Today marks the day when the Sun is above the horizon for exactly 12 hours of the day everywhere on the planet. This astronomical event happens twice a year and is called the equinox. The name equinox comes from the Latin aequus(equal) and nox (night).
For millennia, we have been observing this day in the calendar, and marking the path of the Sun and the tilt in the earth’s rotation around the Sun.
This is also the day when the sun rises exactly in the east and sets in the west, so you can set your sun dial using the sun today. No matter where you are on Earth, you have a due east and due west point on your horizon. That point marks the intersection of your horizon with the celestial equator – the imaginary line above the true equator of the Earth.
According to the astronomical definitions of the seasons, this day marks the beginning of autumn in the northern hemisphere and of spring in the southern hemisphere, so get out and enjoy it!
