A few days ago Michelle asked what time sunrise is here in Helsinki at the moment. Easy to answer – it’s getting light at about 7 am and getting dark around 8 pm. We’re now on daylight savings time (i.e. GMT + 3) and therefore an hour ahead of South Africa. Because we’ve just passed the March equinox, the days are now lengthening very rapidly – nearly six minutes a day (check out this website for more).
But her question got me wondering about how long our summer days will be? And – maybe more importantly – how short will mid-winter days be? Wikipedia suggests that there are only 29 hours of sunlight each December – that didn’t sound good… We’re far north, but surely not that far? Helsinki is at 60 °N – so you’ve got to travels about 700 km further north to reach the Arctic circle (beyond which the sun does not rise in mid-winter).
Well, last year I created some R code to calculate the sunrise and sunset times for a variety of localities (based on formulas from a NASA website dealing with these sorts of calculations) where temperature data had been logged over long periods (we wanted to see what the average day and night temperatures were). I subsequently modified that code to help someone calculate how day length changes throughout the year on Marion Island – and I’ve now quickly run that code for Cape Town and Helsinki. And this is what it looks like: