Did you know that interior Alaska actually receives more sun and twilight than any other place in the U.S.
Alaska is known for long summer days and long winter nights. However, if you average all 365 days together, everyone ends up with 12 hours of daylight and 12 hours of darkness throughout the course of the year no matter where in the world you are, right? Actually, that is not correct.
Looking at the chart that accompanies this post (see Figure 1), the red line at the bottom of the chart shows that daylight at the equator averages about 12 hours and 15 minutes per day over the course of the year. Remember that the sun is a circle (not a point) and we receive daylight from the top of the sun's disc before the middle of the disk reaches the horizon. The same is true at sunset. This makes an average day over 12 hours everywhere.
Figure 1. Hours of daylight and daylight plus Civil Twilight by latitude. All data obtained from the U.S. Naval Observatory's Astronomical Observations Department.
At the equator, the sun moves nearly straight up and down with respect to the horizon. This means the sun rises quickly and sets quickly. As we move to higher latitudes, the path of the sun is more oblique; i.e., the sun moves more and more diagonally with respect to the horizon. It therefore takes longer for the entire disk of the sun to make is across the horizon at both sunrise and sunset.
If the sun were a point and not a circle, the average length of a day would be 12 hours everywhere. Since the sun is a 2-dimensional circle from our perspective, the relative speed of the rising and setting dramatically changes the amount of light we receive. This effect is greatest at the Arctic and Antarctic Circles due to the effective ground speed of the sub-solar point near the solstices. The difference in cumulative day lengths between the equator and the Arctic Circle (for all 365 days) is 225 hours per year – or 37 minutes per day on average.
Figure 2 and 3 show the length of daylight (Figure 2) and the combined length of daylight and Civil Twilight on the summer solstice.
Figure 3. Combined length of daylight and Civil Twilight on the summer solstice.
If we include Civil Twilight, which is when the entire sun's disk is below the horizon but by no more than 6°, the extra light for Alaska dramatically increases. At 69°N latitude, the 365-day average for daylight plus Civil Twilight is 15 hours and 6 minutes. At the equator, the 365-day average is only 12 hours and 52 minutes. The average difference in light (daylight plus Civil Twilight) is a shocking 2 hours and 16 minutes per day. Places just north of the Brooks Range (e.g., Umiat) therefore receive the most usable light of any place in the U.S. Table 1 shows the cumulative length of daylight and Civil Twilight for Fairbanks, Barrow, Anchorage, and Juneau measured in hours.
The final two maps (Figure 4 and 5) show the length of daylight plus Civil Twilight for Alaska and the Lower 48. Again, note how much more light Alaska receives than the Lower 48 over the course of the year.
Figure 4. Average annual length of daylight plus Civil Twilight. The average is for all 365 days of the year. The map perspective is Alaska-centric.
The next time you hear someone complain about how dark it gets in Alaska during the winter, just remind them that interior Alaska is the light champion on the U.S.