More darkness than daylight for the next six months
If you live in eastern Australia anywhere east of about Melbourne, you're probably feeling like you never had too much sunshine this summer. Well, now you're going to have to put up with less daylight too.
It's the equinox today, which is the day that happens twice a year when the centre of the sun is directly over the Earth's equator. It happens around March 20 and September 23, give or take a day depending on the calendar in any given year.
On the equinox, two interesting things happen:
1. The solar terminator is perpendicular between the poles. The term "solar terminator" might sound like a Schwarzenegger movie which is set on the sun, but it actually refers to the line between day and night, which you can see clearly in the image below (its source is here).
2. The other thing that happens around the March equinox in the southern hemisphere is that days start to become shorter than nights.
Now, you'd think that the shorter-day effect would kick in immediately, but it's actually not quite that simple. There's a slight lag until days become shorter than nights. For example:
- Today in Sydney, there will be 12 hrs and 4 mins of daylight. The first day with less than 12 hours (11:58) is Thursday.
- Today in Melbourne, there will be 12 hrs and 5 mins of daylight. The first day with less than 12 hours (11:57) is Thursday.
- Today in Hobart, there will be 12 hrs and 5 mins of daylight. The first day with less than 12 hours (11:59) is Wednesday.
- Today in Darwin, there will be 12 hrs and 5 mins of daylight. The first day with less than 12 hours (11:59) is not till next Monday.
When does the line between day and night become vertical? #Today, March 20 at 15.33 UTC, there will be an equinox on planet Earth https://t.co/tluLebqkQL [more: https://t.co/A9ssesAEFc] pic.twitter.com/irvrVnwuty
— Massimo (@Rainmaker1973) March 20, 2022
So why the lag?
In simple terms, it's because the sun is not a point in the sky, but an enormous celestial object with upper and lower edges.
Did you notice back in the second paragraph of this story where we deliberately referred to the centre of the sun being over the equator on the equinox?
Technically, sunrise begins when the upper edge of the sun appears on the horizon, and sunset ends when the upper edge of the sun disappears in the west. That adds a few minutes.
There's another factor at play too. The earth's atmosphere refracts, or bends, sunlight. That means the light sticks around for a tiny bit longer.
Anyway, the main thing you need to know is that it's all downhill from here in terms of daylight until June, and days will remain shorter than nights until late September. The good news is it might be a little easier to sleep in, if you're lucky enough to have the chance.