PHOENIX -- Saturday morning at 7:49, fall with start in the Northern Hemisphere. And unseasonably hot weather will be on tap with highs around 106 in metro Phoenix.
As the new season begins, Earth’s axis will be tilted neither away from nor towards the sun, allowing the sun’s direct rays to shine overhead at the equator before moving into the Southern Hemisphere.
With the exception of the poles, all locations on Earth experience slightly over 12 hours of daylight and see the sun rise due east and set due west along the horizon. Until the December solstice, the sun continues to rise and set increasingly to our southeast and southwest, respectively.
According to EarthSky.org, 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).
Visit the website: www.earthsky.org for more info.