The shortest day marks the point when the days start to get longer and the nights shorter, and has profound cultural meaning around the world and throughout history.
The shortest day, winter solstice and midwinter are the colloquial terms used to describe the 24 hours around an annual astronomical event which usually occurs on December 21 to 22 each year.
In the Southern Hemisphere this is the Northern solstice, the time at which the Sun is at its northernmost point in the sky, which usually occurs on June 20 to 21 each year, and marks the point when the days are longest and nights shortest.
Worldwide, interpretation of the event has varied from culture to culture, but most northern hemisphere cultures have held a recognition of rebirth, involving holidays, festivals, gatherings, rituals or other celebrations around that time.
UT date and time of equinoxes and solstices on the earth
Why is it called equinox?
On the equinox, night and day are nearly exactly the same length – 12 hours – all over the world. This is the reason it’s called an “equinox”, derived from Latin, meaning “equal night”.
However, even if this is widely accepted, it isn’t entirely true. In reality equinoxes don’t have exactly 12 hours of daylight
The September equinox occurs the moment the sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from north to south.
This happens either on September 22, 23, or 24 every year. On any other day of the year, the Earth’s axis tilts a little away from or towards the Sun. But on the two equinoxes, the Earth’s axis tilts neither away from nor towards the Sun, like the illustration shows.