March promises to be a most eclectic month, with the zodiacal light and aurora borealis, plays of colour and fresh celestial perspectives.
The equinox and all its surprises
Every year, the month of March signals a turning point for night sky observers. As our planet steadily heads toward the spring equinox—which falls on March 20 this year—the nights keep getting shorter and the days longer. The weather is finally mellowing, but with it comes humidity levels that almost make you long for those dry, cold winter nights. But every now and again, March treats us to some lovely, unexpected surprises that provide a whole new perspective of the sky.
The equinoxes—when the ecliptic plane is most favourably inclined to the horizon—are in fact the best times to view the zodiacal light. In spring, the light can be seen due west just after twilight in a moonless night sky, completely free of light pollution, one to two hours after sunset: Look for a hazy, pyramid-shaped cone of light that climbs into the sky (tilted slightly to the left).
Interestingly, statistics show that the period surrounding the equinoxes is when the polar auroras are most numerous and most intense. And solar activity is finally making a big comeback in 2021. After the solar minimum of 2019, activity is expected to ramp up until it peaks in the summer of 2025. Hence, there is much hope of seeing the spectacular auroras dance across the night skies of March.
Taurus impresses with plays of colour and perspective
Although Mars reached opposition about five months ago, it still dominates the night sky. Its steady orange glow (which will continue to exceed magnitude 1 until March 8) is clearly visible high above the west-southwestern horizon in the early evening. Mars then plunges back toward the west-northwestern horizon, setting there in the middle of the night. Come March, the Red Planet will venture into the constellation Taurus, where night after night, between the 6th and the 10th, you can track its journey between the Pleiades star cluster (marking the heart of Taurus) and the Hyades cluster (forming the head of the bull). You’ll be struck by just how similar Mars and Aldebaran, the eye of Taurus, are in colour. Aldebaran is an orange giant star, nearing the end of its life, with a mass comparable to that of the Sun. The nuclear fusion reactions at its core have sped up and heated the gases that make up the star, which is expanding uncontrollably. Today, Aldebaran is 45 times wider than the Sun. When it was still a main sequence star, it may have had its own system of exoplanets, some of which probably resembled our planet Mars. But even if that were true, the system is now largely disrupted by Aldebaran’s orange giant phase, although the star is known to harbour one confirmed exoplanet similar to Jupiter.
Aldebaran shines at one tip of the V-shaped pattern of stars called the Hyades cluster. While its position may suggest it is part of this cluster, Aldebaran is in fact about halfway between our Solar System and the Hyades. And while Aldebaran is at the end of its life cycle, the Hyades is an open cluster of young stars located a mere 150 light-years away, making it the closest cluster to us. Not far from the Hyades is the easily visible Pleiades, an even younger open cluster—about 100 million years old—but slightly further away at about 450 light-years. One look at the constellation Taurus provides us with a portrait of stars at different stages of their life, in order of distance. The Pleiades have shown us that stars form in groups, then gradually disperse and move away from each other, as seen in the Hyades. They also evolve alone or in pairs until their death—the fate currently facing Aldebaran. As Mars enters Taurus this month, we are reminded of the history of our own Solar System and its inevitable fate.
After swinging a mere 2½ degrees south of the Pleiades on March 3 and 4, Mars wanders between the Hyades and Pleiades clusters from the 6th to the 10th. A few days later, on March 18, as Mars hovers above the head of Taurus, the first crescent Moon snuggles up to the bull’s chest, forming a very graceful diamond shape with its eye (Aldebaran) and heart (the Pleiades). The next evening, the Moon inches closer to Mars (about 3 degrees), drawing our gaze to the Moon-Mars-Aldebaran trio.
Mercury poses a challenge
While Mars remains a prominent object in this month’s sky, the same cannot be said for the other bright planets. There is absolutely no chance of spotting Venus, which reaches superior conjunction (behind the Sun) on March 26. Observations of Jupiter and Saturn will be possible but not very easy, given that the two giant planets appear only very low on the horizon, shortly before sunrise. In the first half of the year, the ecliptic’s angle with the horizon is typically not very favourable for observing planets in the morning. Mercury is especially affected this month as it barely gains any significant altitude, although the planet is in theory observable until mid-month. The best chance of spotting the elusive coppery dot is when it meets up with Jupiter on the morning of March 5, very low above the east-southeastern horizon, 30 minutes before sunrise. The two planets will be only one-third of a degree apart. Mercury reaches its greatest western elongation the next day, after which the observing window gradually closes.
An elegant conjunction
Direct your gaze toward the glow of dawn and you’ll be treated to an elegant conjunction on the morning of March 10, about 30 minutes before sunrise. From left to right, Mercury, Jupiter and Saturn form an almost perfect line, slightly tilted in relation to the southeastern horizon, and roughly representing the ecliptic plane. As for the Moon, it orbits Earth along a plane that is tilted by about 5 degrees. On the morning of March 10, at about the same 5 degrees from the ecliptic, an extremely thin lunar crescent hangs like a comma below the imaginary line between the three planets—unfortunately placing the Moon even closer to the horizon. As a matter of fact, this conjunction presents a viewing challenge and will only be visible to observers who have a clear view of the southeastern horizon, under cloudless, haze-free skies.