As the seasons change, be on the lookout for several eye-catching alignments of planets and stars, along with the moon playing hide-and-seek with a stellar bull’s eye. The night skies will also showcase the farthest planet from the sun at its best for the year, while early mornings will offer ghostly views of a serene cosmic pyramid.
So dust off those binoculars, and mark your September calendar!
Neptune at Primetime—September 5
The distant ice giant reaches opposition this month, which means it will sit opposite to the sun in our sky and will be visible all night long. Opposition also marks the planet’s closest approach to Earth, which will make it brighter to our eyes than at any other time of the year.
Over the next few weeks, Neptune will be approximately 2.7 billion miles from Earth, and sunlight reflected off its icy cloud tops will take nearly four hours to reach us. Even shining at its brightest, around magnitude 7.8, Neptune is best seen through mounted binoculars or a backyard telescope.
Start looking in the southeast sky within the constellation Aquarius, the water-bearer. Scan the constellation carefully, and look for a tiny blue-gray disk to pop out against the background of fainter stars. The view may be even better a few days past opposition, though, because the planet may be harder to spot within the glare of the bright full moon on September 6.
Regulus and Mercury—September 10
For a great observing challenge, look for the pairing of the planet Mercury with Regulus, the lead star in the constellation Leo, low in the eastern sky about 45 minutes before your local sunrise. Your best bet is to scan for the duo just above the horizon during the morning twilight.
Moon and Aldebaran—September 12
The waning gibbous moon will be rising near local midnight in the eastern sky, when it will glide through the constellation Taurus, the bull. That will bring the moon near the bright orange star Aldebaran, which marks the eye of the mythical bull. For lucky sky-watchers across North America, the pair will be so close in the sky that Aldebaran will appear to slip behind the lunar disk in an event known as an occultation.
Mars and Mercury—September 16
Look for Mercury this morning teaming up with its fellow planet, Mars. Both worlds will appear like faint orange-hued stars low in the eastern sky about 45 minutes before your local sunrise. The two planets will be separated by less than half the width of the full moon’s disk. Of course, this proximity is an illusion: Mercury, the closest planet to the sun, is about 48 million miles from Earth, on average, while Mars is roughly 55 million miles away.
Venus and Moon—September 17
Morning twilight in the east will show off a stunning lineup of cosmic gems, starting with the thin crescent moon perched just above brilliant Venus. Continuing in a diagonal line, viewers will see Regulus followed by Mercury and Mars. Binoculars may help you hunt down all the members of this celestial alignment and cut through the increasing glare of the rising sun.
Zodiacal Lights—September 18
Starting on this day and lasting for the next two weeks, observers in the Northern Hemisphere will have their best chance to see an ethereal display called the zodiacal lights, thanks to a near-moonless sky in the predawn hours. The phenomenon is caused by sunlight reflecting off countless dust particles scattered between the planets along the plane of the solar system. In the dark countryside far from city lights, look for a pyramid-shaped glow fainter than the Milky Way rising above the eastern horizon.
Venus and Regulus—September 20
Early risers looking toward the eastern sky about an hour before sunrise can catch the brilliant planet Venus parked very close to the star Regulus. The two celestial beacons will appear to be separated by less than the width of the full moon.
September Equinox—September 22
Fall kicks off in the Northern Hemisphere while spring starts in the Southern Hemisphere at 20:02 UT on this day. The September equinox marks one of the four major turning points in the seasonal cycle. Seasons occur on Earth because the planet’s axis is tilted with respect to its orbit around the sun. But during an equinox, the tilt effectively vanishes, and both hemispheres experience equal amounts of sunshine. Day and night are also about the same in length—the word “equinox” derives from the Latin for “equal night.”
Jupiter and Moon—September 22
Act fast on the equinox if you want to get your last glimpse of the king of planets before it disappears in our evening skies. Jupiter is heading for conjunction, when it will be behind the sun from our vantage point. But on the 22nd, speedy sky-watchers will be able to watch the waxing crescent moon as it glides by Jupiter just before the planet follows the sun and sets in the evening.
Saturn and Moon—September 26
After darkness falls, face the southern sky and look for the near quarter moon with bright yellow-hued Saturn hanging below. The ringed planet will about three degrees away from the moon, or about the same span as six lunar disks.