Fall begins every year on the day of the autumnal equinox. Typically, this event takes place on September 22 or 23. This year, the equinox will happen on September 22 at 1:02 p.m. MST.

WHAT IS THE AUTUMNAL EQUINOX?

The autumnal equinox happens when daytime and nighttime are the same length in both hemispheres. The word equinox means “equal night” in Latin, which is the perfect definition of what will happen on September 22. The day will be divided evenly into 12 hours of daylight and 12 hours of nighttime all over the Earth. The day and time of the equinox always varies from year to year because it takes the Earth around 365.25 days to orbit the sun and leap years set the equinox back a day.

WHY DOES IT HAPPEN?

Normally, the Earth is titled by 23.5 degrees. The Southern Hemisphere or the Northern Hemisphere usually tilt a little towards the sun. But twice a year during spring and fall, the sun crosses the celestial equator making the tilt of Earth’s axis vertical. At this point, the sun is directly over Earth’s equator. The celestial equator is an imaginary line through the sky which corresponds to the Earth’s equator. The autumnal equinox is a result of the sun crossing the equator from north to south. The sun will rise precisely on the east and set precisely on the west on this day. Equinoxes happen twice a year, once in September and once in March. The March event is called the vernal equinox and signifies the beginning of spring.

WHAT DOES THIS MEAN?

This astronomical happening marks the end of summer and the beginning of fall. After the autumnal equinox, the days begin to get shorter up until the winter solstice, the shortest day and the longest night in the Northern Hemisphere. The sun will begin to shine more on the Southern Hemisphere and less on the Northern Hemisphere. The Earth will tip gradually until it reaches its maximum tilt, usually around December 21 or 22.

After a hot Arizona summer, colder weather is exactly what we all want. Bring out your coats and cozy up with a blanket because fall is officially here!

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