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+## Copyright (C) 2017-2020 Philip Nienhuis
+##
+## This program is free software; you can redistribute it and/or modify it
+## under the terms of the GNU General Public License as published by
+## the Free Software Foundation; either version 3 of the License, or (at
+## your option) any later version.
+##
+## This program is distributed in the hope that it will be useful, but
+## WITHOUT ANY WARRANTY; without even the implied warranty of
+## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+## General Public License for more details.
+##
+## You should have received a copy of the GNU General Public License
+## along with this program; see the file COPYING.  If not, see
+## <http://www.gnu.org/licenses/>.
+
+## -*- texinfo -*-
+## @deftypefn {Function File} {} earthRadius (@var{unit})
+## Converts the Earth's radius into other units.
+##
+## Input argument @var{units} can be one of the units of validateLengthUnit.
+## The default is meters.
+##
+## @example
+## earthRadius ('km')
+## => ans = 6371
+## @end example
+##
+## @seealso {validateLengthUnit,unitsratio}
+## @end deftypefn
+
+function R = earthRadius (unit)
+
+  radius = spheres_radius ("earth") * 1000;    ## This is the default in meters
+  if (nargin == 0)
+    R = radius; 
+  elseif (nargin > 1)
+    print_usage ();
+  elseif ( ! ischar( unit ) )
+    error ("earthRadius.m: string value expected");
+  else
+    ratio = unitsratio (unit , "Meters");
+    R = radius * ratio;
+  endif
+
+endfunction
+
+%!test
+%! radius = earthRadius / 1000;;
+%! assert (earthRadius ("km"), radius);