Mercury at Greatest Western Elongation

December 15 – Mercury at Greatest Western Elongation. The planet Mercury reaches greatest western elongation of 21.3 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.

Source: http://www.seasky.org/astronomy/astronomy-calendar-2018.html