AP Graphic

The sun will travel directly above Earth’s equator today at approximately 2:18 p.m. PDT, bringing with it the autumnal equinox and precisely 12 hours each of daylight and darkness.

Only twice a year, with the arrival of spring and fall, does the sun intersect celestially to create equal parts day and night, according to Jim Todd, an astronomer at Oregon Museum of Science and Industry.

The equinoxes are moments in time rather than whole days. Generally, they occur around the 22nd or 23rd of September and March each year.

The autumnal equinox has been observed by festival or celebration for more than 4,000 years, according to the Library of Congress.

Persians and Iranians commemorate it as the first day of Libra, while Zoroastrians held a festival of sharing and love.

Pagans of the latter-day Roman Empire brought modern Western civilization the traditional harvest festival to celebrate summer’s farming bounty and held on the weekend of September’s full moon.

A celebration of the autumnal equinox is held at Rooster Rock State Park each year.