What is the term for the day that marks the beginning of fall in the northern hemisphere?

The day that marks the beginning of fall in the northern hemisphere is known as the Autumnal Equinox. This event occurs around September 22 or 23 each year when the plane of the Earth's equator passes through the center of the sun. During this time, day and night are approximately equal in length, which is a defining characteristic of an equinox.

This term specifically refers to the transition into autumn, distinguishing it from other equinoxes like the Vernal Equinox, which signals the start of spring. The word "equinox" itself simply denotes a time when day and night are roughly equal, but the Autumnal Equinox is the particular event that marks the seasonal shift to fall. Similarly, the term "solstice" refers to the points in the year when the sun reaches its highest or lowest point in the sky at noon, which occurs during summer and winter, not fall. Therefore, the Autumnal Equinox is the most accurate term for the beginning of the autumn season in the northern hemisphere.