more use than a chart, and yet precious little help will either provide in these fierce latitudes, and a Holy Book may serve better than both.

Now thus I briefly make an ending of sailing by the Globe.

Ships over the Horizon

Furthermore, that you may better understand that the Earth is a Globe or Circular, you can perceive it thus; if you see a ship at any far distance, you may perfectly see its sails, but not its hull, due to the circularity of the earth and the water of the sea, for the water rises between you and the other ship, according to the distance between both ships. This is because the distance to the centre of the earth or water is the same in every place. And the amount of water between the ships is due solely to the distance between them, in addition to any occasional swells or waves.

I will also say a little whereby you may discern how far it is possible to see a ship upon the sea, by a list of distances and how far the water rises between ships at this distance. And if the water is risen 10 feet between the ships then each observer must also be raised ten feet to see each other, or however much one is lower than this, the other must be likewise raised, and this is why a lookout should be posted on the highest point of the mast.

Water between Ships

Distance between vessels	Height of water
1 mile	3 inches
3 miles	2 feet
5 miles	5 feet 6 inches
10 miles	22 feet
15 miles	50 feet
20 miles	88 feet

The Distance Ships can be Seen

And 20 miles is as far and rather farther than it is possible to see any ship upon the Sea; neither it is possible to see any land further, except land that is very high, wherefore 20 miles is called a ken. Now the circularness of the earth is the reason that you may see a ship or land further out when high on the top mast than upon the hatches; wherefore it is plain that the Earth and Sea are not flat, but circular, as we have already declared.