It was the winter solstice in New Zealand this week, and from now on we’ll be getting longer days and lighter mornings.
Except, for a couple of weeks, we won’t. It’ll get darker, later, for a while. So what gives? I mean, the solstice is meant to be the moment when it all turns around… isn’t it? Well, yes – and no.
The issue is to do with the length of the solar day, also known as the synodic day. This is what counts for sunrise and sunset, and it’s defined by the time it takes for the Sun to return to a given spot in the sky. This is determined not just by Earth’s rotation on its axis, but also by the fact that Earth is also moving a specific distance around the curve of its orbit around the Sun. This means that the Earth has to rotate a shade more – specifically, one degree extra – in order to complete a solar day.
To give that numbers, a day measured purely by Earth’s rotation – its ‘sidereal’ day – is 23 hours and 56 minutes long. However, the solar day is anything from 23 hours 59 minutes 30 seconds at perehelion (December) to 24 hours and 30 seconds at apehelion (June). Why the variation? It’s because the solar day is determined by Earth’s orbital velocity – and that varies, because our orbit around the Sun isn’t a perfect circle.
So far so good. However, our clocks – for convenience – are all geared to measure the day as being precisely 24 hours. The net result is that over a year the solar day runs a bit out of synch with the clocks. Specifically, it means that – as we approach the winter solstice – the day of earliest sunset is actually a couple of weeks before the solstice, and the day of the latest sunrise a couple of weeks after. So, for a couple of weeks after the ‘shortest day’, it actually gets darker later in the morning. Yes, the days are lengthening – it stays lighter for longer in the evening, but it’s that getting up in the morning for work thing that gets the later sunrises noticed.
There are some minor complications. All this happens slightly slower in the southern hemisphere because just now the southern winter solstice happens at apehelion. There are also issues associated with our axial tilt and where you happen to be on the planet. But instead of going on about those, let me point you to my book on astrophysics which doesn’t mention any of that stuff, but does have a lot of interesting gen about science anyway. Go on – you know you want to. Click to buy.
Copyright © Matthew Wright 2017