In many of our hearts and minds, the start to fall is not scientific. It's the moment the air gets cooler, the moment we slip on our first sweater, the moment we order our first pumpkin spice latte.

But fall does not begin in the Northern Hemisphere, officially, until the minute the autumnal equinox strikes. Here's a quick explainer.

 

WHEN WILL IT OFFICIALLY BE FALL?

 

This year, the autumnal equinox falls on Sept. 22, at 10:29 p.m.

 

WHAT IS AN EQUINOX?

 

"Equinox" derives from the Latin word "aequinoctium," which means "equal night." The moment it occurs, the sun shines directly on the earth’s equator. On Sept. 23, the sun will rise exactly in the east and set exactly in the west, resulting in approximately 12 hours of daylight and 12 hours of nighttime, all over the world. After that, the sun will shine more on the Southern Hemisphere and less on the Northern, until the vernal equinox (better known as spring) arrives.

 

HOW DOES IT HAPPEN?

 

The earth is constantly spinning on its polar axis, making a full rotation every 24 hours. It is simultaneously orbiting around the sun, making a full rotation every 365.25 days. The equinox occurs twice a year, at the moment these two journeys overlap.

 

WHAT ELSE AFFECTS OUR SEASONS?

 

The earth's axis is at a tilt of approximately 23.5 degrees as it orbits the sun. When the Northern Hemisphere is tilted towards the sun, we receive more direct sunlight and longer daylight hours, producing the warmth we know and love in the spring and summer months. During fall and winter, we are tipped away fromt he sun, and the Southern Hemisphere gets to enjoy its more direct rays, while we freeze in darkness. (Sharing is caring, you know.)

 

DOES AUTUMNAL EQUINOX ALWAYS OCCUR AT THE SAME TIME?

 

No. The time and date of the autumnal equinox varies year to year depending on the earth's movement in relationship to the sun. Last fall, the autumnal equinox fell on Sept. 22 at 4:44 p.m.

 

WHAT ABOUT SEPT. 1?

 

Meteorogically speaking, Sept. 1 was the first day of fall. Meteorogical seasons begin on the first day of every third month, neatly dividing the year into four seasons with three months each.

 

HOW LONG WILL FALL LAST?

 

Winter solstice will begin on Sunday, Dec. 21. That means you have 88 days to complete our fall bucket list. What are you waiting for?