The Autumnal Equinox and Fall Begins

Red MapleTree (Acer rubrum) leaf. Photo by Donna L. Long.
Red MapleTree (Acer rubrum) leaf. Photo by Donna L. Long.
The autumnal equinox starts the fall season. It occurs on September 22 or 23 each year. The “fall” is my favorite time of the year. The crisp cool air and brilliant colors of the leaves urges me outside with camera in hand. This season is called “fall” because leaves turn colors and “fall” from the trees.
EarthOrbit_solstices_equinoxes
The Earth’s orbit with solstices and equinoxes. Graphic by Nasa.gov.

The Two Equinoxes

There are two equinoxes a year. One in spring and the other in autumn.The spring (or vernal) equinox happens on March 19, 20, or 21.
The seasons are reversed in the northern and southern hemispheres of the planet. When the northern hemisphere experiences autumn, the southern hemisphere is experiencing spring.
On the equinoxes, the sun is directly above the Earth’s equator. Day and night are of equal length all over the planet. The world receives twelve hours of daylight and twelve hours of darkness. The term equinox comes from the Latin and means “equal night”.
Illustration shows the relative positions and timing of solstice, equinox and seasons in relation to the Earth’s orbit around the Sun. N=north hemisphere, S=southern hemisphere. Credit: Colivine, CC0, via Wikimedia Commons.

Equinoxes and the Seasons

The equinoxes signal that changes in length of daylight, temperature, and weather are about to happen.
The seasons are caused by the changing position of the Earth in relation to the Sun. The Earth receives different amounts of sunlight during the year due to the tilt of the Earth.
The Earth is like a spinning gyroscope and always points in the same general direction. The Earth is tilted on a 23.5° degree angle and the sun shines on the planet in a particular way. The north pole is generally tilted toward the north star and the south pole tilts toward the constellation of Octans.
Autumn leaves of Virginia Creeper (Parthenocissus quinquefolia) vine.
Autumn leaves of Virginia Creeper (Parthenocissus quinquefolia) vine.

Autumn: Cooler Days and Short Nights

The nights begin to cool around August 15th, as the northern region of Earth tilts away from the direct rays of the Sun. The air and ground begin to cool.
It takes a few weeks for the weather to change after the beginning of a new season. During autumn, there are alternating warm and cool days and cooler nights for several weeks.
The Autumnal Equinox begins the cold months. During the colder months, the North Pole is at its greatest tilt away from the sun. The northern hemisphere has colder temperatures, short days and long nights.
On the day of the Autumnal Equinox, the Sun is positioned directly over the equator.  After this day, the northern hemisphere begins to tilts away from the sun. The weather becomes colder and the hours of daylight become shorter. This culminates in the shortest day and the longest night of the year, the Winter Solstice on December 22 or 23. Then the days grow longer until the Summer Solstice.
The Circle of Life continues.
collage of orange tree leaves
Autumn collage of orange tree leaves

More on the Seasons

Seasons: Earth’s Natural Rhythms Explained

Why Do Leaves Change Color? with a Video

Why Trees Shed Leaves in the Fall with a Video

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