Definition
Elongation is the angular distance between the Sun and a planet as seen from Earth. Inferior planets have a maximum elongation (east or west of the Sun) beyond which they are lost in twilight — about 28° for Mercury, 47° for Venus.
Elongation
Angular separation between the Sun and a planet on the sky.
3D illustration
Elongation is the Sun–planet angle as seen from Earth.
Use the menu and time controls to explore. Based on Jean Meeus' Astronomical Algorithms.