April’s night sky marks a transition between winter and summer constellations. This year, it also offers two evening planets, and parting glimpses of two comets that are preparing to return to the frozen depths of the outer solar system.
For the past few months we’ve been talking about PanSTARRS (C/2011 L4), a comet that passed within 40 million kilometres of the sun last March 10 — even closer to our star than Mercury. At the time, PanSTARRS was not visible from the Northern Hemisphere. Since then, the situation has improved quite a bit for amateur astronomers in Quebec and elsewhere, even though, while moving away from the sun, the comet has lost in brightness what it gained in altitude. You can look for it above the northwest horizon, about 30 minutes after sunset. Needless to say, a perfectly clear view of the horizon is essential; binoculars will also be necessary if you hope to catch a glimpse of the comet’s coma and tail. Time is running out, so try to catch PanSTARRS during the first few nights of April — after that, it might be too late!
If you don’t succeed in seeing PanSTARRS, another comet — this one named C/2012 F6 Lemmon — might serve as a consolation prize. Discovered in March 2012 by Alex Gibbs at Mount Lemmon Observatory in Arizona, comet Lemmon passed just under 110 million kilometres from the sun at the end of March. It reached its peak brightness around that time, but was only visible from the Southern Hemisphere. We’ll have to wait until the third week in April before it becomes visible from our northerly latitudes, but it’s hard to predict just how bright it will be. By then, comet Lemmon will have already moved substantially farther from the sun, so it won’t be as bright as it was earlier in the month. Contrary to PanSTARRS, comet Lemmon will be visible at dawn, above the eastern horizon shortly before sunrise. Again, binoculars will be essential.
The two largest planets in the Solar System will be visible during the evening throughout the month. Jupiter remains in Taurus, the celestial Bull, and is visible in the southwest as twilight begins. In early April, the giant planet sets around 12:30 A.M., and two hours earlier by the end of the month. Jupiter is always fun to watch through binoculars or a small telescope. In binoculars, its four Galilean moons can be observed as they move around the planet from one evening to the next. Any telescope will show dark- and light-coloured cloud bands in the planet’s atmosphere; larger instruments may even reveal the famous Great Red Spot, a huge cyclone that has been raging in Jupiter’s atmosphere for at least 400 years.
Of course, the other planet visible in the evening sky is Saturn, which is currently in Libra, the Scales. Saturn will be at opposition on April 28. Opposition is the best time to observe a planet through a telescope: That’s when it is closest to earth, and brightest in the sky. Saturn is no exception, and the planet’s magnificent rings certainly add to its appeal. The rings are currently inclined 18 degrees to our line of sight: with their northern face tilted toward earth, they show up beautifully in small telescopes. The rings measure nearly 300,000 kilometres across, but are less than one kilometer thick! When the geometry of the orbits of Saturn and Earth places them edge-on, they appear to vanish almost completely…
April affords us one last opportunity to enjoy the winter constellations, which will soon give way to the stars of summer. Meanwhile, the springtime constellations occupy center stage.
Surrounding the iconic winter constellation, Orion, we can easily find five other constellations, thanks to help from the Hunter. Starting from Orion’s three belt-stars, a line drawn toward the right leads us to the Hyades and the orange star Aldebaran, in Taurus, the Bull. The Pleiades, a compact group of stars, easy to recognize, are just a bit further off to the right. From the same three stars in Orion’s belt, a line drawn toward the left points to Sirius, the brightest star in the sky, in Canis Major, the Great Dog. Above Canis Major, the bright star Procyon, marks the position of Canis Minor, the Lesser Dog. Going back to Orion, and looking above the Hunter’s shoulders, the twin stars Pollux and Castor mark the position of Gemini, on the left, while on the right Capella shines in Auriga, the Charioteer.
The springtime constellations are organized around Leo, the Lion, which the Big Dipper helps us locate. Imagine a hole at the bottom of the dipper: water will trickle straight down onto the Lion’s back, who probably wouldn’t appreciate the unsolicited shower! The Lion’s head looks like a backward question mark, or a sickle, with its opening on the right, and the feline’s hindquarters look like a triangle. The Lion faces toward Cancer, the Crab, a faint constellation that’s home to the Beehive star cluster — a beautiful sight in binoculars. Following the curve of the Big Dipper’s handle, we draw an arc until we come across the bright star Arcturus, in Boötes, the Herdsman; further down, we arrive at Spica, the main star in Virgo. “Arc to Arcturus, and spike to Spica,” that’s how to remember your way to these two stars and their constellations. Boötes is easy to recognize: it looks like a kite, or an ice cream cone, or even a neck-tie — the choice is yours! The stars that form Virgo are less conspicuous, but the constellation is home to a great number of very remote galaxies, which a mid-sized amateur telescope will reveal.