Estimation of the altitude of the aurora. O is the center of the Earth, and North is located on the top. The observation is made on A at a latitude lat_obs and a longitude lon_obs. Considering the stars on the pictures, the blue aurora is supposed to happen at the same longitude and a latitude lat_aur. The line between the aurora and O crosses the surface of the Earth in B. C is the point at the horizon of A, i.e. any point below C is invisible as seen from A. Its altitude of the auora is BD = BC + CD = δ + d. The lenght between A and C is l. R_T is the radius of the Earth, i.e. 6370 km. The latitude of the aurora is that of the observer (geographic 48.3°N) plus the angle θ.