Spherical law of cosines

from math import sin, cos, radians, degrees, acos, asin, pi

point_1 = (1, 1) # in degrees
point_2 = (3, 3) # in degrees
r = 70 # meters
D_in_degrees = 90

# First convert degrees into radians
a = (point_2[0] - point_1[0]) * (pi/180)
b = (point_2[1] - point_1[1]) * (pi/180)
D = D_in_degrees * (pi / 180)
print 'a: ', a
print 'b: ', b
print 'D: ', D

# Use the sperical law of cosines
cosd = (sin(a) * sin(b)) + (cos(a) * cos(b) * cos(D))
print 'cosx: ', cosd
d = acos(cosd) # in radians
print 'd: ', d
d_in_meters = d * r
print(d_in_meters)