How far would you walk for a degree change?

This set of images show the difference between a second, a minute and a degree based on an office in Washington, DC.  The office is located at 38° 53’ 52.59’ N; 77° 02’ 29.60” W.  A couple seconds is about the length of a city block.  As you would expect, the North-South points 1 minute apart along a meridian are 1 nm apart.  The East-West points 1 minute apart along the parallel are only .78 nm apart.  Points that are 1 degree apart North-South are 60 nm apart, yet East-West points 1 degree apart are only 47 nautical miles apart.  This approximation is only valid at this latitude because meridians converge at the poles.  Closer to the poles will make East-West point 1 degree apart closer.  Closer to the equator will make East-West points farther apart.

One second changes.


One minute changes. N/S points are 1 nautical mile apart. E/W points are .78 nautical mile apart.


One degree change:. N/S points are 60 nautical mile apart. E/W points are 47 nautical mile apart


I’m sure that all this talk about degrees, minutes and seconds has made you wonder why there are 60 seconds in a minute, 60 minutes in an degree and 360 degrees in a circle.  We have the Babylonians to thank for that.  They used a base-60 numeric system (sexagesimal) that is used in both time measurements and angles.  You are familiar with a based-10 numeric system (denary), and maybe a base-16 (hexadecimal) if you program computers.  Latitude and longitude are minutes of an arc that originates in the middle of the Earth.

Sexagesimal numbers would name each place past the point in Latin: primus, seconde, tierce, etc.  Minutes are the first position.  Second position is 1/60th of a minute, or seconds as we call them.

While we are off topic, there are 24 hours in a day because the ancient Egyptians used sundials that showed 10 parts day, 12 parts night, 1 part morning twilight, and 1 part evening twilight.  The Egyptians used a base-12 numbering system so it was natural to break day and night each into twelve parts on the sun dial.  Although hours were not the same length of time until the Greeks got involved.

Has anyone ever come up to me in a disaster and asked why our time is a base-60 numbering sequence?  Well, no.  But it is handy knowledge where you’re at a cocktail party, the conversation is in an uncomfortable silence, and you have nothing else to say.


GIS continues: Layering on the data