When is the Fall Equinox?

If you are watching the leaves shift to shades of amber, you might be wondering exactly when the season becomes official.

Before looking at the global clock, there is one quick detail to clear up: solstices are only for summer and winter, marking the longest and shortest days of the year. Autumn actually begins with an equinox, which is the moment day and night are almost perfectly equal in length.

Even if we use the correct term, a major question remains: why does the calendar date keep shifting?

Does Autumn Start on the 22nd or 23rd of September?

The short answer is that it starts on both dates. The confusion is not caused by a planetary mystery, it is entirely down to human time zones.

The equinox is not a gradual, day-long transition. It is an instantaneous event that happens at the exact second the Sun crosses directly over the Earth’s equator. Because it is a singular moment, the entire planet crosses the threshold into autumn at the exact same second.

However, because the globe is split into different time zones, that single moment lands on two different calendar days depending on where you live.

Here is how the exact timing of the equinox looks across different regions:

How the Time Zones Divide the Date

The global clock tracks the equinox at precisely 00:05 UTC on September 23rd.

Because London and Tokyo are ahead of the prime meridian, their clocks have already ticked past midnight. For someone living in the UK, Europe, or Asia, autumn officially begins on September 23rd while they are asleep or starting their morning.

Now look at North America. Because New York and Los Angeles are several hours behind UTC, the sun has not even set on their Tuesday yet. For an American, the shift happens on the evening of September 22nd.

Both dates are completely correct. Your calendar is not broken, it simply depends on what your watch says when the earth hits its orbital milestone.

Why the Time Shifts Every Year

The time zone difference explains why the date splits on any given year, but the exact time of the equinox also moves from one year to the next. This happens because our calendar does not perfectly match the physical orbit of the Earth.

A standard calendar year is exactly 365 days. However, it takes the Earth roughly 365.24 days to complete its journey around the Sun. That extra quarter of a day means the equinox arrives nearly six hours later each year on the calendar.

To prevent the seasons from drifting completely out of place over time, a leap year is added every four years. Adding February 29th pulls the calendar back into alignment, resetting the clock by shifting the equinox back by about 18 hours.

Here is how this four-year loop looks in Universal Time (UTC):

Because of that steady six-hour forward creep, the equinox time pushes past midnight UTC, moving the date to September 23rd for the UK, Europe, and Asia. When the next leap year arrives, the reset button is pushed, and the date moves back to September 22nd.

How to Celebrate the Start of Fall

If you live in the Western Hemisphere, your astronomical autumn lands on September 22nd. If you live in the Eastern Hemisphere, it is September 23rd.

If you prefer not to tie your seasonal plans to strict astronomy, there is an entirely separate way to measure the season using weather patterns instead of space science.

How the Weather Team Defines Fall

If you want to see how meteorologists bypass the time zone headache and lock down a fixed date every single year, you can read our full guide: [Astronomical vs. Meteorological Autumn: Which One is Right?] (/blog/meteorological-vs-astronomical-fall/)