Spring Equinox 2020 in Southern Hemisphere will be at11:30 pm onTuesday, 22 September

Equinox happens when sun shines directly on an equator. There is an equinox if daytime hours equals to night time hours. Every year, there are two events of equinoxes—the March equinox and the September equinox. In the northern hemisphere, the September equinox is known as the autumnal or fall equinox. In the southern hemisphere like Australia, September equinox is known as the spring or vernal equinox.

September equinox in Australia usually happens on the 21st, 22nd, or 23rd of September. Though September equinox is a highly-regarded event in ancient times, this does not make it a public holiday in Australia.

“Equinox” literally means “equal night”, giving the impression that the night and day on the equinox are exactly the same length; 12 hours each. But this isn’t entirely accurate.

Even if the name suggests it and it is widely accepted, it is not entirely true that day and night are exactly equal on the equinox – only nearly.

The Earth spins around its own axis approximately every 24 hours (a sidereal day is 23 hours, 56 minutes, 4 seconds).

The time it takes Earth to orbit once around the Sun is around 365 days, 5 hours, 48 minutes and 45 seconds (365.242189 days). And, like two spinning tops connected at the tips, it also wobbles around on its axis, making a complete rotation every 26,000 years (axial precession).

This axial procession is why the Northern Pole Star changes every 26,000 years and why the Sun’s location in the signs of the Zodiac are different today than they were 4000 years ago. That however is a post for another day.

Earth is actually tilted at an angle of around 23.4 degrees toward the celestial pole, which is a certain point in the sky. As Earth makes its yearly orbit, one hemisphere faces the sun more than the other, the side that has summer.

On any other day of the year, the Earth’s axis also tilts a little away from or towards the Sun. However, the equinoxes marks the exact moment twice a year when the Earth’s axis is not tilted toward or away from the Sun at all. However, the axial tilt of around 23.4 degrees, remains the same.

It is your actual Latitude that determines the length of the day and can vary by as much as several weeks from place to place.

Even if day and night aren’t exactly equal on the day of the equinox, there are days when day and night are both very close to 12 hours.

On the equator, the day and night stay approximately the same length all year round, but the day will always appear a little longer than 12 hours, due to the reasons below.

On the equinoxes, the geometric centre of the Sun is above the horizon for 12 hours, and you might think that the length of the day (hours of daylight) would be 12 hours too.

However, ‘sunrise’ is defined as the moment the upper edge of the sun’s disk becomes visible above the horizon – not when the centre of the sun is visible. In the same sense, ‘sunset’ refers to the moment the Sun’s upper edge, not the centre, disappears below the horizon. The time it takes for the sun to fully rise and set, which is several minutes, is added to the day and subtracted from the night, and therefore the equinox day lasts a little longer than 12 hours.