Welcome to the first day of fall

HOLT, Fla., Sept. 22, 2017—The astronomical calendar switches from summer to fall today as Earth celebrates the autumnal equinox.

It may not seem like fall around here, but Sept. 22 is the official end of summer and start of the fall season, according to the stars.

The first day of fall, also known as the September equinox, isn’t determined by cooler temperatures or changing leaves. Cooler temps won’t happen until much later around here and a mass region-wide leaf change is rare this far south.

Instead, the start of autumn is determined by the tilt of the earth’s axis.

Tilt

Tilt of the earth showing the celestial equator at equinox. (illustration by Dennis Nilsson)

The earth is tilted 23 degrees on its axis as it moves around the sun each year.  When the northern hemisphere is tilted toward the sun, it’s summer.  Conversely, when the earth tilts away from the sun, it’s winter.

Twice a year, on the summer and autumnal equinoxes, the earth is midway through its planetary tilt, halfway between the longest and shortest days of the year in June and December, respectively.

On the equinox, daytime and nighttime are roughly equal in length.  Daylight hours following the autumnal equinox begin to get shorter and nighttime hours get longer as winter nears.

Equinox happens at the exact same time all around the world:  8:02 p.m. GMT.  That’s 3:02 CDT for the Holt area.

This is because the equinox is determined when the sun crosses the celestial equator, the imaginary line above the Earth’s equator.  At that precise time, sun is directly overhead at noon as seen from the equator.

The sun rises and sets exactly in the east and west on the equinox.

And as a reminder, the change in seasons is followed by the end of Daylight Savings Time across most of the continental United States.  Clocks change back an hour to Central Standard Time Nov. 5:  “Spring forward, Fall back.”

Stephanie Holcombe

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