The Sky Tonight Update: Jan. 19, Mercury at Greatest Western Elongation
This January 19th, the planet Mercury reaches its greatest western elongation of 24.1 degrees from the Sun. This is the best time to view Mercury since it will be at its highest point above the horizon in the morning sky. Look for the planet low in the eastern sky just before sunrise.
From Baton Rouge (click to change), it will be difficult to observe as it will appear no higher than 14° above the horizon. It will be visible in the dawn sky, rising at 05:22 (CST) – 1 hour and 38 minutes before the Sun – and reaching an altitude of 14° above the south-eastern horizon before fading from view as dawn breaks at around 06:43.
Mercury’s orbit lies closer to the Sun than the Earth’s, meaning that it always appears close to the Sun and is very difficult to observe most of the time.
It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.
These apparitions take place alternately in the morning and evening skies, depending whether Mercury lies to the east of the Sun or to the west.
When it lies to the east, it rises and sets a short time after the Sun and is visible in early evening twilight. When it lies to the west of the Sun, it rises and sets a short time before the Sun and is visible shortly before sunrise.