In the northern hemisphere, today is the Winter Solstice, the shortest day of the year.
The Solstice normally falls on either the 21st or the 22nd, the date changing based on the exact position of the north pole in relation to the sun. This is the same reason why we have leap years – our calendar year doesn’t match up with the solar year, and so we have to add a day on every four years in order to recalibrate our calendars with our position in space. This year, 2015, the point at which the north pole is furthest from the sun falls on the 22nd December.
Interestingly, though today is the shortest day of the year, it is not the day with the latest sunrise or the earliest sunset. These fall on the 30th December and the 12th December respectively.
The changing day length is one of the ways in which animals and plants can tell what time of year it is without being able to ‘tell’ the time using a clock. Plants know what time of year to flower or animals know what time of year to start breeding. This process is called photoperiodism, which basically means measuring the total amount of light in a day. In somewhere like London, the day length changes throughout the year, so is a meaningful signal for animals and plants to tell the time of year. At the equator, it is less variable and so is less meaningful. Animals and plants here could use other signals instead, like the onset of the rainy season or the variation in temperature.
There is significant amount of evidence that photoperiodism is regulated by a circannual clock, in the same way that the 24 hour day is timed inside a cell by the circadian clock. Researchers in Strasbourg, France and Tromsø, Norway, propose that this circannual clock is located in the brain, in a region called the pars tuberalis.
So, enjoy the short day! It’s your signal for nesting, relaxing, and enjoying the Christmas holidays.