
$$
   {\rm distance\ } = \sqrt { (a_x - b_x)^2  + (a_y - b_y)^2 }
$$

$$
   p_{1} = (\alpha_1, \delta_1) 
$$
$$
   p_{2} = (\alpha_2, \delta_2) 
$$

$$
\alpha
$$

$$
\delta
$$

$$
   \cos(\gamma) = \cos(90^\circ - \delta_1) \cos(90^\circ - \delta_2) +
         \sin(90^\circ - \delta_1) \sin(90^\circ - \delta_2) \cos(\alpha_1 - \alpha_2)
$$

$$
   \gamma \simeq \sqrt { \lgroup  (\alpha_1 - \alpha_2) \cos(\delta_1) \rgroup ^2 +
                            (\delta_1 - \delta_2)^2 }
$$

\bye
