function a = deltaphi(phi1,phi2)
%
% The function a = deltaphi(phi1,phi2) gives as output the
% distance (in km) of two latitude circles (assuming the 
% earth is a sphere).  phi1, phi2 should be the latitude
% of each cirlce, in decimal degrees.
%
%---------------------------------------------------------------------
%  Written by Vassilis Zervakis, in June 1996,
%  while at AINCO-Interocean, Madrid, Spain
%---------------------------------------------------------------------

% Let R be the ratio of a sphere with a volume equal to that of the Earth.
R = 6371.;
dphi = phi1 - phi2;
a = (2*pi*R/360.)*dphi;
return
