For Northern Hemisphere observers the last week of February and the beginning half of March offers the best opportunity in 2012 to observe the elusive planet Mercury in the evening sky. Opportunities to view Mercury are limited by it closeness to the Sun; the innermost planet never ventures more than 28 degrees from our star, is easily visible on only a few occasions per year, almost always in twilight and never against a black midnight sky.
Greatest Elongation East
On March 5th, Mercury reaches greatest eastern elongation. At this point it is at its furthest easterly position - this time 18 degrees - from the Sun when viewed from Earth. Eighteen degrees does not sound much at all considering that Venus, the other inner planet can appear as far as 47 degrees from the Sun. However, this elongation is especially good for Northern Hemisphere observers who will see Mercury set nearly 2 hours after the Sun. For Southern Hemisphere skywatchers the elongation is not so good; Mercury sets only about 30 minutes after sunset, is very low in the dusk sky and therefore an extremely challenging object to locate.
Finding and observing Mercury
The diagram below shows the position of Mercury, one hour after sunset as seen from London, England. The best time to view Mercury is a few days either side of March 5th. For other mid-latitude northern hemisphere locations the view is similar to that shown below. During this period the magnitude of Mercury decreases rapidly as it proceeds sunwards. On February 21st it shines at a healthy magnitude -1.0 - nearly as bright as Sirius - but just over 3 weeks later on March 15th, Mercury will dim considerably to only magnitude 2.3.
It is best to start looking for Mercury about 20 to 30 minutes after sunset. Allow your eyes to relax in the evening twilight and then you should be able to pick out Mercury hovering gently above the western horizon.
At greatest elongation on March 5th, Mercury shines at magnitude -0.3. Also easily visible are Venus (mag. -4.2) about 25 degrees above Mercury and Jupiter (mag. -2.1) 5 degrees above Venus. Telescopically Mercury is a difficult object. Even with large telescopes it is difficult to notice any features. The surface appears essentially bland but one thing you can easily spot is Mercury's illuminated phase. This is achievable even with a small 80mm (3.1 inch) telescope. Using medium to high magnifications you will see that the disk is just less than half illuminated and measures 7.4 arcseconds across. Also on this date, Uranus at magnitude 5.9 is located only 2.5 degrees south of Mercury, providing a challenging conjunction for keen observers and imagers.
Mercury fast motion means that it soon reaches inferior conjunction on March 21st.