Celestial Navigation

By Theresa Williams 

We’re lucky to have Second Mate Logan on this ship.  He is knowledgeable about historical sailing techniques, has sailed across both the Atlantic and Pacific using celestial navigation, and he designed the rigging for the movie Master & Commander.  We asked him to teach us about celestial navigation and the use of a sextant.

Celestial navigation was used up until the 1990s when GPS first became widely available.  It’s based on taking measurements that allow the calculation of latitude and longitude.  Latitude is the measured declination from the horizon in degrees and minutes north or south of the equator.  Longitude is the distance west of the prime meridian in Greenwich, England.

The angle between the north star and the horizon at the equator is close to 90⁰ and the north pole has a latitude of 90⁰.  In the northern hemisphere, you can find your latitude by measuring the angle from the horizon to the north star then subtracting that angle from 90.  During the day, you can use a sextant at high noon to gauge latitude using the sun.  Humans have been able to figure out their latitude on the planet for thousands of years.  The tricky thing is to figure out longitude.

Because Earth is nearly circular at the equator, there are 360 degrees around it.  One revolution of Earth takes about 24 hours, so Earth moves about 15 degrees per hour (360/24=15) and 15 degrees is one time zone around the planet.  One minute of longitude is 15 nautical miles at the equator.  This means that clocks could be used to calculate longitude.  Unfortunately, pendulum clocks don’t work at sea.  In response to the offer of a Longitude Prize in 1713, a guy named Harrison invented the H1 Chronometer and it worked!  The prize-givers didn’t like the simple solution, so they refused to pay him the prize.  Over time, better and better clocks were invented to work on ships allowing sailors to calculate hour angle and determine distance from the prime meridian.

To learn about how Logan became a sailor, what he likes about it and some of the challenges, watch the video.

Don’t get lost!

Slackjaw Sally