Mar 2017

Mercury (Evening Star) and Crescent Moon

by: John Hlynialuk

The variable weather of March prevented stargazers in the Bruce and Grey area from seeing Venus and Mercury as double evening stars on March 19. Though the daytime sky was a spectacular blue, a dense cloud moved in above the western horizon at sunset, hid the planets and even obliterated the Sun as it sank below the edge of Lake Huron. Amateur astronomers were again teased by the goddess of astronomy, Urania, but, on that occasion, she perversely hid her beauties from sight.

Though this is not the first time that clouds have defeated local stargazers, (weather cancellations average about 50%) we all understand the following: "whether or not it is clear to you...the universe is unfolding as it should":
Desiderata, Max Ehrmann. If you read "weather" instead of "whether", the line has a slightly different meaning to be kept in mind as we attempt to observe future sky sights.

The next test of local stargazer’s dedication comes when Urania offers a series of crescent moon appearances near Mercury and Mars in the western sky from Mar 29 to Apr 1 (weather permitting, of course). The thin crescent Moon is a very pretty sight and should be high enough above the western horizon on Mar 29 to stand out in bright twilight. (See diagram). Right beside it, the only bright “star” in the area is the planet Mercury. This is an opportunity to spot that elusive planet if you have never seen it before, -it will be in the western sky playing the role of “Evening Star” for a few weeks. On March 30, the crescent Moon moves upwards and will be just left of Mars. On March 31, it appears close to Aldebaran, the “angry red eye” of Taurus the Bull. Aldebaran is indeed a red giant star, 44 times larger than the Sun, and does have a slight tinge of red to it. After April 1, the Moon continues moving eastward, increasing its phase (waxing) and brightening the sky as it does so. The Moon is first quarter on April 3 and full on April 11. April’s Full Moon is also called the “Egg Moon”, the “Grass Sprouting Moon” or the “Paschal Moon”.

Moon Mar29-31
Diagram from Starry Night Simulation Curriculum

Unlike Venus which had a three month period of visibility, the two planets presently in the western sky will not be around for long. On April 1, Mercury starts back towards the Sun and will be lost in the Sun’s glare by the time of full moon. Mars follows Mercury and is hidden in the Sun’s glow a few weeks later. The show is over by the third week of April.

Most of Mercury’s disappearing act is due to the rapid decrease in the amount of sunlight that reflects our way from its surface. Both Venus and Mercury exhibit phases as they orbit the Sun and on March 29, Mercury is half illuminated, -a first quarter phase. By Apr 6, this is reduced to a thin crescent (like the crescent Moon on March 29 and 30) and with less light reflected our way, the brightness of Mercury drops rapidly. With the weather in March so changeable, if you want to add Mercury to your life-list of astronomical objects, you need to observe on any clear night in the next two weeks or so.

If you join the Bluewater Astronomers on Saturday, March 25 at the Fox Observatory for the Messier Marathon, and you arrive before Mercury sets, ask one of the guides to show you the planet in a telescope. At medium power it is possible to see the planet as a thin crescent like a miniature crescent Moon. If you have a telescope of your own, Mercury is a neat sight, -this month is your best opportunity to see it for the entire year. Don’t miss it!