Autumn Equinox Signals Start of Season as Day and Night Near-Equal

Autumn officially begins tonight as the sun appears to shine directly above the equator, marking the autumn equinox at 19:20 BST Monday. The timing means day and night are nearly equal in length worldwide, with days expected to shorten in the weeks ahead as winter approaches. NASA notes that the September equinox signals later sunrises and earlier sunsets in the Northern Hemisphere, along with cooler days, breezier winds and leaves turning color.

During the equinox, the Sun crosses the plane of the Earth's equator, illuminating the northern and southern hemispheres more or less equally. The equinoxes occur twice a year, in March and September, and are distinct from the solstices, which mark the longest and shortest days. Astronomers at the Royal Museums Greenwich note the 2025 dates for reference: spring equinox on March 20 at 09:01 GMT; summer solstice on June 21 at 03:42 BST; autumn equinox on September 22 at 19:20 BST; winter solstice on December 21 at 15:03 GMT.

For the Northern Hemisphere, autumn shifts mean later sunrises and earlier sunsets. Meanwhile, in the Southern Hemisphere, the equinox signals springtime, bringing warmer days and blossoming flowers in parts of Australia and elsewhere. In the Northern Hemisphere, tonight's equinox kicks off a countdown to the winter solstice on December 21, the shortest day of the year in many locations.

Solstices and equinoxes are two ways scientists describe the changing seasons. Solstices occur in June and December when the tilt of the Earth's axis is most extreme toward or away from the Sun. Equinoxes occur in March and September, when the Sun crosses the plane of the Earth's equator and day and night are nearly equal in length. NASA and Royal Museums Greenwich provide the standard references for these dates and definitions.

Beyond the astronomy, definitions of seasons vary by discipline. Meteorologists split the year into four seasons for statistical purposes, aligning them with the calendar months: spring (March 1–May 31), summer (June 1–August 31), autumn (September 1–November 30), and winter (December 1–February 28 or 29). The Met Office explains that this approach makes it easier to compare seasonal and monthly statistics across years, even as astronomers point to the tilt of the planet and the Sun’s relative position as a more physical basis for seasons.

Are day and night truly equal during an equinox? In practice, not exactly. The Old Farmer's Almanac notes that daytime begins when any part of the Sun is over the horizon and ends when the last part disappears below it. Atmospheric refraction and the Sun's finite size also mean that, even on an equinox, day and night are not perfectly equal. Royal Observatory astronomer Dr. Ed Bloomer has explained that equinoxes are commonly thought to mark exactly equal daylight and darkness, but the general pattern—longer days toward summer and shorter days toward winter—remains the practical takeaway as the year progresses.

Looking ahead, the astronomical definitions align with a predictable cycle. For 2025, the sequence begins with the spring equinox on March 20, followed by the summer solstice on June 21, the autumn equinox on September 22, and the winter solstice on December 21. In the Northern Hemisphere, the autumn equinox signals the transition toward shorter days and cooler weather, while the Southern Hemisphere heads toward spring and warming temperatures.