Equinox

 Yes, I'm writing this on the March Equinox, that magical day when everywhere day and night are supposedly each 12 hours in length. It's not really so, though, it turns out. Depending on where you are on the planet it can vary considerably. Here in Perth, Western Australia, daylight lasts 12 hours and 9 minutes while further south in Hobart in Tasmania, it ist 2 minutes longer. That wasn't a surprise but when I looked at the figures for McMurdo Base in Antarctica I was surprised to see that they have 12 hours 40 minutes of daylight. It's probably more accurate to say that day and night at the Equinox are in the vicinity of 12 hours all over the planet. Oh, and as well that day is only when the sun is above the horizon and doesn't include other periods when there's still some light such as twilight.  

This, of course, led me into wanting to find out more about twilight. If, like me, you thought twilight was just that period of the evening which we call dusk you may be surprised to learn there are several periods of twilight. There's what we call dusk and dawn for starters but there are also technical terms for different times during dusk and dawn. There's morning Civil Twilight which begins at the instant when the Sun's geometric centre is 6 degrees below the horizon and ends at sunrise while evening Civil Twilight begins at sunset and ceases when the Sun's geometric centre is 6° below the horizon. This is that time when the sun is just below the horizon and there's still enough light to be out and about to work or play. Civil Dusk and Civil Dawn it turns out aren't time spans at all but the specific moments when Civil Twilight ends and Civil Dawn begins. Then there's Nautical twilight which precedes Civil Dawn and follows Civil Dusk. It begins when the Sun's geometric centre is 12° below the horizon in the morning and ends when it reaches the 12° point in the evening. It gets its name because both the horizon and brightest stars are visible enough to allow navigation at sea. Phew!

If that's not enough there's also Astronomical Twilight which is when the geometrical centre of the Sun is 12-18 degrees below the horizon and is the earliest part of dawn and the latest period of dusk. For most of us living in cities with their light pollution there's little difference between Astronomical Twilight and night. The sky is dark and many stars are visible. Astronomical Dawn and Dusk are the moments when the sun reaches that 18° position. Astronomers can make out more stars in this period but to view fainter and more distant ones they have to wait for night.

I found this all fascinating as you can see and if you want to find out more about such things I found the Time and Date website very informative. It has lots of other interesting topics. The National Weather Service website was also helpful.