When I was a youngster, I used to walk about a quarter mile to my bus stop every morning. Over the course of the autumn, there would be less and less light each day. By early November, I’d basically be walking by the light of houses’ windows and the odd headlight. Then daylight savings time would end, giving back an hour of sunlight to the morning, and it would be bright again. However, that procession of increased darkness wasn’t stopped, it was just delayed. Sure enough, by the time winter break rolled around, I was walking by the light of outdoor Christmas lights.
It’s easy enough to say that today was darker than a month ago when each day has gotten darker without interruption. However, I was never quite sure if the extra hour afforded by the switch back to standard time was gobbled up and exceeded by the next six weeks or if the end of daylight savings time was, ironically, the morning with the least day light.
How do you measure darkness?
Trying to do so directly would require actual physical equipment, would be highly sensitive to my exact location, and would be very noisy data-wise based on cloudy days and motion-activated flood lights.
What is easy to measure, however, is when the sun rises. This is governed by the dance of celestial bodies, not the Gibsons’ reindeer display, so I think it’s a lot more reliable, and generalizable to boot. My intuition tells me that thirty minutes before sunrise is darker than fifteen minutes before sunrise and fifteen minutes after sunrise is darker than thirty minutes after sunrise.
NOAA (the federal weatherboys) have a handy sunrise calculator that can tell us exactly what time the sun comes up on any day in history, so we’ll be using that. Whichever day has the latest sunrise, we will declare as having the darkest morning.
So which day is it?
Our two candidate dates are the morning of the winter solstice and the morning of the day before daylight savings time ends. The first because it is the day with the least total sunlight, the latter because it is the day closest to the winter solstice which is set up to sacrifice an hour of morning light for an hour of evening light. This year, those dates are November 6th and December 21st.
I plugged in roughly my hometown coordinates and found that, for me, the darkest morning of the year was the day before the end of daylight savings time*. It simply had a later sunrise than the solstice. Okey dokey, end of story, right?
Is it ever?
Generalizing the answer
I started playing around with the calculator, you know, finding out when the sun rose on the day I was born, when sunset would be on my fiftieth birthday, that kind of thing. Then I started plugging in other coordinates to see how the sun treated them.
The first thing I discovered when messing around with the calculator was that longitude didn’t have much of a seasonal effect. Since the Earth rotates on its axis at a constant speed, it makes sense to me that if Point A is twenty-fourth of the Earth due west of Point B, Point A would always have its sunrise one hour after Point B, time zones notwithstanding.
The second thing I noticed is that latitude had a very strong seasonal effect. When approaching either the North or South Pole, eventually you reach a point where you can go days or weeks or even months without sunlight, centered around the solstice. Then, it hit me: if at a certain latitude the sun doesn’t rise at all for a month around the solstice, then surely the day before daylight savings time ends would have a brighter morning by virtue of having any sunlight at all. The question was not fully answered.
This figure shows the amount of sunlight seen at midnight GMT throughout the year. Shadow on top is winter in the northern hemisphere, shadow on the bottom is summer in the northern hemisphere.
At the equator, the sun rises at the same time every day, but the farther you move from the equator, the more variable your sunrise times become. That means that there should be a point where the variance between the sunrise on November 6th and December 21st should be one hour. With daylight savings time aiding November 6th, this would mean that their sunrise would be at the exact same time.
I will note that daylight savings time moves around each year based on days of the week (and some countries change their clocks at different weeks altogether) and November 6th is the latest that day could fall in the US and Canada.
Sure enough, after a little poking around, I found the breakeven latitude: 51° North. For reference, this line runs straight through Calgary.
Below 51° North, the darkest morning of the year is the day before the end of daylight savings time, November 6th. Above 51° North, the darkest morning of the year is the winter solstice, December 21st.
What about the other brightness extremes?
First we’ll tackle the brightest and darkest evenings.
Without switching the clocks, the latest sunset would be the summer solstice. The way that daylight savings time works, it exacerbates the timing of sunsets, so the sunset on the summer solstice is an hour later than it would be otherwise. So the brightest evening of the year is the summer solstice, or June 21st.
The darkest evening is again aided by the changing of the clocks. The winter solstice would be the darkest in a vacuum, and since daylight savings helps part of the year to have brighter evening, then the winter solstice, December 21st, is the darkest evening of the year.
That leaves the brightest morning. I had never really given this one much thought as I guess sunrise was early enough from March onwards that I never really noticed the difference. But I can certainly see how someone who woke up earlier than I did might pick up on the brightness subtleties.
At first thought, I would’ve guessed it would just be the summer solstice, end of story. Plugging into the calculator, I can see that for my home coordinates that I was right. But again, we must look to the extremes. Near the equator, the sun rises at nearly the same time every day. With daylight savings time, that sunrise would occur an hour later, meaning that our brightest morning would occur not on the solstice, but the day before daylight savings time begins, this year March 13th.
Therefore, just as there was a latitudinal breakeven point for the darkest morning between the day before daylight savings time ends and the winter solstice, so too should there exist a breakeven point for the brightest morning between the day before daylight savings time begins and the summer solstice. With some fiddling, I found that latitude to be 25° North, which goes through Key Largo, FL.
Below 25° North, the brightest morning of the year is the day before the beginning of daylight savings time, March 13th. Above 25° North, the brightest morning of the year is the summer solstice, June 21st.
For the entirety of the mainland contiguous United States, the brightest morning is the summer solstice and the darkest morning is the day before daylight savings time ends.
So I guess all of those dark November mornings where I was cursing the school for making me go or my parents for having a son, I really should have been cursing the person who came up with the entire hokey clock changing idea in the first place, one Mr. Benjamin Franklin.