September is always one of the best months for stargazing in the northern hemisphere—and this month is better than most.
The nights lengthen as we edge towards the autumnal or fall equinox later this month, but they stay reasonably warm. It’s therefore a fine month to look for rising constellations and asterisms in the east—such as the Great Square of Pegasus and Perseus—and an excellent time to get eyes-on (through binoculars) with Andromeda Galaxy (M31) nearby.
About two hours after sunset you’ll see the “Summer Triangle” stars drifting to the western sky, where Venus shines brightly yet briefly after sunset. In the east you’ll find Jupiter and its incredible eclipsing moons, and Saturn, while close to them the tiny Neptune comes into “opposition.”
However, there’s always one major celestial highlight in September, and that’s the rise of the famously bright “Harvest Moon.”
Here’s everything you need to know about stargazing and moon-watching in September 2021:
1. Warm, dark night skies—and the Milky Way!
When: Wednesday, 1 September through Wednesday, September 8, 2021
Where: All-sky
This is one of the best weeks of the entire year for stargazing from the northern hemisphere. With the Moon waning to a New Moon on Tuesday, September 7, 2021, there’s a long stretch of reasonably warm and dark moonless nights in which to go stargazing, or to take a stroll after dark.
It’s the perfect time to get a last look at the Milky Way, which is visible in the south and southwest after dark, before it sinks into the horizon for fall and winter.
2. A crescent Moon and Venus
When: After sunset on Thursday, September 9, 2021
Where: Western sky
Look low in the west about half an hour after sunset and you’ll see the lovely sight of the bright planet Venus just 4° from a 11%-lit crescent Moon. Just below will be Spica, the brightest star in the constellation of Virgo.
You may need binoculars for the latter. Although it will make no difference to what you see, know that tonight the Moon is at its annual perihelion—its closest point to the Sun of the entire year.
3. Neptune at opposition
When: 14 September
Where: close to Circlet Of Pisces
Earth is between Neptune and the Sun tonight, which makes the other “pale blue dot” as bright as it gets. In practice any night this month is a good time to look for the eighth planet from the Sun, though you’ll need a pair of binoculars or a small telescope to see it close to the “Circlet Of Pisces.” It’s level with Saturn and Jupiter, in the southeast.
4. Northern Lights season begins
When: September-March
Where: Northern sky
Also known as the aurora borealis, the phenomenon of mostly green lights in the night sky are caused charged particles from the Sun being accelerated down the field lines of the Earth’s magnetic field.
To see them you really need to be around the Arctic Circle in northern Canada, Alaska, Iceland or northern Scandinavia. They’re often especially active around the date of September’s equinox.
5. An iconic ‘Harvest Moon’ rising
When: Dusk on Monday, September 20, 2021
Where to look: Rising due east
At 23:55 UT today sees perhaps the most famous full Moon of them all—the “Harvest Moon.” The prize is to see it appear in the eastern horizon draped in autumnal oranges and yellows.
Look due east about half an hour after sunset or get the exact times of moonrise and moonset for your location.
6. ‘Equal night’
When: Wednesday, September 22, 2021
Where to look: N/A
At precisely 19:21 UT today it’s the fall or autumnal equinox. It marks a point when the midday Sun is directly above the equator, giving every location on the planet 12 hours of daylight and 12 hours of darkness. The Sun will rise due east, follow an arc along the celestial equator and then set due west.
An important seasonal waymarker, it’s when the Sun crosses the celestial equator heading south to create shorter, cooler days as the northern hemisphere begins to tilt away from our star.
7. The Andromeda Galaxy
When: All month
Where to look: High in the the east after dark
Although it’s been visible for a good few weeks if you know where to look, anyone with either inky-black night skies or (more likely) a small pair of binoculars should be able to make-out the famous Andromeda Galaxy (M31) high in the eastern sky after dark.
The most distant object you can see with your naked eyes at around 2.5 million light-years, M31 is our sister galaxy and contains around a trillion stars. Sure, it’s a mere smudge, but ... wow. We only know what our own Milky Way looks like because we can see the similarly giant spiral Andromeda Galaxy.
Times and dates given apply to mid-northern latitudes. For the most accurate location-specific information consult online planetariums like Stellarium and The Sky Live. Check planet-rise/planet-set, sunrise/sunset and moonrise/moonset times for where you are.
Wishing you clear skies and wide eyes.
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