The 2012 September equinox comes on September 22, at 9:49 a.m. CDT (14:49 Universal Time).
What is an equinox?
The earliest humans spent more time outside than we do. They used the sky as both clock and calendar. They could easily see that the sun’s path across the sky, the length of daylight, and the location of the sunrise and sunset all shift in a regular way throughout the year.
Our ancestors built the first observatories to track the sun’s progress. One example is at Machu Picchu in Peru, where the Intihuatana stone, shown at right, has been shown to be a precise indicator of the date of the two equinoxes and other significant celestial periods. The word Intihuatana, by the way, literally means for tying the sun.
Today, we know each equinox and solstice is an astronomical event, caused by Earth’s tilt on its axis and ceaseless orbit around the sun.
Because Earth doesn’t orbit upright, but is instead tilted on its axis by 23-and-a-half degrees, Earth’s Northern and Southern Hemispheres trade places in receiving the sun’s light and warmth most directly. We have an equinox twice a year – spring and fall – when the tilt of the Earth’s axis and Earth’s orbit around the sun combine in such a way that the axis is inclined neither away from nor toward the sun.
Earth’s two hemispheres are receiving the sun’s rays equally now. Night and day are approximately equal in length. The name ‘equinox’ comes from the Latin aequus (equal) and nox (night).
So enjoy the 2012 equinox – September 22 or 23 depending on your time zone – a seasonal signpost in Earth’s orbit around the sun!
- EarthSky
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