We’ve reached that time of year again — people meet and say, often with a touch of anxiety, ‘I think there’s a bit of a stretch in the evenings’.
For people like us who live at high latitudes this has always been a subject of great importance. After all, our ancestors went about the monumental task of building Newgrange so that they could know that winter had reached its climax and from now on there would be a stretch in the evenings.
We are a bit under four weeks from the winter solstice, which occurred just before Christmas. According to data I got from the internet, sunrise today in Dublin was at 08.35 and sunset will be at 16.32, giving a day length of 7 hours and 57 minutes. This day next week sunrise will be at 08.28 and sunset at 16.43, giving a day length of 8 hours and 15 minutes. This is an increase of 18 minutes in one week — a considerable stretch in the evenings.
Unfortunately calculating day length is rather complicated because the increase is not uniform as spring progresses. Fortunately other people have made the astronomical calculations and done the maths.
The effect of increasing day length on plants and animals is also a rather complicated subject which biologists call photoperiodism. The first studies into plants naturally assumed that the factor controlling things like leaves breaking bud or flowers opening was the length of the day. It was subsequently discovered that it was the length of the night, not the day, that was crucial. But despite this plants are still classified as long-day plants and short-day plants. The long-day plants are further divided into obligate photoperiodic plants, which absolutely require a long day (actually a short night) before they’ll flower and facultative photoperiodic plants which are more likely to flower under the appropriate light conditions but will eventually do so regardless of night length.
Examples of long-day obligate plants are carnations, henbane and oats. Examples of long-day facultative plants are peas, barley and lettuce. Short-day plants cannot flower after short nights or if a pulse of light is shone on them for several minutes during the course of the night. In Ireland most of these are autumn flowering plants. Some important commercial crops, such as rice, cotton and hemp, are short-day plants. Some plants are day-neutral and flowering is controlled by factors such as temperature or overall maturity. They include cucumbers, roses and tomatoes.
Birds and animals are also affected. It controls things like migration, mating and the growth of fur and feathers. The extent to which human beings are affected is the subject of argument among scientists but my suspicion is that it’s more important than most of them think.
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