The first day of fall marks the autumn equinox, which is different from a solstice

Today is the fall equinox and first day of Autumn, when the sun rises directly in the east and sets directly in the west. (Don Ryan/AP)

Fall starts at 9 p.m. ET Thursday, a day officially known as the autumn equinox.

Both equinoxes and solstices only happen twice a year — the first days of fall and spring are equinoxes, while the first days of summer and winter are solstices.