For the northern hemisphere, March 22 (Note: This will be on March 23 for Philippine observers) marks Mercury’s best apparition in the evening sky for all of 2011. Mercury, the solar system’s innermost planet, never strays far from the sun. It never appears against an black midnight sky. But Mercury does appear in twilight, and today this innermost world reaches its greatest elongation east of the sun. Mercury swings to the end of its tether, at 19 degrees east of the sun. (For reference, your fist at an arm’s length to approximates 10 degrees of saky.)
To find this elusive planet, all you need is a clear evening and a viewing site with an good view down to the west horizon. Note the spot where the Sun sets, and then start scanning above (and slightly to the left) of there for Jupiter. Jupiter may be visible immediately if the air is very clear, but it will be more obvious 15 or 30 minutes later when the sky is darker (though Jupiter will also be lower).
Once you’ve found Jupiter, look for Mercury near it.
During March, Mercury will appear higher each evening until the 22nd, while Jupiter appears ever lower. So by the end of that period, Mercury may actually be the more obvious of the pair, despite the fact that it’s slowly fading. Starting around March 25th, Mercury plunges back toward the Sun, fades rapidly, and soon becomes hard to locate with the unaided eye.
As seen from the northern hemisphere, Mercury stands above the setting sun. It sets about one and one-half hours after the sun.
From the southern hemisphere, Mercury sits to the side of the setting sun. From there, this planet could be seen setting only about one-half hour after the sun.
That’s why Mercury sets so much later after sunset in the northern hemisphere than in the southern hemisphere during March of 2011. And it’s why this apparition of Mercury is the best evening apparition of this planet for all of 2011.
But why does Mercury stay out so much longer after sunset in the northern hemisphere? It’s because the ecliptic – the pathway of the planets – hits the horizon at a steep angle on early spring evenings. But in the southern hemisphere – where it’s early autumn – the ecliptic intersects the horizon at a narrow angle at evening time.
In a telescope, however, the later part of this apparition is most interesting, because that’s when Mercury grows into a long, thin, crescent. It’s 7″ wide and 50% illuminated on the 20th, 8″ wide and 30% illuminated on the 24th, and 9″ wide and 20% illuminated on the 28th.
In the northern hemisphere, Mercury should remain in good view until the end of the month. Look for Mercury to shine low in the west some 45 to 75 minutes after sunset.