At one moment or another during the course of a year, we can see seven wandering celestial objects to which the weekdays owe their names. In English, Saturday, Sunday and Monday are obviously dedicated to Saturn, the Sun and the Moon, respectively; in French, the subsequent weekdays are: mardi for Mars, mercredi for Mercury, jeudi for Jupiter, and vendredi for Venus. If you are at all interested in observing the night sky, you’ve probably seen most of these objects. But have you ever seen Mercury? Of the seven, it’s the most difficult to spot.
This year, the first three weeks of April will offer the best opportunity to view Mercury. On the evening of April 1, Mercury is just 8 degrees above the western horizon at sunset, and the sky is too bright to spot the furtive planet, which sets as night falls. But conditions improve over the days that follow, until April 18 when Mercury reaches its greatest separation from the Sun: On that date, when sunset occurs over Montreal at 7:45 P.M., the planet will be 18 degrees above the horizon—high enough for the sky to darken sufficiently, enabling Mercury to be viewed before it sets at 9:39 P.M. (See illustration for proper orientation.)
Spotting Mercury at twilight remains a challenge none-the-less. But clear skies, an unobstructed horizon and a little patience, will ensure success… allowing you to check this noteworthy observation off your list of astronomical quarries.
Mercury is a challenging object to spot, not because of its small size, but because it is the closest planet to the Sun: Most of the time, solar glare and a twilit sky hinder observations. But the orbits of Earth and Mercury allow their positions to change rapidly with respect to the Sun. When Mercury appears between the Earth and Sun, the planet is in conjunction and is impossible to see, except on rare occasions when it passes directly in front of the solar disk, revealing itself in silhouette… which will be the case this May 9. Following a conjunction, the apparent gap between the Sun and Mercury, known as elongation, increases. The planet is easiest to observe when it is at greatest elongation. These elongations of Mercury alternate between the eastern and western side of the Sun—when the planet is visible in the evening, and morning, respectively. In 2016, Mercury is at greatest eastern elongation on April 18, August 16 and December 11; and at greatest western elongation on February 7, June 5 and September 28.
As for the other wandering celestial objects mentioned earlier, Jupiter remains bright and easy to find, in Leo, this month. Mars and Saturn rise around the middle of the night, in Ophiuchus and Scorpius, and will be at their highest and best in May. Meanwhile, Venus, the brightest planet of all, is currently very close to the Sun, low in the east at dawn, and is difficult to observe. And in closing, the Moon will be new on April 7, and full on the 22nd.