Elongation
When an inferior planet (Mercury or Venus) 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°. This value varies because the orbits of the planets are elliptical, rather than perfect circles.