The Griffith Observatory will celebrate the autumnal equinox -- the official start of fall -- on Tuesday. (Fall officially begins at 2:19 Pacific time.)
At 12:46 p.m., the projection of the Sun’s image crosses the engraved meridian arc of the Gottlieb Transit Corridor; then, a 6:50 p.m., the Sun aligns with the engraved equinox line on the Observatory’s West Observation Terrace.
So what's the Autumnal Equinox? The museum explains:
The autumnal equinox occurs when day and night are of nearly equal length ("equinox" is Latin for "equal night"). This is true throughout most of the world at only two times a year.
The autumnal equinox is the moment when the Sun crosses the celestial equator (the projection of Earth’s equator into space) from north to south. From the autumnal equinox and until the start of winter – the winter solstice on December 21 – days will continue to get shorter, and the noon elevation of the Sun will decrease. This happens because the Earth's axis is tilted 23½ degrees to its orbit.
The Sun will rise and set on September 22 nearly due east and west, respectively. Around the world, ancient ruins include features oriented to the rising, setting, and passage of the Sun through the sky on solstices and equinoxes. Many ancient sites suggest that ritual was an important part of observing solstices and equinoxes. Griffith Observatory has several architectural features that align with celestial events.