"Here comes the sun, doo doo doo doo
Here comes the sun
And I say, "It's all right"
Sun, sun, sun, here it comes
Sun, sun, sun, here it comes
Sun, sun, sun, here it comes. . . ."
The Beatles, "Here Comes the Sun," Abbey Road, 1969
Today's post is not about politics.
If you are noticing that it has been getting dark early for a long time, it is not your imagination. This is really happening. Early gloom isn't a literary device, nor a metaphor for the era of Trump, nor a commentary on our democracy. It is the reality of a tilted Earth.
Today's post is about the length of daylight for Medford, Oregon and for people who live near the 42nd parallel. Go to this site and enter your own location: Click.
First of all, let's ignore daylight savings time. That is the sharp break in time in March and November on this sun graph.
We all recognize that sunsets come early now: 4:42 p.m. on December 21, at the winter solstice. At the summer solstice sunsets occur at 7:51 p.m. standard time or 8:51 p.m. daylight time. It doesn't get dark for another half hour in the summer because of the long twilights, which exaggerates our perception of late nightfall. Twilights are short in the winter, which exaggerates our perception of early darkness.
Days at the winter solstice are nine hours and five minutes long. Days at the summer solstice are 15 hours and 17 minutes long, a difference of six hours and 12 minutes -- 372 minutes -- over the 183 days between the two dates. That is two minutes and three seconds per day, on average. Sunsets come earlier or later by exactly half that amount.
Day lengths don't change on average. There are big changes in the length of daylight around the equinoxes and then, at either solstice, the changes are tiny. By November 15 the sun is setting at 4:48 p.m. December 8 has the year's earliest sunset: 4:38. On New Year's Day it sets at 4:48. There is a six week period of early darkness and a change of only a few seconds a day. We aren't just imagining this. It is real.
The six hour difference in day length between summer and winter takes place in the middle months. By January 5 daylight begins lengthening by a minute a day and by January's end it is two minutes a day. In March days lengthen by almost three minutes daily. That pace of lengthening daylight slows only a little until May, when we re-enter the stable six-week period of near-constant late sunsets around the summer solstice.
I think the websites charting daylight hours are fascinating. The different lengths of day through the seasons aren't intuitive so it is fun to look at data. Google your location and a keyword like "sunset" or "length of day." Some religions want to know an exact time of sunsets to be observant, so there are lots of places to go.
Good news for people who like the holiday season but dislike the dark: Sunsets are already getting later. Because Earth's orbit is elliptical, the earliest sunset isn't on the shortest day.
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I wrote this app for the Palm Pilot along time ago. It shows areas of day and night on a map of the world. The shape of the “shadow“ changes with the seasons, and expresses how the length of the day changes as the year progresses.
ReplyDeleteThe link takes you to a Palm Pilot emulator that will run the app for you on the webpage in your browser.
I did this app because I saw a Geochron Clock in a bank and wanted one, but could not afford the $1,700 it would have cost. So instead, I made one for myself.
I always marked Jan 26th as the end of real winter because the sun came up in Barrow Alaska. In Anchorage 4 hours 29 minutes was the shortest time of sun being up and it remained low on the horizon. Glad I live in Washington where it is less extreme.
ReplyDelete