Altitude of three hypothetical candidates (T1, T2, T3) from La Silla on the spring equinox. The current time t_obs is indicated, and t_SS and t_SR are the sunset and sunrise time respectively. The shaded grey regions are day and twilight (the Sun’s altitude a > −18°, when astronomical observations are not possible). The shaded region under each altitude curve between t_obs and t_SR is A_vis, computed in Eq. (12). The reference altitude a_ref for normalisation is also indicated.