About this calculator:

The radius of the earth is a problem. There are several. This calculator

• defaults to 6371 km. for the radius of the earth. This is considered the mean radius.

  However, you may choose one of the other frequently used numbers.

  6367 is derived from the fact that 1 nautical mile is 1.852 km. This makes the circumference
  360 degrees * 60 minutes/degree * 1.852 = 40,003.2 km

  The implied radius is then
  40,003.2 / 2Pi = 6367 km or 3956 mi

  ...or you can chose the WGS ellipsoid number 6378.137. This is the equatorial radius.

  Then of course there is the polar radius of 6356.788 km, but I don't even offer that. You may not have known it was so complicated.

• uses the Haversine Formula from R.W. Sinnot, "Virtues of the Haversine", Sky and Telescope, 68/2 (1984), 159. See discussion at www.census.gov/cgi-bin/geo/gisfaq?Q5.1

• calculates all values in kilometers and then converts to other values

Differences in the distance among the various calculators may be caused by

• different values for the latitude and longitude. You would be surprised by how many there are.
• round off error
• different values for the earth's radius. Here I give you a choice.

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