PHOENIX -- It's official! Tuesday at 5:18 p.m. marks the beginning of fall.

The start of the season, called the autumnal equinox, happens when the sun is directly over the equator.

The result -- for just a short while at this time of year -- is that days and nights are approximately equally long.

From here, the sun starts to move south for the winter, creating longer nights and shorter days in the Northern Hemisphere (and the reverse in the Southern Hemisphere).

The term "equinox" is actually derived from the Latin aequus (equal) and nox (night).

For those of you who do not know what this event really is, you should know that an equinox happens twice a year. We have the vernal (spring) equinox and an autumnal (fall) equinox.

The equinox doesn’t last one whole day; the event only represents a single moment in the day when -- at the equator -- the sun’s center can be observed to be exactly vertically overhead.

The equinox occurs when the tilt of Earth’s own axis is inclined neither away from nor toward the sun.

There is no "winter" or "summer" at this moment -- there is simply the changing of seasons.

The equinox can happen on March 20 or 21 (for spring), and on Sept. 22 or 23 (for fall) each year.

A taste of autumn will actually arrive in northeast Ohio next week with lower temperatures.

Happy equinox day!