There are two times each year when day and night are almost exactly 12 hours long: one in March and one in September – both called equinoxes. Because of the Earth’s irregular orbit around the Sun, these dates vary from year to year, with the first occurring between March 20 and 21, and the second between September 21 and 23.
The September equinox marks the first day of autumn in the Northern Hemisphere and the first day of spring in the Southern Hemisphere, which this year will be on Sunday (22). According to the website InTheSky.org, the word equinox is derived from the Latin words aequus (equal) and nox (night).
Equinoxes occur because Earth’s axis of rotation is tilted at an angle of 23.5° to the plane of its orbit around the Sun. The direction of Earth’s axis of rotation remains fixed in space as it transits around the Sun, while the line of sight from the planet to the Sun moves through the constellations.
On the day of the spring equinox (and the autumnal equinox too), day and night are almost exactly 12 hours long. Credit: KajaNi – Shutterstock
As a result, sometimes the north pole of the globe is tilted towards the Sun (in June), and sometimes it is tilted in the opposite direction (in December) – the so-called solstices. This gives rise to the planet’s seasons.
At the intermediate points between the solstices (March and September), the Sun is directly over the Earth’s equator. In March, the Sun is traveling north across the equator, and in September, it is traveling south.
Each year, spring begins on a different day, between September 21 and 23 – a variation that also occurs in other seasons. Credit: LedyX – Shutterstock
Why the dates of the equinoxes are not fixed
The Earth orbits the Sun once every 365 days, 5 hours and 48.77 minutes, and this is the period during which the cycle of solstices and equinoxes, and consequently the beginning of the Earth’s seasons, repeat from one year to the next.
In any non-leap year, the equinoxes occur about 5 hours and 48 minutes later from one year to the next. Thus, the seasons would creep ever further forward if it weren’t for an additional day inserted every four years—February 29—that “resets” the cycle.
At the September equinox, the Sun has a right ascension of almost exactly 12 hours. This is because the zero point of right ascension is defined by the central position of the Sun at the time of the March equinox. In September, it is almost exactly opposite this point in the sky.