*E-mail: bjorn.sorensen@ntnu.no

In the final editing of the manuscript an error in the typing of equation (1) occurred. The equation should read:  
L=cos2ϕsin2(τϕ)·sin2τ·sin2(Γλ180°)
(1)
These corrections are in in accordance with the original equations in Bloss (1999). Note that angles are in degrees. The author offers apologies for any inconvenience.