The switch between summer and autumn is marked by the autumnal equinox. Here is everything you need to know about this event.
- What is it?
The autumnal equinox (as well as spring equinox) is the moment when neither of the Earth’s hemispheres is inclined towards or away from the Sun.
- When is it?
In the Northern Hemisphere, the closest autumnal equinox will take place on September 23, 2023, at 06:50 GMT (02:50 a.m. EDT). In the Southern Hemisphere, it will occur on March 20, 2024, at 03:07 GMT (March 19, 10:07 p.m. EST).
- Why is it important?
In astronomy, the autumnal equinox marks the change of seasons — the end of summer and the beginning of fall. People from different parts of our planet celebrate this day with different traditions and festivals.
What is the autumn equinox?
Equinoxes happen when the Earth’s hemispheres aren’t tilted towards or away from the Sun. As a result, the Sun stays right above the equator, giving both hemispheres almost the same amount of sunlight.
There are two equinoxes a year — spring and autumnal ones. Once the autumnal equinox happens, the relevant hemisphere (the northern one in September and the southern one in March) is tilting away from the Sun. This leads to fewer daylight hours there, with the Sun rising later and setting earlier. The hemisphere will keep tilting away until it reaches the farthest point from the Sun at the winter solstice.
When is the first day of fall?
Based on the astronomical method of determining the seasons, autumnal equinox marks the beginning of fall. In 2023, this happens on September 23, at 06:50 GMT (02:50 a.m. EDT) in the Northern Hemisphere. But this date is not fixed and can vary from year to year, falling anywhere from September 21 to 24. In the 21st century, September 22 is the most common first day of fall, happening 76 times, while September 21 only occurs twice. September 23 is the date for the other 22 times. The next September 24 equinox won’t come until 2303.
In the Southern Hemisphere, the start of fall will be on March 20, 2024, at 03:07 GMT (March 19, 10:07 p.m. EST). There, the specific date of the autumnal equinox can change between March 19 and 21. Throughout the 21st century, March 20 will be the most common equinox day, happening 78 times. On March 21, it occurs only twice, and the remaining 20 times, it takes place on March 19.
Keep in mind that the dates mentioned are based on Greenwich Mean Time (GMT), and the equinox date might differ in various time zones.
There is another method of determining the seasons — meteorological. It suggests that autumn begins on September 1 (or March 1 in the southern latitudes). This approach relies on yearly temperature patterns. It aligns with the Gregorian calendar and employs consistent dates to evenly divide a year into four quarters.
What time is the autumn equinox?
In the Northern Hemisphere, the autumnal equinox takes place on September 23, 2023, at 06:50 GMT (02:50 a.m. EDT).
In the Southern Hemisphere, the autumnal equinox takes place on March 20, 2024, at 03:07 GMT (March 19, 10:07 p.m. EST).
The exact time for your location depends on your time zone. Here are examples:
- Eastern Daylight Time (GMT -4): autumnal equinox occurs on September 23, 2:50 a.m. EDT.
- Central Daylight Time/CDT (GMT -5): autumnal equinox occurs on September 23, 1:50 a.m. CDT.
- Mountain Daylight Time/MDT (GMT -6): autumnal equinox occurs on September 23, 12:50 a.m. MDT.
- Pacific Daylight Time/PDT (GMT -7): autumnal equinox occurs on September 22, 11:50 p.m. PDT.
If you want to know exactly when the event will happen where you live, try the Sky Tonight app. Just open the search window in the app and type “Equinox”. You’ll see equinoxes’ dates, and times, all set for your local time.
When does Daylight Saving Time end?
As you can see, each time zone listed above contains the word “daylight”. Daylight Saving Time is when we move our clocks ahead by one hour in the summer and then back again in the fall. The main idea is to make better use of natural daylight. When we move the clocks ahead in spring, we get more daylight in the evenings, and when we move them back in fall, we get more daylight in the mornings.
