KNOXVILLE, Tenn. (WATE) — Season start dates seem arbitrary, especially considering that the weather may feel like a season before the season begins. With Autumn beginning soon, there’s a reason why the specific date was chosen to kick off fall.

This year’s fall begins on Sept. 22 aligning with the Autumnal Equinox. This equinox is when the Earth’s Axis is tilted either toward or away from the sun, resulting in a day with equal daylight and darkness at all latitudes, according to Sky & Telescope, which provides information used in WATE’s weekly Star Watch. This year, the sun passes through the equinox at 9:04 p.m.

Seasons are determined by the tilt of the Earth on its axis, which is 23.5°. This tilt causes the daylight and temperature changes associated with the seasons.

But the reasoning for why seasons start on specific dates, and why fall is starting on the Autumnal Equinox, is not as much of a coincidence as it might seem. According to National Geographic, the observance of the first day of certain seasons is based on astronomical seasons. These seasons each begin with an equinox or a solstice.

Equinoxes are when the days have equal amounts of daylight and darkness, while solstices are the brightest and darkest days of the year. This is why the beginning of fall is always on the Autumnal Equinox, winter with the Winter Solstice, spring with the Vernal Equinox, and summer with the Summer Solstice. The dates of each equinox are as follows:

  • Vernal Equinox: between March 20-21
  • Summer Solstice: between June 20-22
  • Autumnal Solstice: between September 22-23
  • Winter Solstice: between December 21-22

In this week’s Star Watch, WATE 6 Meteorologist Victoria Cavaliere said Thursday was the first day of astronomical fall. This term refers to the start date of fall being dependent on the positioning of celestial objects, according to National Geographic. In the late 18th century, some scientists also began using meteorological seasons, attempting to find a simpler model than astronomical seasons.

Meteorological seasons divide the year into three even months with each season beginning at the start of the month, according to National Geographic. Using this method, fall begins in September, winter in December, spring in March and summer in June in the northern hemisphere. This aligns more with temperature cycles and calendar dates than with astronomical events, which may seem more useful to some even though the astronomical season is still more used officially.