The equinox arrives on September 23. That’s when the equinox sun is exactly above Earth’s equator, moving from north to south. 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.
Meanwhile, south of the equator, spring begins.
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 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). Bottom line: The equinox – a seasonal signpost in Earth’s orbit around the sun – will arrive on September 23. The sun will be exactly above Earth’s equator then, moving from north to south.
Irene W. Pennington Planetarium 