Elongation
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Elongation is an astronomical term that refers to the angle between the Sun and a planet, as viewed from Earth.
When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation (west or east), for Mercury, is between 18° and 28°; and for Venus between 45° and 47°.
Unfortunately for observers in the Northern hemishere the largest elongations of Mercury took place when the ecliptic has a low inclination toward the horizon, so it is in areas nortwards approximately 40° not to see Mercury at its greatest elongations with the naked eye.
See also: astronomical conjunction