Despite the shorter nights, summer is a great time to admire the starry sky. At this time of year, you’ll easily spot a large triangle of stars if you look up: For many, the return of the Summer Triangle at nightfall is a sure sign that the warm weather has arrived.
Strictly speaking, the Summer Triangle is not a constellation, but rather an asterism made up of three brilliant stars: Vega, Deneb and Altair. In July, it is visible due east in the early evening and can be seen all night during the balmy season.
Vega, in the constellation Lyra, is the brightest star in the Summer Triangle; the other stars of this constellation form a small parallelogram just below it. Vega is the fifth-brightest star in the night sky and one of the first to appear after dusk during the summer. It is three times more massive and 50 times brighter than the Sun. In about 12,000 years, due to the precession of the equinoxes, Earth’s axis will point directly at Vega, making it our North Star.
The second star in the Summer Triangle is Deneb, in the constellation Cygnus (the Swan), which looks like a large cross in the sky. Although Deneb is located at the top of the cross, it actually represents the tail of the celestial bird; the star Albireo is at the head of the Swan, at the opposite end of the cross. Aim a small telescope at Albireo and you’ll discover one of the most beautiful double star systems—one star appearing distinctly bluish and the other orange.
Altair, the alpha star in the constellation Aquila the Eagle, completes the Summer Triangle. It marks the head of the celestial animal and means “the flying eagle” in Arabic. Given that Altair is located only 17 light-years from Earth, a team of astronomers was able to photograph its surface in 2007. This star has the unique characteristic of being a rapid rotator, completing a revolution in just 7 h 46 min, compared to almost 25 days for the Sun. Its rotational speed at the equator reaches 314 kilometres per second, making Altair very flattened at the poles.
Regulus joins Venus, Mars and Mercury
Three planets are currently hovering near Regulus, the alpha star of Leo. On the evening of July 10, Mars can be found less than 1 degree to the upper right of Regulus, whereas the very brilliant Venus will hang to the lower right of the star. Admire the pair above the west-northwestern horizon at nightfall.
The crescent Moon joins this celestial trio on the evening of July 20, about 30 minutes after sunset. This time, Mars can be spotted to the upper left of Regulus, with the Moon above both celestial bodies; Venus shines directly below Regulus. The elusive Mercury also makes an appearance, a dozen or so degrees to the right of Venus.
In fact, Mercury is in close conjunction with Regulus on July 28 at twilight, when the planet lies just one-eighth of a degree to the lower left of the star. This observation will require a clear view of the west-northwestern horizon, since the encounter takes place very low in the sky, half an hour after sunset. Binoculars can help you locate them.
Saturn and Jupiter visible after midnight
Saturn will be visible in the second half of the night this month. The ringed planet rises around 11 p.m. in the east-southeast and culminates in the south at the crack of dawn. The night of July 6 to 7, the waning gibbous Moon will pass a mere 3½ degrees below Saturn.
Jupiter rises a little over two hours after Saturn and will also be visible due east in the second half of the night: It’s so bright you can’t miss it! The waning Moon will be its neighbour on the mornings of July 11 and 12.