The equinox is when the length of night and the day are almost equal. The September equinox is when the sun crosses the celestial equator, according to thetimenow.com.
The celestial equator is an imaginary circle that runs on the same plane as the earth's equator.
The official date for the first day of fall changes from year to year. In 2015, the first day of fall was September 23. Next year, the first day of autumn will take place on September 22 for the second year in a row.