You might think that Daylight Saving Time would end when astronomical fall starts. But that’s not quite right. In the United States, Daylight Saving Time actually starts on the second Sunday of March and ends on the first Sunday of November. In 2023, Daylight Saving Time in the US goes from March 14 to November 5. In the United Kingdom and some European countries, summer time begins on the last Sunday in March and ends on the last Sunday in October. There, Daylight Saving Time in 2023 goes from March 26 to October 29.
Full Moon and autumn equinox
Even though you can’t directly observe the equinox, you can observe some effects it brings. For example, you might notice that the Full Moon rises earlier around the time of the autumnal equinox compared to other times. This happens because the angle of the Moon’s path in the sky and the tilt of the Earth’s axis align during this period.
The Full Moon that occurs close to the autumnal equinox is traditionally called the Harvest Moon. In the past, before electricity was common, this Full Moon was really helpful for farmers. It would rise right around sunset, giving them extra light hours to complete their harvesting. Interestingly, this is one of just two Full Moons (the other one being the Hunter’s Moon) with a name linked to an astronomical event rather than specific months’ characteristics.
In 2023, the Harvest Moon occurs on September 29 at 09:57 GMT. You can find more details about this Full Moon, its other names, and interesting aspects in our dedicated article.
Aurora borealis and autumn equinox
Another visible outcome of an equinox is the increasing activity of aurora borealis, also known as the Northern Lights. Around equinoxes, your chances of seeing the aurora go up.
To simplify it, during equinoxes, the Earth’s magnetic poles are situated in a way that they align well with the solar wind (the cause of the Northern Lights), making it more likely for the Earth to “accept” it.
Are day and night truly equal on the equinox?
They’re quite close to being equal, but not precisely. For instance, on September 23, 2023, in New York, the daylight will last for 12 hours and 7 minutes. There are two reasons behind this slight discrepancy — first, atmospheric refraction (that delays sunsets by about 8 minutes), and second, the specific definition of sunset and sunrise. For a visual explanation of why day and night aren’t exactly equal during equinoxes, you can check out our infographic about equinoxes.
By the way, there are indeed days when day and night are exactly the same lengths. They’re called equiluxes and usually occur a few days after the autumnal equinox. However, the exact dates for equiluxes can vary for different locations. Learn more about equiluxes from our article.
Do other planets have equinoxes and solstices?
Except for Mercury, all planets in our Solar System tilt on their axes, leading to their own equinoxes and solstices. Venus and Jupiter have small tilts of 2.6° and 3.1°, respectively. Venus, with its quicker orbit, experiences these events every few months, while Jupiter’s slower orbit means they occur roughly every three Earth years.
Mars, Saturn, and Neptune share tilts similar to Earth’s: 25.2°, 26.8°, and 28.3°. Saturn’s equinoxes present its rings edge-on to Earth.
Uranus stands out with its extreme tilt of 98°. It causes day and night to switch every 17 hours around equinoxes, but during its summer and winter, half of the planet remains either in darkness or daylight.
What’s the difference between an equinox and a solstice?
There are three key differences between an equinox and a solstice:
- Day and night balance vs. longest day or night. Equinoxes mark nearly equal lengths of day and night, while solstices bring the longest day or night of the year, depending on the season.
- Timing of events. Equinoxes occur in March and September, while solstices take place in June and December.
- Tilt angle. Equinoxes happen when the Earth’s hemispheres aren’t tilted toward or away from the Sun. Solstices occur when one hemisphere is tilted maximally toward the Sun (summer solstice) or away from it (winter solstice).
Now that you’ve gone through this article, you should be a pro at understanding equinoxes. Take our quiz to test yourself! If you paid attention while reading, you’ll find it easy to tell the difference between solstices and equinoxes.
In the Northern Hemisphere, the autumnal equinox happens on September 23, 2023, at 06:50 GMT (02:50 a.m. EDT). Meanwhile, in the Southern Hemisphere, it occurs on March 20, 2024, at 03:07 GMT (March 19, 10:07 p.m. EST). This event signals the shift from summer to fall. Following the equinox, temperatures drop, and the nights grow longer. During the equinox, the duration of daylight and nighttime is almost the same.