The calculation of the angle between the day-night line on the Earth’s surface, a terminator, and the meridian at a given point, at dawn or at sunset, can easily be performed either by means of flat geometry or, for those familiarized with astronomy, through formulae for the Sun’s azimuth and elevation, both derived from spherical trigonometry. The two procedures are compared. An auxiliary angle obtained in the process of derivation via planar geometry becomes useful to represent the daylight map on a terrestrial planisphere.