First day of Fall—what does it mean?


What is the Autumnal Equinox?
Fall begins on September 23, early in the morning at 4:21 A.M. The autumnal equinox is when the Sun appears to cross the celestial equator from north to south. (The celestial equator is the circle in the celestial sphere halfway between the celestial poles. It can be thought of as the plane of Earth’s equator projected out onto the sphere.)

Another definition of fall is nights of below-freezing temperatures combined with days of temperatures below 70 degrees Fahrenheit.

The word equinox means “equal night”; night and day are about the same length of time. This occurs two times each year: Vernal in late March and Autumnal in late September.

In addition to the (approximately) equal hours of daylight and darkness, the equinoxes are times when the Sun’s apparent motion undergoes the most rapid change. Around the time of the equinoxes, variations in the position on the horizon where the Sun rises and sets can be noticed from one day to the next by alert observers.

From here on out, the temperatures begin to drop and the days start to get shorter than the nights (i.e., hours of daylight decline).

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Questions and Answers About Fall
Question: Why aren’t there exactly 12 hours of daylight and 12 hours of darkness on the fall equinox?

Answer: On the equinoxes, the very center of the Sun sets just 12 hours after it rises. But the day begins when the upper edge of the Sun reaches the horizon (which happens a bit before the center rises), and it doesn’t end until the entire Sun has set. Not only that, but the Sun is actually visible when it is below the horizon, as Earth’s atmosphere refracts the Sun’s rays and bends them in an arc over the horizon. According to our former astronomer, George Greenstein, “If the Sun were to shrink to a starlike point and we lived in a world without air, the spring and fall equinoxes would truly have ‘equal nights.'”

Question: The autumn leaves seems to be hanging on longer than usual in my neck of the woods. Is this an indication of winter weather to come?

Answer: There’s an old weather proverb that states, “If autumn leaves are slow to fall, prepare for a cold winter.” Or perhaps you just haven’t had the kind of wind or rain needed to shake the leaves loose from their branches. But look on the bright side—you get to look at the beautiful autumn foliage for a little bit longer