What’s Up in the Sky in September?
A seasonal shift is on the way as the September equinox arrives on the 23rd, marking the start of astronomical autumn in the Northern Hemisphere and spring in the Southern Hemisphere. The equinox also brings an extra bonus: the possibility of a boost in aurora activity.
Even if auroras don’t make an appearance, it’s still worth looking up. Cassiopeia, one of the most recognizable constellations, is easy to spot in the evening sky in the Northern Hemisphere.
Toward the end of the month, the Harvest Moon brings extra evening moonlight for a few nights. There are also plenty of close encounters to look out for throughout September, as the Moon meets Mars, Jupiter, Venus, and Saturn.
Lunar occultations of Jupiter and Venus, during which the Moon appears to pass in front of the planets, will also be visible from some parts of the world.
*Naked-eye events—although you should NEVER look directly at the Sun without proper eye protection. The Night Sky Map links use New York City, USA, as an example location.
A Good Month for Auroras?
September can be a particularly good month for aurora activity, and not only because it gets dark earlier.
The position of Earth’s axis relative to the Sun around the equinox brings some interesting factors into play in terms of the frequency and brightness of the auroras. These effects are not limited to the day of the equinox itself, but extend for several weeks before and after the equinox.
Scientists think this increase in northern and southern lights displays is mainly due to two factors:
-
The Russell–McPherron effect—the Sun’s and Earth’s magnetic fields are more likely to line up in a way that allows them to connect.
-
The equinoctial effect—Earth’s magnetic poles are tilted at a right angle to the flow of the solar wind, effectively presenting a bigger target for charged particles streaming toward our planet.
Together, these effects make it easier for charged particles and energy from the solar wind to enter Earth’s magnetosphere—our planet’s invisible shield—creating more frequent and sometimes brighter auroras.
September Equinox: “Equal Night”
So, what exactly is an equinox?
Astronomers define the equinox as the moment the Sun crosses the celestial equator. In other words, the Sun is directly above a point on Earth’s equator. The September equinox occurs when the Sun crosses the equator from north to south.
This year, the September equinox falls on September 23 at 00:05 UTC.
The September equinox marks a seasonal turning point. In the Northern Hemisphere, it is the autumnal (fall) equinox, after which nights become longer than days. South of the equator, it is the spring equinox, after which days become longer than nights.
The word equinox comes from Latin words meaning “equal night,” which can give the impression that day and night are exactly 12 hours long everywhere. But that isn’t quite the case.
The date on which day and night are closest to 12 hours each is known as the equilux. It occurs a few days before the September equinox in the Southern Hemisphere and a few days after it in the Northern Hemisphere, though the exact date depends on your latitude.
On the day of the equinox, the amount of day and night is almost equal in both hemispheres.
The Harvest Moon in September
Towards the end of the month, the Harvest Moon lights up the night. This year, September’s Full Moon falls on September 26.
The Harvest Moon gets its name from an unusual pattern of moonrises around this time of year. Normally, the Moon rises about 50 minutes later from one day to the next. But around the September equinox, that gap becomes much smaller in the Northern Hemisphere.
For example, around the Harvest Moon, successive moonrises are only about 25 minutes apart in New York, and the interval can be even shorter farther north.
This means the Moon rises at a similar time for several evenings in a row, providing more moonlight shortly after sunset. Traditionally, this extra evening light gave farmers more time to bring in their crops, which is how the Harvest Moon got its name.
In the Southern Hemisphere, the effect is reversed because September marks the spring equinox. Around Full Moon, the gap between successive moonrises is greater than the usual 50 minutes, producing the opposite of the Harvest Moon effect.
Look Out for Cassiopeia
Cassiopeia is one of the most recognizable constellations in the Northern Hemisphere, thanks to its distinctive W or M shape, depending on its position in the sky.
From locations north of about 34° N—the latitude of Los Angeles—Cassiopeia* is circumpolar, meaning it never sets below the horizon and appears to circle Polaris, the North Star.
During September evenings, it climbs higher in the northeastern sky, making it easier to spot. It will remain well placed for viewing throughout the autumn (fall) and winter months. Since it’s so recognizable, it’s also a useful reference point for finding trickier constellations and other features in the night sky, including the Milky Way.
Many cultures have their own stories associated with this star pattern, but the name Cassiopeia comes from Greek mythology, in which Cassiopeia was a vain and boastful queen.
Where Is the Sun in the Zodiac in September?
As Earth completes one orbit around the Sun each year, the Sun appears to travel through constellations within the zodiac, which astronomers define as a strip of sky on either side of the ecliptic.
These include the 12 “traditional” constellations of the zodiac—such as Aquarius and Leo—plus the lesser-known constellation Ophiuchus, through which the Sun passes each year around December.
In September 2026, the Sun begins the month in the constellation Leo before crossing into Virgo on September 17.
Readers familiar with horoscopes may notice these dates don’t match the zodiac sign dates they’re used to—in Western astrology, Leo typically runs from the final third of July to the final third of August.
This offset is a consequence of axial precession, a slow wobble in Earth’s rotational axis that has shifted the stars’ apparent positions since Western astrological signs were defined more than 2,000 years ago. Back then, the signs matched the constellations the Sun appeared in more closely—but the two have since drifted apart by about 30°, or roughly one month.
* Here, “Cassiopeia” refers to the constellation’s familiar W-shaped star pattern. Parts of the official constellation boundary may lie below the horizon even when the W star pattern itself remains visible.
Frequently Asked Questions
Why are auroras more common around the equinox?
Auroras are often more frequent around the March and September equinoxes because of two effects: the Russell–McPherron effect and the equinoctial effect. Together, they make it easier for charged particles from the Sun to enter Earth’s magnetosphere—increasing the chances of northern and southern lights displays.
When is the Harvest Moon?
In 2026, the Harvest Moon falls on September 26 at 16:49 UTC. The Harvest Moon is the Full Moon closest to the September equinox, which takes place on September 23 this year.
What is an occultation?
An occultation occurs when one celestial object passes in front of another and temporarily hides it from view. For example, the Moon can occult a planet or star as seen from certain locations on Earth.