Equinoxes and solstices, via NASA Earth Observatory.
According to EarthSky, the September equinox arrived on September 22, 2016 at 10:21 a.m. EDT. At the equinox, days and nights are approximately equal in length. For us in the Northern Hemisphere, the sun is rising later now, and nightfall comes sooner. We’re enjoying the cooler days of autumn. Meanwhile, south of the equator, spring is about to begin. Learn more about this equinox by following the links below:
What is an equinox? The earliest humans spent more time outside than we do. They used the sky as both a clock and a 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 above, has been shown to be a precise indicator of the date of the two equinoxes and other significant celestial periods. The wordIntihuatana, 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 throughout the year 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.
Around the time of an equinox, Earth’s Northern and Southern Hemispheres are receiving the sun’s rays about equally. Image via Wikipedia
Earth’s two hemispheres are receiving the sun’s rays about equally around equinox-time. The sun is overhead at noon as seen from the equator. Night and day are approximately equal in length.
The name equinox comes from the Latin aequus (equal) and nox (night).
Of course, Earth never stops moving around the sun. So these days of approximately equal sunlight and night will change quickly.
Read the rest of this at EarthSky.