It's the Southern Hemisphere's vernal (or spring) equinox today, which means the sun sat directly above the equator early this morning, giving roughly equal amounts of day and night in most parts of the world – but why does the date of the equinox differ by a day or so each year?

They don't change the date of New Year's Day or your birthday. But the vernal equinox day bumps around the calendar by a day or two. Why?

In simple terms, it's largely to do with the Gregorian Calendar, which is the official name for the calendar traditionally used in the Western world, and which is used in everyday life pretty much everywhere these days – with the exception of religious calendars in certain faiths other than Christianity.

As you know, the Gregorian calendar has 365 days each year, with an added day each four years on a leap year. That's because the earth actually takes about 365-and-a-quarter days to orbit the sun, so we take those four quarters and throw them into an extra day in years that are multiples of four.

So in real terms, each vernal equinox happens about six hours later than the previous year's. For example in Sydney:

• Last year's vernal equinox happened at 11:31 pm on September 22.
• This year's vernal equinox happened at 5:21 today, September 23.

So basically, leap years are the reasons that the equinox does not always fall on the same date.

Today marks the spring equinox in the Southern Hemisphere ☀�

The word 'equinox' comes from Latin and means 'equal night', referring to the roughly 12-hour day and 12-hour night. They occur twice a year—around late March and late September.

Learn more : https://t.co/hhtVws3wkx pic.twitter.com/dT8N1MZjLK

— Bureau of Meteorology, Australia (@BOM_au) September 23, 2021

Wait, what exactly is the equinox again? Here’s a little more information from Weatherzone meteorologist Ben Domensino

"On the day of the equinox, the solar terminator – which is the line between day and night – runs perpendicular to the Earth's equator. This causes day and night to be roughly equal in length around the world on the date of the equinox.

"After the Southern Hemisphere's spring equinox, the south pole gradually tilts towards the sun and causes days to slowly become longer than nights. This imbalance persists until the autumn equinox in late March, at which point nights will start to become longer than days once again."

Image: It's all about sunshine and dandelions on the vernal equinox. Source: AdinaVoicu via Pixabay.

And what about the seasons? Do they start on the equinox or not?

We've written about this before here at Weatherzone.

The main point is that in Australia, we start the seasons on the first of the month. The Bureau of Meteorology is responsible for this, and explains why here:

Says the Bureau:

"These definitions reflect the lag in heating and cooling as the sun appears to move southward and northward across the equator. They are also useful for compiling and presenting climate-based statistics on time scales such as months and seasons."

In the USA and numerous other countries, seasons start on the equinoxes and the solstices (solstices are the shortest and longest days).

So which one is more meteorologically accurate? For example, is it colder in Australia between June 1 and August 31, or between June 21 and September 21 (or whatever day close to the 21st the equinox and solstice falls upon)?

We actually crunched the data to answer that precise question in this story back in June, and the bottom line is there's virtually nothing in it.