Usually you come to The Nest for quirky art, nifty flashbacks, and your daily squirrel fix. But today, you just might learn something!
As you know, here in the Northern Hemisphere, the Winter Solstice is only a few weeks away, officially falling on December 21st this year. No doubt, you were taught in school that the first day of winter is also the shortest day of the year…. and conversely, the first day of summer is the longest day of the year. And by “shortest” and “longest”, of course I mean the length of time the sun is visible that day. Your pets know what I’m talking about…
OK, you haven’t learned anything you didn’t already know yet… but here’s something cool that I didn’t discover until a few years ago, and this is the best time of the year (again, in the Northern Hemisphere) to notice this strange phenomenon….
Question: What day of the year has the earliest sunset?
If you said December 21st, go to the back of the class with all the cool kids!
Surprisingly, the earliest sunset time will occur later on this week! If you don’t believe me, go dig up an almanac for your city with sunrise and sunset times.
Yet the days continue to get shorter because at this time of the year, the sunset time is changing very little from day to day. Meanwhile, the sunrise time is still chugging forward later and later in the morning… right up until the first week of the new year!
Yes, the latest sunrise actually occurs in early January! This part of the bizarro nature of sunrise and sunset times is the one I can most relate to since it is definitely the darkest when I leave work in the morning during the first few days of the year.
This phenomenon also takes place around the summer solstice in June as well, but it isn’t as pronounced as in December… and again, we are talking about north of the equator here. In the Southern Hemisphere, this happens in reverse… with earliest sunrise and latest sunset times a couple weeks removed from the summer solstice, with just a slight variation during the winter solstice.
OK, so why does this happen? Why don’t sunrise and sunset times correlate better with the solstices? Well, it’s because we don’t live at the poles!
If your school had one of those nifty old-style globes, you may have noticed a strange figure-8 design that was sitting out in the middle of the Pacific Ocean. It wasn’t some strange island formation, it actually had a name and a purpose! It was called an analemma.
The analemma is essentially a diagram that displays just how skewed our view of the sun is as the year goes by. You may have noticed the sun isn’t in the exact same position of the sky at the same time each day… if you plotted the sun’s position for the same time throughout the year, you would have an analemma! At the poles, the analemma one could “plot” in the sky would be perfectly vertical, like the one on the globe. But as you venture into the middle latitudes where most of us live, the analemma begins to tilt more horizontally…. until you hit the equator where it is literally laying on its side.
Let me steal a photo from the Wiki page for analemma, and demonstrate how this helps explain my fun fact of the day….
This is an illustration of where the sun’s position would be at the same time each day throughout the year in the middle latitudes of the Northern Hemisphere. Notice, it shows the tilted analemma design one would expect to find between the poles and the equator. Time moves counterclockwise from the top of the figure 8, and the time between each white dot is one week.
It is intuitive that the latest sunrise is going to come at the time of year when the analemma curve is at its lowest point in the sky… and that occurs about two weeks after the solstice. To balance things out so that the shortest day is still on the solstice itself, then the earliest sunset should occur at a point the same distance from the solstice on the opposite side. Following the same logic for summer, the earliest sunrise should occur at the point where the analemma curve is the highest in the sky…
And notice that because the summer end of the loop is smaller than the winter end… this is why the lag between sunrise and sunset extremes is most noticeable to us Northern Halfers around the winter solstice!
If you really want strange sunset and sunrise extremes, try moving around the equator! Because the analemma curve is now mostly horizontal, you actually get two times of the year where the sunset and sunrise times will reach a peak and a valley!
One last tidbit just to further stuff your mind with useless astronomical weirdness…. if you’re one of those old fashioned people who uses a sundial, you can use the analemma to figure out whether it’s running too fast or too slow…
As messed up as that sounds, sundials are not perfect tellers of time because the orbit and tilt of the earth vary the timing of the solar day throughout the year in a pattern with the analemma. Take another look at the picture of the globe analemma I posted above, and notice where it says “clock ahead of sun” on one side and “clock behind sun” on the other. A sundial is only going to be “correct” four times a year… around each solstice, in mid April, and in late August. During the Spring and Fall, a sundial’s time will be “fast” relative to our clocks, and in winter and summer, a sundial’s time will be “slow” relative to our normal methods of keeping time. Around Valentine’s Day and Halloween are when it’s most out of whack, so keep this in mid should you ever find yourself relying on a sundial for the time…
So when you’re heading home from work this evening, or just going out to grumble about the afternoon sunset, keep in mind that after this week you will be guaranteed more and more PM sun right on up until the end of June! Just make sure to keep putting those headlights on in the morning well into January….