If you live in the Northern Hemisphere, today is the shortest day of the year. Here, in Minneapolis, sunrise was at 7:48 am and the sun, obscured by the cloud of winter, is setting as I write this at 4:34 pm. While it is common practice to mark the season with Christmas, Hanukkah, Kwanza or the New Year, the Winter Solstice delineates the astronomical transition into next year.
If you were a red-necked wallaby, patterns of gestation and oestrus are linked to this annual cycling between long and short days (Tyndale-Biscoe and Renfree, 1987); such responsiveness to daylength is known as photoperiodism. Many biological behaviors can be partially or entirely attributed to photoperiodic changes. When accompanied with shifts in temperature, photoperiodism explains annual biological phenomena like molting, migration, hibernation, reproduction and the development of sexual organs. While not directly related to vision, it should be no surprise to learn that the mammalian response to day length is linked to retinal light-sensitive cells ( http://en.wikipedia.org/wiki/Photoperiodism ).
While the human tendency towards lethargy in winter and promiscuity in summer is probably an evolutionary relic, the astronomical solstice holds deep cultural meaning that extends back to neolithic times. The layouts of archaeological sites like Stonehenge in England and Newgrange in Ireland are aligned with the winter solstice sunset for the former and summer solstice sunrise for the latter. Historical ruins suggest that the early months of winter were linked to periods of famine, and knowledge of this impending loss of daylight had real implications for survival. Punctuated by a final feast, the winter solstice was an opportunity to celebrate life and a good harvest prior to the onset of coldest months. So while everyone around me is busy shopping online and in stores in search of the perfect gift, I prefer to take this time to reflect on the year that was and the year that will be now that the days are getting longer.
Happy Winter Solstice from Minnesota.
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