Derek – Friday 11th December 2020

Plants are dependent on day length. It determines the emergence and growth of flowers and leaves. To be expected when you think about it, as plants get their energy for growth, that is for making new cells, from sunlight. As well as energy, the sugar produced is combined with nitrates to make protein, vital components of cells.

These short winter days with a low sun give less light than we realise, as our eyes accommodate to low light levels. When day length gets below 10 hours, many plants stop growing, as we witness in winter dormancy.

The winter solstice on 21 December is the shortest day of the year. The Sun will rise at 8.03 am and set at 3.53 pm, giving us a day of just 7 hours 50 minutes. Contrast this with the longest day on June 21 in 2021, the summer solstice, when the sun will rise at 4.43 am and set at 9.21 pm, a day of 16 hours 38 minutes. The difference between the shortest and longest day is 8 hours 48 minutes.

The reason for the changes in day length is the Earth’s angle to the Sun, 23.4 degrees, which is also responsible for the seasons. The Earth leans away from the sun at the winter solstice and towards it at the summer solstice, and intermediary positions between these extremes give us the variation in day length and the seasons.

The time difference over the 6 months between winter and summer solstice equates to roughly 3 minutes a day. But it isn’t regular, day to day, due to the difference between our 24 hour day and the solar day. The latter is the time between the sun at its highest, the zenith, on one day and the next. It varies by up to 15 minutes on either side of 24 hours. The reason for the variation is that the Earth’s orbit round the sun is an ellipse and not a circle.

An added difficulty is pinpointing sunset and sunrise. Sunrise is when the arc of the sun is just visible on the horizon. There is, though, a refraction effect which complicates matters, in the way a ruler seems to bend in a bowl of water. You actually see the sun a few minutes before it has risen because of refraction. Sunset is when the last arc of the sun is about to disappear and refraction affects this too.

Following the shortest day on 21 December, the days get longer. On 31 January, the day will be 9 hours and 8 minutes long. And on 28 February it will be 10 hours and 41 minutes, as we head towards the equinox on March 21, when day and night will be equal.

One hardly notices the lengthening days until about mid January, but by mid February day length is over 10 hours, and plant dormancy turns off. And, as if to prove it, there will be snowdrops and crocuses, a sure sign that spring is on the way.

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