An “elongation” is the term used to describe the position of the inner planets Mercury and Venus with respect to the Sun in the sky. It can be a Greatest Eastern Elongation in which the planet appears at its greatest distance east of the setting Sun in the evening—or a Greatest Western Elongation where the planet is at its greatest distance west of the Sun in the morning sky.
Elongations of Mercury or Venus are fairly common. But it is unusual for them to both be at elongation in the sky at nearly the same time. Here’s a celestial treat for those willing to get up before dawn this month—seeing both inner planets in prime positions. Mercury will be at Greatest Western Elongation 27 degrees from the Sun on March 6th and Venus 47 degrees from the Sun on March 20th. Although that’s two weeks later for Venus, it will be an unmistakable brilliant object keeping company with Mercury. Viewing them about 45 minutes before sunrise is the best time around Mercury’s greatest elongation.
Because of its rapid orbital motion (the fastest of all the planets, being closest to the Sun) Mercury will be visible for a couple weeks but soon disappears into the sunrise. Venus on the other hand will remain visible in the morning sky for several months. Due to its swiftness,
Mercury itself has an undeserved reputation of being very elusive and difficult to see. Actually when at elongation it is fairly bright and an easy naked-eye object. (The often-repeated story that the famed astronomer Copernicus never saw it is almost certainly a myth.)
While Venus continues its leisurely motion across the morning sky, Mercury rapidly moves between us and the Sun in late April and will next appear as an “evening star” in May, coming to greatest eastern elongation on the 17th. This amazing movement is the result of being the closest planet to the Sun and having an orbital period of just 88 days. Talk about being the “Swift Messenger of the Gods” as the ancients considered this little world!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.