When Does Fall Start? Autumnal Equinox Marks First Day of the Season

While many people associate fall with the leaves changing colors and an end to hot weather, the first day of the new season will likely arrive before any noticeable changes in many areas.

Regardless of weather conditions or the emergence of Starbucks' pumpkin spice latte, fall will officially begin Monday. The start of the season and the end of summer is marked by an equinox, a planetary event that occurs twice a year.

Equinox derives from two Latin words, aequus and nox, meaning "equal" and "night," respectively. Residents of the Northern Hemisphere may have noticed their long summer evenings shrinking recently, while those in the Southern Hemisphere are currently enjoying increased daylight hours. On the day of an equinox, every part of world day sees almost exactly 12 hours of daylight and 12 hours of darkness due to the position of the Earth's axis.

Over the course of 12 months, Earth experiences two equinoxes. The vernal or "spring" equinox takes place around March 20, and marks the end of winter in the Northern Hemisphere.

The second takes place in September and, although the date varies from year to year, it usually falls between September 22 and September 24.

The autumnal equinox is set to take place at 3:50 a.m. in New York City, according to the National Weather Service. This means Chicago will experience the equinox at 2:50 a.m., Phoenix at 1:50 a.m. and Los Angeles at 12:50 a.m., all local time. At this moment, the equator will be pointed directly at the center of the sun.

when does fall start autumn equinox
A man sleeps on grass covered with leaves in Central Park on November 8, 2018, in New York City. On Monday, the autumnal equinox will take place, marking the first day of fall in the Northern Hemisphere. Eduardo Munoz Alvarez/Getty

The fall season will last until the winter solstice, which occurs around December 21. A solstice differs from an equinox and during the winter solstice, the Northern Hemisphere is tilted away from the sun, resulting in the shortest day and longest night of the year.

Normally, Earth tilts on its axis at 23.5 degrees as it orbits the sun, which accounts for the hemispheres' alternating warm and cool seasons. However, during equinoxes, the Earth's orbit and its axial tilt combine, causing the sun to sit right above the equator, according to National Geographic.

While Earth is unique in many ways from other planets, the equinox is not one of its defining characteristics. NASA has identified autumnal and vernal equinoxes on almost every planet in the solar system. However, since other planets orbit the sun at different rates to Earth, they experience equinoxes differently. In the year 2000, for example, autumnal equinoxes on Jupiter, Venus and Mars equinox occurred during March, May and June on Earth, respectively.

In 2008, NASA launched the Cassini Equinox Mission, which concluded in 2010. Aimed at studying Saturn's rings under new lighting conditions, the spacecraft visited the giant planet during its autumnal equinox, which occurred in August 2010.

For those north of Earth's equator, the sun has been up for less time each day since the summer solstice in June, a phenomenon that will continue after the equinox. Daylight will continue to lessen until the winter solstice, when daylight hours will start to increase again.

However, it's not just a looming cloud of darkness. In November, those who observe Daylight Savings Time will get the chance to roll the clock back and get an extra hour of sleep.