Of the five naked-eye planets in our Solar System, Mercury is undoubtedly the most difficult to see. It rarely makes much of an appearance and when it does, the tiny planet tends to stay close to the horizon. In fact, Mercury never strays far from our star, whether it’s in the early morning before sunrise or shortly after sunset. That said, we’ll have an excellent opportunity to spot Mercury in the evening sky this May.
Like Venus, Mercury is what is known as an “inferior” planet since its orbit lies within that of the Earth. For both these planets, the apparent angle between the planet and the Sun, also called elongation, determines how well they can be seen. When that angle is at its narrowest and the planet appears directly in line with the Sun (as seen from Earth), we call that a conjunction; the phenomenon is termed an inferior conjunction when the planet passes in front of the Sun and a superior conjunction when it passes behind.
When the angle of elongation is at its maximum, either to the east (left) or west (right) of the Sun, the separation between the planet and our daytime star makes it easier to spot the planet in the sky. Thus, Mercury is only ever visible at dawn or dusk—for just a few minutes, one hour tops—over the few days or weeks around the time of greatest elongation.
But not all elongations are equal. Because of the eccentricity of its orbit, Mercury’s greatest elongation varies between 18 and 28 degrees, depending on the time of year it occurs. Moreover, the planet will appear brighter before or after it reaches greatest elongation, depending on whether this occurs in the east or in the west. This month, for example, Mercury shines much more brightly at the start of its apparition and then gradually dims as the days pass.
There’s one other factor that affects the visibility of a planet close to the Sun: the angle of its orbit relative to the horizon, at dusk and at dawn. After the Sun sets in our northern latitudes, this angle is steepest (almost vertical) in the winter and spring but becomes shallower (hence closer to the horizon) in the summer and fall. For the same angle of elongation, a planet’s visibility at dusk is considerably better in the first half of the year, since the planet appears higher above the horizon. The opposite is true in the second half of the year, when the geometry favours greater visibility at dawn.
Planets and lunar occultation
In May, all the conditions are favourable for viewing Mercury: During the first few evenings of the month, the planet is particularly bright above the west-northwestern horizon, where it makes its appearance 30 minutes after the Sun sets. As night gets underway on May 3, binoculars will provide a fine view of the beautiful star cluster known as the Pleiades, just above the tiny planet. When it reaches greatest elongation on May 17, Mercury will be visible for slightly over one hour at nightfall. By month’s end, Mercury’s brightness dims noticeably and the planet spends less and less time above the horizon; the best viewing period extends from early May to the 21st.
But don’t confuse Mercury with dazzling Venus, which also makes an appearance shortly after sunset, much brighter but lower above the west-northwestern horizon. The crescent Moon cozies up to the planetary duo on the evenings of May 12 and 13. Mercury and Venus will be in conjunction on the 28th, less than ½ a degree from each other—a distance equal to about the apparent diameter of the Moon. Use binoculars to locate Mercury, which will have waned considerably by this time.
The Red Planet can be found slightly higher in the sky, in the constellation Gemini. As soon as darkness gathers, Mars lights up along with the first stars and remains visible throughout the first half of the night. On the evening of May 15, the crescent Moon parks itself next to the Red Planet, making it easier to spot.
The following evening, on May 16, we’ll be treated to a fine lunar occultation: At 9:29 p.m. (approximate Montreal time, since the exact timing depends on the viewer’s precise geographic location), our natural satellite will pass in front of Kappa Geminorum—a +3.4 magnitude star located near Pollux—temporarily blocking its light. The Moon continues its journey and reappears in the night sky a little over an hour later, at 10:30 p.m.
By night’s end, the two gas giants will emerge above the southeastern horizon. Magnificent Saturn appears first, followed closely by brilliant Jupiter. The duo will be visible until dawn. A quarter Moon pays them a visit on the mornings of May 3, 4 and 5, and again on the morning of May 31.
The famous Halley’s Comet leaves behind a stream of dust as it repeatedly swings by the Sun. Every May, our planet’s orbit intersects with that of the comet, tiny bits of which plunge into the Earth’s atmosphere, where they burn up in a flash of light. The resulting meteor shower, known as the Eta Aquarids, is expected to peak this year on the night of May 5-6. If the weather cooperates and the sky is dark enough, it may be worth going out between 3 a.m. and dawn to catch some “shooting stars.” You may even get to glimpse a few meteors from this particularly special shower during the two or three nights around its peak.