On Longest Days

The Summer Solstice for 2020 just passed last Saturday (June 20, 2020). For those of us in the Northern Hemisphere, the longest day of the year*. And now for those of us in the North, the days get shorter until late December. The hottest days of the year here in the North are yet to arrive. For those of us who work outdoors a good bit, longest days might not coincide directly with the longest time in the sun. For me anyway, working an eight-hour shift outdoors at 98 degrees Fahrenheit feels much longer than the same number of outdoor hours at 80F. So even as the days of July get progressively shorter by a daylight measure, they do seem so much longer.

The longest day of the year is more than a mere mark on our calendars (indeed, as a mark it is pretty fluid… moving about from year to year – more on that below). The longest day is not some time hack of concern to we humans. Much of the planet’s plant life carefully measures the daylight length (or the length of the night). Measuring the time in the light helps daylength sensitive plants determine an appropriate time to switch from a juvenile to a reproductive phase of development. The phenomenon of tracking daylength is called photoperiodism. Soybean is a classic example of such a sensitive plant. [so classic that it was one of the first plants in which the phenomenon was recognized] See here: https://garden.org/courseweb/course1/week4/page14.htm

Quoting from the site:

The phenomenon of photoperiodism is a fairly recent discovery. Scientists first linked the onset of flowering with day length in the 1920s, during experiments with soybeans and tobacco. During one experiment, plots of soybeans were planted at two-week intervals throughout the spring and early summer. Surprisingly, all the plants flowered at approximately the same time, no matter what their age. Based on this result, scientists postulated that an environmental factor was triggering the flowering.

The time shifting some of us do with daylight savings time is so completely artificial that I wanted to skip over it completely. But as a human artifact like leap days and leap seconds, jumping hours back and forth during the same year does confound “real-time” scheduling of communications with others around the world. With a few colleagues I was recently on a call with folks in Israel, Japan, and Ohio at the same time. Thanks to computers (and a colleague willing to set up the call) all I had to do was show up (on time). We did talk about soybean, but photoperiodism didn’t come up.

We have to do so many artificial things to make the world and the universe fit into our ways of thinking and living. Above I mentioned how the summer’s solstice slides about along our calendar. Most of the change between years is due to “leap correction” – accounting for leap year effects. But because we also have leap seconds, there is another correction every 400 years. Just had one of those twenty years ago. I must confess – I missed that one.   The following figure is from this site:  https://www.calendarpedia.com/when-is/summer-solstice.html

summer-solstice-leap-shifting

June date on the Y, calendar year on the X

There one sees day of June on the Y axis, and year on the X (ordinate and abscissa axes, respectively). Because 2000 is divisible by 400, it still had a leap day. 2100 will not have a leap day. 1900 didn’t have a leap day either. And yes, I missed that one. [For those so inclined, bonus points for the first commenter to indicate which centurial year was the first for which we humans, having adopted a leap day correction, decided to skip a leap day.]

What a day.

* The longest day, for those paying attention, is a misguided use of the language… what is meant is the longest stretch of daylight hours. But one would also be mistaken to presume that all days are exactly 24 hours in length. Just as we have a leap year in order to get our calendars back in sync, we have leap seconds to deal with every few years. We could have a leap second at the end of the month, but those in charge of such things have put it off. See here: https://endruntechnologies.com/support/leap-seconds

Quoting from the site:

            The next possible leap second event is June 30, 2020.
            There will NOT be a leap second introduced into UTC on that date.
            December 31, 2016 was the most recent leap second event.

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