Read an Excerpt
From Chapter 1:
The sextant will measure the height above the horizon of a celestial object, (sun, moon, planet or star), in degrees, minutes and fractions of a minute. Some sextants show seconds instead of a fraction of a minute.
What good is this to us? How does this help us find our position? To demonstrate, let's look at Polaris, the North Star.
Our latitude in Southeast Texas is about 30 degrees North. When we use our sextant to shoot a sight of Polaris we face north and find the star is about 30 degrees above the horizon. If we were in Nebraska, our latitude would be about 45 degrees North and facing north our sextant would show Polaris to be 45 degrees above the horizon. In fact if we move all the way north to the North Pole we would be at 90 degrees North latitude and our sextant would read 90 degrees height for Polaris, regardless of which direction we faced.
Therefore, we can determine our latitude by measuring the altitude of Polaris.
We can determine the distance we are away from the North Pole, by measuring the height of Polaris. The point directly below a celestial object is called the GP. The GP of Polaris "wobbles" around the North Pole every twenty-four hours, so a sextant reading at dusk will be slightly off, as will a morning reading. An average of the two readings will be exact since the morning error and the evening error offset. The nautical almanac gives corrections for both.
POLARIS
How to find it
EQUATOR TO POLE
If a sextant reading is 30 degrees, subtract that from 90 degrees, which gives the co-altitude of Polaris. In this example, that would be 60 degrees. Since each degree is equal to 60 nautical miles distance, 60 degrees times 60 miles equals 3600 miles, the distance from the North Pole.
Co-altitude x 60 miles = 3D distance away.
Figure 1-1 Jack Davis
The early ocean navigators understood that you could find your latitude using the North Star.
They didn't have the very accurate sextants we have today. They had a crude instrument called an astrolabe, which would give them a rough reading. When Columbus discovered the entrance to San Juan Harbor, Puerto Rico, he knew (from the astrolabe) the entrance was at about 19 degrees North latitude. He didn't have a clue what his longitude was and for his purposes it didn't matter.
When he sailed back toward his home port, he sailed north until he arrived at the latitude of the Mediterranean entrance. There, he turned east and ran that latitude line until the Rock of Gibraltar came into view.
On his return trip to San Juan, he sailed from the Mediterranean, past the Rock of Gibraltar and sailed south to 19 degrees North. He followed that latitude line until Puerto Rico came into view.
The thousands of sailors who followed in Columbus's wake, said when leaving the
Mediterranean to go to the West Indies, you sail south until the butter melts, then turn right.
This "latitude sailing" has been used by sailors (and aviators) well into the electronic age and in fact I still use the technique on occasion.
