The September equinox announces the official arrival of autumn in the Northern Hemisphere, and as the nights grow rapidly longer, several planets draw our attention…
Two of them are visible in the evening sky during September. Dazzling Venus is the first to appear in the glow of twilight above the west-southwest horizon, thirty minutes after sunset. The Evening Star gradually descends and sets an hour later. On the evening of September 5, Venus is just a degree-and-a-half above Spica, the brightest star in Virgo. You’ll have to wait for the sky to darken to see Spica shining next to the brilliant planet, though a pair of binoculars will help you spot it sooner. On September 8, it’s the Moon’s turn to appear next to Venus: A thin, three-and-a-half-day-old lunar crescent will be positioned a degree-and-a-half away from the dazzling planet — a spectacular scene set against the colours of twilight 45 minutes after sunset. Also note the reflected light of earthshine illuminating the dark part of the Moon’s face. The following day, on September 9, the Moon will be to the left of Saturn, which is the other planet visible in the evening sky.
Saturn and Venus move toward one another at the beginning of the month; from September 16 to 19, they are less than 4 degrees apart. But from evening to evening Saturn gets closer to the Sun and becomes harder to see at twilight. The ringed planet disappears in the glare of sunset during October… Until what point will you still be able to see it?
Another planetary duo occupies the morning sky: Jupiter and Mars. Around mid-month, Jupiter appears above the east-northeast horizon at 12:30 A.M., next to the “twin” stars, Pollux and Castor, in Gemini. By dawn, the giant planet is high in the southeast, about 55 degrees above the horizon. The lunar crescent will appear near Jupiter on the morning of September 28.
Mars rises a few hours after Jupiter. The Red Planet is visible toward night’s end, from three-and-a-half hours before sunrise till dawn: Look for a tiny orange “star” shining on the east-northeast horizon beneath Pollux and Castor. The crescent Moon will appear to the lower right of Mars on the morning of September 2, and to the right of the planet on the 30th. Mars moves rapidly against the background constellations as it heads toward Leo: It will shine near Regulus on the morning of October 15.
We occasionally tend to forget that the sky is not a 2D screen onto which the stars are affixed: It has depth. For example, on the mornings of September 8 and 9, Mars will pass close to the Beehive cluster (M44), in Cancer. As you watch this event unfold with binoculars or better still, a small telescope, consider the following: Mars is 338 million kilometers (2.8 au) from Earth, but the stars of the Beehive are 16.5 million times farther away, at a distance of 590 light years! This serves to illustrate the dizzying depths of space that separate objects in the cosmos…
Concerning the equinox
The autumn equinox takes place on September 22 at 4:44 P.M. EDT. At that time, the Sun will be exactly above Earth’s equator, and the line dividing day and night will extend from the North Pole to the South Pole. In principle, the length of day and night will be equal everywhere on Earth — the basis for the term “equinox.” However, in Montreal, the Sun will actually rise at 6:42 A.M. and set at 6:51 P.M., and in reality, the length of day and night will be equal a few days later, on September 25. (In Montreal, the Sun will rise at 6:46 A.M. and set at 6:45 P.M.) Why the offset? It’s caused by Earth’s atmosphere, which bends sunlight a small amount — the same effect that flattens the setting Sun when it’s low in the sky. When we “see” the Sun touch the horizon, it has actually already set by a few minutes; and the same thing applies at sunrise. At our latitude, we gain about ten minutes of sunlight due to this effect, which shortens the night slightly: If our atmosphere didn’t exist this effect would not occur. (In fact, it’s the same case throughout the year, but more noteworthy on the equinoxes.)
Around the equinoxes, the length of day also changes most rapidly. This month, at mid-northern latitudes, we lose about three minutes of daylight every 24 hours. Between the beginning and end of September, daytime is shortened by more than an hour-and-a-half!
Finally, statistics show that the period surrounding the equinoxes is most conducive to auroral displays. The reason is poorly understood, but it may be due to the Earth-Sun geometry, which creates an alignment between our magnetic field and that of the solar system, causing breaches in Earth’s magnetic shield. So, keep an eye out for the northern lights, especially if you’re under a dark sky... You never know.
Clear skies !