Quote of the Day
The road to wisdom? Well, it's plain and simple to express. Err and err and err again, but less and less and less.
— Piet Hein
I have lived most of my life in Minnesota, so you would think that I would be used to cold weather by now. The key to having a pleasant winter is dressing properly. The one thing that I still struggle with is the short duration of our daylight in winter. Because of our limited daylight, I need to know the time of sunrise and sunset to plan my outdoor activities. So every morning I listen to the radio when they list our local sunrise and sunset times.
I recently noticed that the date of our earliest sunset was coming six days before the winter solstice. I then checked the date of our latest sunrise and that date was twelve days after the winter solstice. I know that the winter solstice has the shortest daylight duration of all the days of the year, but it has neither the latest sunrise nor the earliest sunset. This seemed odd to me. Let's investigate these observations.
I will be focusing my discussion on Minneapolis, which is the largest city near my home. I obtained my sunrise, sunset, and daylight duration times for Minneapolis from this web site.
I will not derive the formulas used to compute sunrise and sunset times as these formulas are well document elsewhere (e.g. formula for both). For this post, I chose to obtain my data from a web page (I am getting lazy in my old age).
The terms sunrise and sunset are defined in terms of the Sun's position relative to the horizon. The fact that things start getting dark as the Sun rises and sets is associated with atmospheric light scattering. We use terms like dusk and twilight. to describe the light level present. Figure 1 illustrates the Sun's position relative to the horizon for the terms sunrise, sunset, dusk, dawn, and twilight (Source).
My analysis will be focusing on sunrise and sunset time.
I just grabbed the sunrise, sunset, and daylight duration numbers for Minneapolis from this web page and plotted them in Excel.
Figure 2 confirms that the winter solstice (Dec 21 in 2013) has the shortest daylight duration.
Sunrise and Sunset Times
Figure 3 shows the sunrise and sunset times around the time of the winter solstice. Notice how the earliest sunset occurs six days before the winter solstice, and the latest sunrise occurs twelve days after the winter solstice.
While the time of earliest sunset occurs six days before the winter solstice, our days continue to get shorter because the sunrise time is getting later faster than the sunset time is getting earlier. After the winter solstice, the duration of daylight begins to increase sunset time is getting later faster than the sunrise time is getting earlier.
Here is what I accomplished with this exercise:
- I confirmed that the day of shortest daylight duration is the winter solstice.
The interesting thing about this fact is that the winter solstice has neither the latest sunrise nor the earliest sunset.
- I see that the date of earliest sunset is six days before the winter solstice.
Sunset times are later for the six days after Dec 15th. You would think that later sunsets means more total daylight, but sunrise is coming so late as to more than compensate for the later sunset.
- I see that the date of latest sunrise occurs twelve days after the winter solstice.
At this point, we now start seeing the duration of daylight begin to increase more rapidly.