Winter is Here

The winter solstice is today, when the northern hemisphere marks its shortest day length and longest night length. (It’s the summer solstice in the southern hemisphere.) Of course, the Earth doesn’t actually tilt on its axis, it’s just reached that part of its annual journey around the sun where the northern hemisphere is tilted farthest from the sun. Here’s a photo we’ve all seen going back to elementary school, from the National Weather Service:

We all know these basic facts about our existence on the planet, but rarely think about how we evolved here and so are deeply tuned to these cycles of life.

1. The Earth takes ~365 days to go around the sun once. That’s our year. In reality, it’s a bit longer than that.
2. The moon, our loyal satellite, has an orbit about the Earth that takes approximately ~30 days, which is a lunar month. (It turns out that the moon makes this journey 13 times over the course of one Earth orbit around the sun.)
3. The Earth spins on its axis and one complete spin is a day, pretty much 24 hours. Not exactly, but close.

Ancient astronomers took careful measurements of the sun, moon and star positions to figure out these celestial patterns, and then societies simplified them so that our activities could be coordinated. The modern agreed-upon time scales are not scientific, but were decreed by the pope (Pope Gregory the XIII) in 1582 as the Gregorian calendar. To make it all work out with the actual astronomy of it all, we have to have leap years every four years, some months are 30 days, some 31 days and one is 28 or 29 days, and a day is 24 hours, where an hour is 60 minutes, a minute is 60 seconds. The day length was basically empirically determined by the Babylonians dividing a full “day” into those increments and giving them names. Even the 12 hours of day (AM) and 12 hours of night (PM) were determined on an equinox. These civilizations basically just observed stars and the sun and made up a calendar that most folks in the world have agreed on (Saudi Arabia approved the calendar for official business use in 2023!). We also have crazy situations in the US where we “move” our clocks forward and back a couple times a year.

I didn’t really want to go into all the finer points of our calendar year. Basically that humans created it to help us organize our activities. For example, global trade and corporations work best if there’s some agreed upon algorithm for time so we can make our meetings and collaborate.

The reason I bring up the various calendar categories is that, as biological creatures on this planet, we have evolved rhythms that tune us to the astronomical cycles.

Daily rhythms are called circadian rhythms. All living organisms respond to this ~24 hr cycle. We have proteins that respond to sunlight, which helps tune us to day and night. By us — humans, fruit flies, plants, algae. All of us. I’ll devote a newsletter in the new year to some of the biology of our circadian rhythms.

We also have monthly (ish) cycles, called circalunar rhythms– most well-known is the menstrual cycle of human females. This is not tuned by the moon (as far as we know), but there’s some interesting biology I’ll show you in an upcoming newsletter. Many intertidal species time their spawning to either the new moon or the full moon.

Then, there are circannual rhythms. These are annual cycles tied to aspects of day length and/or temperature. I’ve written about some of these in past newsletters. Events like the annual migrations of many birds and monarch butterflies, and hibernation. Things like flowering or leaf emergence in many plants and trees. While there are intriguing differences depending on whether it’s at the equator (where the day length and temperature vary much much less), even tropical organisms experience circannual rhythms. I’ll write some more about these in an upcoming newsletter.

Clearly TIME is an important feature regulating life on Earth. And, of course, how we Earth inhabitants respond to other events and signals like chemicals, will be different at different points in these cycles. Earlier newsletters featured aspects of time like developmental stage in a lifespan. Next year, we’ll look more at these other aspects of time- our rhythms and cycles.

As a college professor, I experience another type of rhythm- that of the academic year. It’s finals time here at Vassar and we’re rapidly approaching a crucial time of year. Winter break! So, I will be taking a short hiatus until sometime in January.

I hope you’ve enjoyed reading my newsletter this year. I’d love any comments you might want to throw my way and I’ll reconnect soon. And I hope you’ll consider sharing it with others you think might enjoy it.

This newsletter is devoted to explaining how biology works and how it is relevant to our daily lives. Most of us stopped learning about biology in high school or even middle school. And the way we learned it was as isolated concepts and vocabulary to memorize. I hope that this newsletter helps you rekindle that love of biology and might even help with better understanding of some of the important biology all around us. Please share this with anyone you think might want to take a look.

Thanks for reading!