Greatest Elongation

Greatest elongation is the moment when Mercury or Venus, the two planets that orbit closer to the Sun than Earth does, reach their maximum apparent angular distance from the Sun as seen in our sky, making that the best possible time to observe them. Because these two "inferior" planets orbit inside Earth's own orbit, they can never appear to wander far from the Sun the way outer planets like Mars or Jupiter can; instead, they swing back and forth between appearing in the evening sky just after sunset and the morning sky just before sunrise, always trailing relatively close to the Sun's glare. Mercury's greatest elongation ranges from about 18 to 28 degrees due to its markedly elliptical orbit, while Venus, with a more circular orbit, reaches a more consistent 45 to 47 degrees, which is why Venus can appear brilliantly high above the horizon after sunset while Mercury remains a much more challenging, low-lying target. Astronomers distinguish greatest eastern elongation, when the planet appears in the evening sky after sunset, from greatest western elongation, when it appears in the morning sky before sunrise. Around these moments, the planet is angularly as far from the Sun's overwhelming glare as it will get, giving observers their darkest possible sky and longest viewing window to spot it.