Every year in September, we say a sad farewell to summer as students head back to school and nights grow chillier. September is also the perfect time to change how you observe the skies. This year, early birds can be confident they’ll catch their worm.
A longing for summer
Summer slowly fades to fall as leaves turn yellow, days grow shorter and the sky also undergoes some gradual changes. The famous Summer Triangle, which some people see as a big V for “vacation,” still appears in the evening sky throughout the month. The Milky Way also continues to form a beautiful arch overhead in the first half of the night.
Yet Jupiter, which has shone near Spica in the constellation Virgo since early in the year, makes its final appearances. For the first half of September, you can still easily spot the planet near the west horizon where it lingers up to 90 minutes after sundown before setting. Beyond the second week, a longing for summer might prod you into looking for the planet at twilight, but don’t expect to see any surface details. On the 21st, during Jupiter’s last hurrah, a thin, elegant crescent Moon lies 6.5 degrees to its right very low in the west-southwest.
A fine year for the ringed planet
In 2017, Saturn displays its rings at their maximum tilt. Make sure you take advantage before the humidity kicks in for good this fall. Saturn appears early in the night in the south-southwest and disappears in the southwest well before midnight. This window of time is quite narrow and the planet is very low on the horizon, but if atmospheric conditions are stable during your observations, you just might push your instruments to their limit and make out the Cassini Division.
To the naked eye, Saturn is a yellowish, non-twinkling dot in the constellation Ophiuchus. It’s the most noticeable celestial body between Scorpius and Sagittarius, practically at the heart of the Milky Way. The ringed planet receives a visit from the Moon on the 26th.
In September, though, the skies reveal their true beauty at the first light of dawn. Early birds can thrill to the waltz of Mercury, Venus and Mars near Leo. The Moon also joins the party now and then.
Waltz of the planets at dawn
Early in the month, Venus, the bright, easy-to-spot Shepherd’s Star, rises three hours before the Sun and shines up to 20 degrees high at dawn. But as the days go by, its path along the ecliptic causes it to rise later and later till it’s just one hour ahead of the Sun by the end of the month. During these four weeks, Venus moves quickly from the heart of Cancer to the belly of Leo. During its travels, watch it through an instrument as the planet appears one degree to the south of the Beehive Cluster on the 1st and brushes past the star Regulus on the 20th just a half degree to its left.
As of the 8th, Mercury enters its best observation period of the year in the morning sky. Its visibility peaks around mid-month when you can make it out in the east 30 to 45 minutes before sunrise. On the morning of the 15th, Mercury appears the same distance from Regulus as Venus, although on the other side of Leo’s heart. Rarely does shy Mercury upstage its fellow planets, but it does in September. Mars, which is present in the same region of the sky throughout the month, is just a dim reddish dot often lost in the first light of dawn. To spot the red planet, use other celestial bodies as your guide. On the 5th, Mars lies just left of Regulus. It also forms a beautiful cosmic duo with Mercury from the 15th to 18th.
A rare encounter in the zodiacal light
As of the 15th, check out Venus, Regulus and the Mercury-Mars pair at dawn as they align perfectly along the ecliptic. Between the dawns of the 17th and 19th, you’ll note that the Moon, now shrunk to a very thin crescent, moves from above to below this lovely alignment after passing through the centre on the morning of the 18th. In fact, during the 24 hours surrounding this unusual encounter, the Moon occults Venus, Regulus, Mars and Mercury one by one, although, unfortunately, none of these occultations are visible from North America.
The alignment at dawn on the 18th is striking nonetheless. Also, if you’re lucky, the whole event may be bathed in the zodiacal light. The last two weeks of September, around the fall equinox (on the 22nd at 4:01 p.m. this year), are always a good time to observe this whitish glow. The zodiacal light is shaped like a long triangle or a pillar and stands out in a sky free of light pollution. You can see it in perfect conditions along the ecliptic as it emerges from the east horizon, about one hour before sunrise. The glow is caused by light from our star reflecting off the interplanetary dust in the plane of the solar system. It’s one more reason to be an early bird this month.