elongation

The angle between a planet and the Sun in the sky. Because Mercury and Venus are inside the Earth's orbit, their elongation angles are never more than 23 degrees (for Mercury) or 46 degrees (for Venus), which is why they appear as "morning" or "evening" stars, never as "midnight stars." All other planets, though, with orbits outside the Earth's, can appear at any elongation angle. When a planet reaches the greatest angular separation east of west of the Sun in the sky, it is said to have reached greatest elongation. The maximum elongation for Venus and Mercury is 46 and 23 degrees, respectively. The maximum elongation of planets outside the orbit of the Earth is 180 degrees, when the planet is said to be at opposition. At these points in the sky, planets are visible for the longest period of time.