Today marks the Autumnal equinox and the first day of fall. At 14:49 Universal time, the earth will mark the point in its journey around the sun at which day and night are equal. The word equinox comes from Medieval Latin equinoxium meaning equality of night (and day) from the Latin word aequinoctium meaning the equinoxes from aequus meaning equal and nox (genetive singular form noctis) meaning night. Many European languages that share common Proto-Indo European roots share this form-the word to express the equality of night and day but only refers to night. A few (the Russian равноденствие for example) express it as equal-day.
Image of the earth at equinox courtesy dna-webmaster under a Creative Commons 3.0 license.