I love maps. I really love maps.  Hours were spent with the world atlas at the public library as a kid. My professional career has been spent as a user of maps; I’ve even been involved in making maps, in other words working as a cartographer.  I know that to the average person this may sound a little nerdy, but wait, there’s more.  I also love map coordinate systems, the ways in which we can navigate by use of maps.  There are numerous different coordinate systems one can use for purposes of navigation, but I’d like to go back to one of the earliest: latitude and longitude.

Navigating by latitude and longitude was developed primarily to help mariners navigate deep waters once you were out of sight of land.  If there are no landmarks to guide you, what are you going to do?  Turning to the sky – the sun by day and the stars by night – was the solution.

This was most easily accomplished by mariners sailing in the waters of the Northern Hemisphere since Polaris, the North Star, was available.  Sitting directly over the north end of the rotational axis of the Earth (in other words, directly above the North Pole), it presents a fixed mark for the mariner.  Picture it this way:  If you are standing directly on top of the North Pole, where is Polaris in the night sky?  Answer: directly above your head.  If you are standing at the Equator while facing north, Polaris will be found sitting right on the line of the horizon.  As you move farther and farther north of the Equator, Polaris will climb higher and higher into the night sky.  If you are driving north on Interstate-5 through Salem, Oregon, you will come across a road sign just south of the Keizer Station exit which informs you that you are crossing the 45th Parallel, halfway between the Equator and North Pole.  This takes us to another point:  Lines of latitude are called parallels.  This is because as they circle the Earth they stay a constant distance from one another (roughly 70 miles/113 kilometers between one degree of latitude and the next). The Equator is found at 0°, Salem, Oregon is found at 45° North Latitude, and the North Pole at 90° North Latitude. It works the same way south of the Equator all the way to the South Pole.  Speaking of the Southern Hemisphere, establishing your latitude was a bit more difficult without Polaris.  Mariners had to develop a somewhat more sophisticated process using a couple of stars called the Southern Pointers and the Southern Cross Constellation. The primary tool was the mariner’s sextant and its predecessors.  Basically the idea is to measure the angular separation between a celestial object (like Polaris) and the horizon.

And then there’s longitude.  This is where the story gets interesting.  The Equator makes a great starting point north and south (remember: 0° Latitude), but how is this going to work going east and west? You might be able to keep Polaris at a certain height in the night sky as you sail west from England, but how far west?  And west of what exactly?  It was at this point navigating by the sun and stars alone became extremely difficult.  In the Age of Sail, this led to a great many shipwrecks (as well as some serendipitous discoveries).  The great sailing powers of the 16th-18th centuries (the British, Spanish, Dutch et al) were determined to develop a method for more easily determining longitude, making a contest of it.

Enter the English Board of Longitude in 1714, and from that the story of Englishman John Harrison (1693-1776), the inventor of the Chronometer.  The solution to knowing your longitude at sea, as it turned out, was to be able to tell time.  It works like this:  Take chronometers (basically clocks that keep time with great precision), using a sextant, take a sightings of the sun to determine noon while at anchor on the Thames in London. Set both clocks to the exact same moment (high noon). Set sail for your destination (say North America).  The next day, with one of your chronometers safely stowed away (but operating) in the ship’s chart room, take your sightings and reset the other clock, as noon in your new location is not at the same time as at your previous location the day before. Repeat each day.  The farther away from London, the greater this difference in time. This difference in time can be translated into a difference in longitude west of London.

The English Board of Longitude established a cash prize for the first person to develop a means for accurately determining longitude.  That person was the clockmaker John Harrison.  For a great account of the entire story, try Dava Sobel’s “Longitude: The True Story of a Lone Genius Who Solved the Greatest Scientific Problem of His Time”.  It’s a great read.

Because lines of longitude converge as they approach the North and South poles, they are called meridians rather than parallels.  The Prime Meridian (0° Longitude) runs through the Royal Observatory in Greenwich, England.  Appropriate, don’t you think?  The Prime Meridian is the longitude analogue to the Equator for latitude, that is, the starting line (it is also the home of Zulu Time, or Greenwich Mean Time, which is the clock for astronauts in space).  However, unlike latitude, which only needs 90° to get from the Equator to either pole, a full 360° must be accounted for in longitude, therefore we have 180° of longitude to the west of Greenwich and 180 of longitude to the east of Greenwich.  180° west and 180° east are the same line and you know it by its common name:  the International Dateline.

This reliance on time to determine distance from a reference carries into the numerical system used.  Time:  one hour = 60 minutes; one minute = 60 seconds.  Distance: one degree = 60 minutes; one minute = 60 seconds.  The location of downtown Los Angeles is then:  34° 03’ 08”  (34 degrees, 3 minutes, 8 seconds) North Latitude; 118° 14’ 37” (118 degrees, 14 minutes, 37 seconds) West Longitude.

With modern navigational aids, especially satellite-based systems, navigating by sextant and chronometer to find latitude and longitude is something of a dying art. But this surely is a great story of our creativity and powers of observation.

Adapted from lecture notes (2014), Harrison and the determination of longitude