Thought Leadership

OFF-TOPIC: The change of day length

By Colin Walls

On the worst of Winter days – when it is gray and damp – I long for Summer. I fantasize about warmth coming from that strange light in the sky. I do not think that I am alone in these thoughts. I have pondered what it might be like to live further South, where Summer is longer; I am certainly attracted to that idea. What about further South still, where Summer is perpetual and there are no seasons? I am not so sure.

Maybe it is because I have never known anything different, but I feel that there is something right about the progress of the seasons. I think that everyone has a favorite season. I think that mine is Spring. Perhaps because my birthday is in April [feel free to mark your calendars for the 3rd], but I think mainly because it is a portent of Summer. Having said this, a bright, crisp Winter’s day is wonderful and who cannot wonder at the glorious colors of the Fall?

There is something intriguing about the solstices and equinoxes:

  • Vernal equinox (about 21 March): day and night of equal length, marking the start of spring
  • Summer solstice (20 or 21 June): longest day of the year, marking the start of summer
  • Autumnal equinox (about 23 September): day and night of equal length, marking the start of autumn
  • Winter solstice (21 or 22 December): shortest day of the year, marking the start of winter

Clearly they have fascinated people for a long time. For example, there are many stone circles that are thousands of years old and accurately track these “magic” dates. I recall going to Stonehenge for the Summer solstice in 1979 [I was a student – it seemed like a good idea …].

The shortest day, which is fortunately just before Christmas, always seems particularly grim to me. I do not mind the dark mornings, but the way that dusk seems to fall before the day has really got started gets to me. But I always tell myself that all is well, as the days will now get steadily longer. But that is not strictly true …

Although the days do start getting longer after the Winter solstice, it happens quite slowly at first. Then it picks up speed up to the Vernal equinox, when the rate of lengthening is at its fastest. Then it tales off until the Summer solstice when day lengths are changing slowly again. A plot of day length over time is actually a sine wave.

The geometry that leads to this result is not too complex, but too much to describe here. Suffice it to say that the reason for the curve being this shape is exactly the same as the waveform of mains electricity, which comes out of a rotating generator at 50 or 60 cycles per second [Hz].

Although most people say that they prefer the longer days, it is odd that we do not exploit them. Why, in countries that even at Midsummer have at least 6 hours of darkness, do so many people still go to bed well after sunset and get up way after sunrise. Our ancestors knew better. Of course, the totally ridiculous idea of Daylight Savings Time, which I have written about before, is a foolish attempt to accommodate this. However, it really is pointless.

All I can say just now is: roll on the Vernal equinox and the rapidly lengthening days …

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This article first appeared on the Siemens Digital Industries Software blog at