Autumnal Equinox Day

Calculate the Autumnal Equinox Date and Time

The autumnal equinox marks the astronomical beginning of autumn. It occurs when the Sun crosses the celestial equator from north to south (in the Northern Hemisphere) or south to north (in the Southern Hemisphere). Our Autumnal Equinox Day Calculator uses high-precision astronomical algorithms to determine the exact date, time, and countdown of the equinox for any year between 1000 and 3000.

The Astronomy Behind the Equinox

An equinox occurs twice a year when Earth's axis is tilted neither toward nor away from the Sun, resulting in nearly equal amounts of daylight and darkness worldwide. The term "equinox" is derived from the Latin words aequus (equal) and nox (night).

Due to orbital variations, the gravitational pull of other planets, and leap year cycles, the exact time of the equinox shifts slightly each year. To calculate these astronomical moments, we employ the standard polynomial expressions formulated by astronomer Jean Meeus in his work Astronomical Algorithms.

Mathematical Formulation

We first compute the time parameter $T$ in Julian millennia from the J2000.0 epoch (January 1, 2000, at 12:00 Terrestrial Time):

$$T = \frac{\text{Year} - 2000}{1000}$$

For the September Equinox (which is the Autumnal Equinox in the Northern Hemisphere), the Julian Ephemeris Day ($JDE$) is calculated using the following fourth-degree polynomial:

$$JDE_{\text{September}} = 2451808.41777 + 365242.01767 \cdot T - 0.11575 \cdot T^2 + 0.00337 \cdot T^3 + 0.00078 \cdot T^4$$

For the March Equinox (which is the Autumnal Equinox in the Southern Hemisphere), the Julian Ephemeris Day ($JDE$) is computed as:

$$JDE_{\text{March}} = 2451623.80984 + 365242.37404 \cdot T + 0.05169 \cdot T^2 - 0.00411 \cdot T^3 - 0.00057 \cdot T^4$$

Once the Julian Ephemeris Day is calculated, it is converted to Coordinated Universal Time (UTC) and then adjusted to your selected display timezone.

Explore Related Date & Time Tools

If you are interested in seasonal boundaries, try our Find Season by Date tool to determine which season any calendar date falls in. You can also explore calendar irregularities using the Check Leap Year tool to see if a year has 366 days.