Every year, the September sky heralds a transition to fall, a time when nights become much longer and the cooler weather forces us to change our summer routine. In 2018, the planets tag along during this transition and try to preserve their summer glow, although with varying degrees of success. Mercury steals the show and promises some excellent observations for early risers.
Jupiter and Venus on their way out
As summer comes to an end, Venus and Jupiter struggle to stay above the west-southwestern horizon. Jupiter now rises only about 15 degrees at nightfall. Venus is even lower on the horizon and eventually disappears in the twilight glow near the end of the month (in October, it’s Jupiter’s turn). However, in the first weeks of September, we can still spot them both just after sunset thanks to their intense glow. On the 24th, Venus reaches its maximum brightness of magnitude –4.8. The Moon also swings by the two planets, its last visits for several months. The crescent lies 9 degrees above Venus on the 12th at twilight and comes within 4.5 degrees of Jupiter on the 13th.
Mars and Saturn, unobtrusive but still lovely
After taunting Sagittarius with its bright red glow in August, Mars returns to Capricornus as of September 1 but loses some of its lustre during the month. As the weeks pass, its brightness fades by half, going from magnitude –2.1 to –1.3. The optimal period for viewing Mars (which recurs every two years) is now over, and the red planet culminates only about 20 degrees high. Nonetheless, Mars remains easy to make out in the first hours of the night in Capricornus, where no other celestial body upstages it. On the night of September 19–20, it’s visited by the waxing gibbous Moon, which passes 4.5 degrees above it. The pair culminates in the south around 9:30 p.m.
Saturn also remains very visible in Sagittarius till around midnight in early September. The planet sets earlier and earlier as the days go by and becomes difficult to spot after 10 p.m. late in the month. Saturn has kept a low profile this year because it’s in the lowest part of the ecliptic, but its superb rings, which are tilted about 26 degrees, are still worth checking out. They’re easily observable even with a small instrument. Since Saturn’s glow doesn’t stand out much among the stars of its host Sagittarius, take advantage of the Moon passing by on the evenings of the 16th and 17th for a more interesting tableau.
Mercury rewards early risers
Though the glow of the other planets dims in September, Mercury offers a very good viewing period. Admire the planet in the morning sky above the east-northeastern horizon 45 minutes before sunrise. Still fairly close to Cancer very early in the month, Mercury gradually drifts toward the star Regulus and overtakes it on the 6th. Look for the tiny coopery ball between the 5th and 9th while it’s close to the Lion’s Heart. On the 8th, a very thin crescent Moon joins the party.
If you’re an early-morning stargazer, the Moon offers a challenge on the 7th, starting when it rises around 3:30 a.m. and lasting till the first light of dawn. During these hours, a very thin crescent Moon lies less than 5 degrees under M44, the Beehive Cluster. Though visible with the naked eye, this encounter is certainly more impressive through binoculars (with a maximum magnification of 8x).
Zodiacal light, the challenge of the equinox
Still not convinced to set your alarm for the wee hours of the morning? Each year, the weeks surrounding the fall equinox (September 22 at 9:54 EDT this year) are ideal for observing the zodiacal light. Sunlight reflecting off dust between the planets along the plane of the solar system can be seen more easily during this period when the angle between the ecliptic and the eastern horizon is quite steep. The zodiacal light appears as a white glow shaped like a long triangle or pillar and stands out in a sky free of light pollution. It can be spotted in the morning in the east, about an hour before sunrise above a clear horizon. This year, focus on the two weeks before the equinox so you avoid the light of the Moon, which is waxing during this period. Observing this phenomenon is tricky, but you can also try your luck at twilight close to the spring equinox.