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Twilight Treats

by Perry Pezzolanella

Truly warm weather may still be a few months away, but a planetary observer always looks forward to spring evenings when the ecliptic is at its steepest in the west after sunset. This is prime time for observing one of the most elusive planets of all, Mercury. This year it will be amazingly easy to find and follow Mercury. If you have never seen it, your chance has finally arrived.

The steep ecliptic after sunset allows Mercury to appear higher, and take longer to set, than normal, which keeps it from being buried deep in the bright twilight as it is the rest of the year. This is especially critical for Mercury since it orbits so close that it hardly strays from the blinding Sun. This year Venus will guide the eye to the area where Mercury lurks. Venus becomes visible in the evening sky shortly after sunset by late February, so start looking by February 27. If Venus can be seen, then Mercury may be visible to its lower right. If Mercury cannot be seen, a perfect opportunity arrives during the evenings of March 3 and 4 when Mercury passes hardly 1º to the right of Venus for a beautiful conjunction! It is rare to see these two innermost planets so close together; it should not be missed!

Venus sparkles like a bright silvery star at magnitude -3.9. It appears rather small at 10 arcseconds across and roundish at 98% lit. Mercury shines duller, more like a spark, but still bright at magnitude -1.3, smaller at 5.7 arcseconds across and 90% lit. Both planets will be fun to follow as Mercury travels to the upper right of Venus as both move slightly higher. Venus will not change much in appearance during March, but Mercury will demonstrate its swiftness as it grows larger, approaching 10 arcseconds across, and dimmer at magnitude +1 while changing phases from gibbous to half, to a thinning crescent, like a tiny, coppery version of the Moon, all within the month of March, plus it will rapidly sink lower and disappear into the twilight before March even ends.

Mercury will be at its highest and best on March 15, meaning it will remain up the longest, about an hour, before plunging into the murk. A rare photo opportunity comes soon afterwards on March 18, which also should not be missed unless weather compromises the chance. A thin crescent Moon, possibly bathed in earthshine, will line up neatly with Venus and Mercury. The Moon will be to the lower left of Venus, while Mercury will be to the upper right of Venus.

Viewing any detail on these worlds is nearly impossible. Venus always appears dazzling white and featureless due to its clouds. However, using an ultraviolet filter and photographing it may tease out a few dusky streaks. Mercury is equally as challenging with only vague hints of dusky surface features. Finding and following Mercury through its phases is rewarding in itself, and unless there is a monster snowstorm like last March that wrecked observing for weeks, this March will finally be the perfect opportunity to discover and explore Mercury.