A Little Bit about Celestial Navigation


Can you find your way by the Sun, Moon, and stars?  Most of us will never have a need to do that in our age of GPS and other aids to keep us from getting too badly lost.  In the age of exploration, though, it was essential.  The captain of a ship must know when he is coming close to land and approach only where there is a port suitable for his ship.

The North Star (Polaris)

One way to find latitude is by looking at how high in the sky the North Star is. Polaris is not an especially bright star, but it is very close to the true North Celestial Pole and there are star patterns that help locate it {a picture and a link to details would be nice here}.   With a sextant, a skilled seaman could measure the angle between the horizon and the star with an accuracy of (?), allowing the ship to sail along a fixed latitude and come close enough to its destination port for landmarks on the shore to be used for the final course adjustment.

 With a simple home-made sextant, you can probably measure the angle to within two degrees or so {picture and details, please}.  When the altitude (angle above the horizon) of the North Star is known to sufficient accuracy, the latitude (how far North or South on the Earth) of the observer can be determined to within a degree {40 minutes, or 2/3 of a degree today; how much better or worse within recent centuries?}.  The computation couldn't be any simpler.  The observer's latitude is the same as the altitude of Polaris {picture and explanation}.

Other Stars

Captains could not always depend on their ability to see the North Star.  Although using it was the simplest method for finding latitude, it might not be visible for days in bad weather, and might not be the easiest to measure even when it could be seen.  Any star (or planet or the Sun) can be used to determine latitude provided the position of the body in the heavens is accurately known, and its exact position as seen in the sky -- both altitude, angle above the horizon, and azimuth, direction from the ship, is known.  But on a ship rolling with the waves, it is difficult enough to measure the altitude and generally impossible to find the azimuth of a body in the sky.

The complete position of the body in the sky can be known if the altitude is measured at the time when the body is at its highest point in the sky.  The highest point, known as the transit, always occurs when the body is either directly North or South of the ship {or directly above, at the zenith}.  To determine the altitude at the time of transit, it is sufficient to begin measuring the altitude before transit, while its altitude is increasing, and repeat the measurement until the altitude begins decreasing.  The greatest altitude measured should be used.

Because of the extreme importance of knowing the location of the ship, captains usually assigned some of their best seamen to make celestial measurements night and day, whenever the weather permitted.  There was the noon sighting of the sun, and a number of the brightest stars would be measured during the night whenever one reached its transit point and could be seen.

Next: measuring longitude