Autumn Equinox: Why Fall Begins on September 22 or 23, Depending on the Clock

Ask three people when fall begins and you may hear two dates. In 2026, September 22 and September 23 are both right. The difference is the clock, not the Sun. The September equinox is a single moment, and that moment falls on different calendar days in different time zones.

This guide sorts out the date and explains why day and night are nearly equal, but not exactly. It then links the equinox to the height of the noon Sun and ends with a task in the Orbit Dial. The date comes from the U.S. Naval Observatory table. The science comes from NASA, NOAA and the Naval Observatory.

What the September equinox is

NOAA's satellite team describes an equinox as the time when Earth's axis is neither tilted toward nor away from the Sun. The result is nearly equal hours of daylight and darkness across the globe. At the equator, NOAA says, the Sun sits directly overhead at noon during both equinoxes.

NASA's Night Sky Network gives the geometry. The equinox marks the exact moment when the center of the Sun crosses the plane of our planet's equator. In September that crossing goes from north to south. NASA says the Sun's path then continues traveling south until the December solstice, when it heads back north once again.

For people in the Northern Hemisphere, this moment starts astronomical fall. NOAA calls the autumnal equinox the official start of astronomical fall.

The drawing above shows the September equinox for a place at 40 degrees north latitude.

The 2026 date and time

The Naval Observatory's table for 2026 lists the September equinox on September 23 at 00:05 Universal Time (UT). Set to Eastern Time, the same table gives September 22 at 20:05 DT, which is 8:05 p.m. Eastern Daylight Time. When this guide was published on November 10, 2026, that moment had already passed.

NASA's Night Sky Network lists the same moment as September 23 at 00:05 UTC, or September 22 at 5:05 PM PDT. So one moment shows up as two dates.

In the Eastern and Pacific time zones, then, fall began on the evening of September 22. By UT, it began just after midnight on September 23. The National Centers for Environmental Information (NCEI, part of NOAA) gives the approximate date as on or around September 22 and says the exact dates vary because an extra day is needed every fourth year.

Day and night around the equinox

NOAA says the equinox brings nearly equal hours of daylight and darkness. It adds that refraction, the bending of sunlight by the atmosphere, along with the slower sunrise and sunset at higher latitudes, makes the day appear slightly longer. The Naval Observatory states it plainly: day and night are not exactly of equal length at the time of the March and September equinoxes.

The exact day of equal length depends on where you are. NASA says the date for that split depends on your latitude and may occur a few days earlier or later than the equinox itself. The Naval Observatory gives two examples.

At 40 degrees north, then, the day is still a little longer than the night on the equinox date. Equal length comes about September 26, a few days later. Near the equator the shift is bigger. At 5 degrees north, equal day and night come about three weeks after the September equinox.

How high the Sun stands at noon

At an equinox the Sun sits on the celestial equator, says the University of Nebraska-Lincoln, and that is true for both equinoxes. NASA's Radio JOVE training module turns this into a rule: the noon Sun's altitude, its angle above the horizon, equals 90 degrees minus your latitude. Its example is a person at 40 degrees north, where the noon Sun stands 50 degrees high.

That is the same number you get in March. What differs is the direction of travel. After the September equinox the Sun's path moves south, so for a northern place the noon Sun drops a little each day. By the December solstice, 40 degrees north gets a little over 26 degrees. Our winter solstice guide explains that low point. For more on why days change length, see the guide on day length.

Common mistakes

• Thinking fall starts on one date for everyone. The moment is the same, but the date depends on the clock. The 2026 equinox is September 23 in UT and September 22 in the Eastern and Pacific times.
• Expecting exactly 12 hours of day on the equinox. The Naval Observatory says day and night are not exactly equal. At 40 degrees north, equal length comes about September 26.
• Mixing astronomical and meteorological fall. NOAA says meteorological fall runs from September 1 through November 30. NCEI describes it as September, October and November, set by the annual temperature cycle. Astronomical fall starts at the equinox.
• Calling it fall everywhere. At this equinox the Southern Hemisphere's season is the reverse of the north's. NASA says that in March the north welcomes spring while the south notes its autumn, and that in September the reverse is true.
• Treating the equinox as a whole day. NASA calls it an exact moment, when the center of the Sun crosses the plane of the equator.

Try it in the instrument

Open the Orbit Dial and set the sky to the September equinox, the first day of fall in the Northern Hemisphere. The dial takes a date in 2026 and a latitude, and it shows the height of the noon Sun and the length of the day. The target is September 22 or 23, the pair of dates in the table, since the equinox is one moment between them. The game accepts a window of dates, not one exact minute, and the window gets narrower from Bronze to Gold.

Use the rule from this guide to check yourself. Slide the latitude on the target date. The noon Sun height should follow 90 degrees minus your latitude, and the day should stay close to 12 hours at every latitude. Then compare with March 20. The noon heights should match, because the Sun is on the celestial equator at both equinoxes. The only difference is that after September the heights fall in the north, and after March they rise.

Questions people ask

Are "fall" and "autumn" the same season?

Yes. NASA's Night Sky Network pairs the words in the phrase "autumnal, or fall, equinox." NOAA's article about the September equinox is titled with "Autumnal Equinox" and describes the start of astronomical fall.

Does the Sun really rise due east at the equinox?

Almost everywhere. NOAA says the Sun rises due east and sets due west for nearly every location on Earth on the equinox, and that this does not happen at any other time of year.

When does astronomical fall end?

At the next solstice. NOAA says astronomical fall continues until the winter solstice. In the Naval Observatory's 2026 table, that is December 21 at 20:50 UT, which is 3:50 p.m. Eastern Standard Time.

Why does the noon Sun get lower after the September equinox?

The Sun's path keeps moving south. NASA says it continues traveling south until the December solstice, when it heads back north. For a place in the Northern Hemisphere, a more southern path means a lower noon Sun.

Sources

• Earth's Seasons - Equinoxes, Solstices, Perihelion, and Aphelion, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/calculated/seasons?year=2026&tz=0&tz_sign=1&tz_label=true&dst=false&submit=Get+Data. Opened November 10, 2026.
• Length of Day and Night at the Equinoxes, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/equinoxes. Opened November 10, 2026.
• Embracing the Equinox, NASA Science, Night Sky Network. https://science.nasa.gov/solar-system/skywatching/night-sky-network/embracing-the-equinox/. Opened November 10, 2026.
• NOAA Satellites Observe the Autumnal Equinox, NOAA NESDIS. https://www.nesdis.noaa.gov/news/noaa-satellites-observe-the-autumnal-equinox. Opened November 10, 2026.
• Citizen Science Training Modules, Training Module 0.0: Introduction to the Sun, NASA Radio JOVE Project (radiojove.gsfc.nasa.gov). https://radiojove.gsfc.nasa.gov/citizen_science/training/TM0.0_The_Sun_Introduction.pdf. Opened November 9, 2026.
• Paths of the Sun - Motions of the Sun - NAAP, University of Nebraska-Lincoln, Astronomy Education (NAAP). https://astro.unl.edu/naap/motion3/sunpaths.html. Opened November 10, 2026.
• Meteorological Versus Astronomical Seasons, NOAA National Centers for Environmental Information. https://www.ncei.noaa.gov/news/meteorological-versus-astronomical-seasons. Opened November 10, 2026.

We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.

Read next

• Spring EquinoxAt the March equinox the Sun stands over the equator and day and night come out nearly equal, though not exactly.
• Why Day Length ChangesDay length is set by two things, where you are and what day it is.