Autumnal equinox, or “First Day of Fall”, is September 22

After three sizzling months, the Earth's trip around the sun now brings it to the point of the autumnal equinox, or the first day of fall. For the Tennessee Valley, the autumnal equinox will take place at 9:21 a.m. Thursday, September 22.

The word "equinox" harkens back to Medieval Latin, and it means "equal night and day."

This is close, but not exactly true. In fact, sunrise in Huntsville Thursday will occur at 6:35 a.m. and sunset will occur at 6:43 p.m. -- that comes out to 12 hours 7 minutes and 37 seconds of daylight on September 22. After that point, we will gradually lose daylight time until the Winter Solstice, which will take place on December 21 at 4:44 a.m.

Seasonal change from summer to fall

Recall that the earth orbits around the sun in an elliptical path that lasts approximately 365.25 days (every 4 years there is an extra day -- February 29 -- and the year is called "Leap Year"). As the earth travels around the sun, it is also spinning on its axis, causing half the earth to spend a period in daylight with the other half experiencing darkness, or night.

Seasonal changes with respect to the Northern Hemisphere (Source: NOAA)

Seasonal changes with respect to the Northern Hemisphere (Source: NOAA)

But the earth does not "sit" perfectly vertical on its axis. Instead, it is tilted 23.5 degrees from the vertical.

As a result, sunlight is concentrated on the hemisphere "tilted" towards the sun, which increases temperatures. We call this season "summer", and in the case of the Northern Hemisphere (where the United States is located), this half of the earth is tilted towards the sun. The opposite is true for winter: the Northern Hemisphere is tilted away from the sun, resulting in less sunlight and colder temperatures.

During the vernal (spring) and autumnal equinoxes, the earth remains tilted 23.5 degrees from its vertical, but the earth's orbit puts in it in place such that an equal amount of sunlight reaches both the Northern and Southern Hemispheres despite the tilt. For those located near the equator, they would observe the sun reaching straight overhead at noon.

From this point forward until the Winter Solstice, daylight would continue to decrease the further north you go in latitude (for example, the further north you travel in the Northern Hemisphere). An extreme example of this is Barrow, Alaska: For 60 days, the town experiences "polar night", or darkness for as many as 21 of 24 hours in the day.