On September 23rd, the equal length of day and night is known as the Autumnal Equinox. This phenomenon occurs twice a year, in March and September, in which the tilt of the Earth`s axis is neither leaning towards nor away from the sun. Option 2, Celestial Equinox, is not the correct term, as celestial refers to anything relating to the sky. Option 3, Winter Equinox, and Option 4, Vernal Equinox, are both incorrect as they refer to the solstices that occur in December and March respectively. The Autumnal Equinox marks the beginning of fall in the Northern Hemisphere and spring in the Southern Hemisphere, and it is an important event for many cultures and traditions around the world.