Daylight Saving Time (DST) officially ends tomorrow and everyone in my little corner of the world will set their clocks back and get a well-deserved extra hour of sleep. We all know that this odd modern ritual is suppose to save energy (or candle wax or some such thing) but just how does it work?
First of all let’s look at what happens to the Earth’s day over the course of a year. Because the Earth rotates on its axis at a slight tilt, there are times where the North Pole leans toward the Sun (this is summer in the Northern Hemisphere) and there are times where the North Pole tilts away from the Sun (this is the Northern winter). A city or town located in the Northern Hemisphere experiences longer days during the summer because of the additional exposure it gets as it rotates in this position. The reverse is true for places in the Southern Hemisphere.
When you plot out the the sunrise and sunset times for different areas of the U.S., you can see that the daylight pattern varies depending on your latitude. Honolulu, one of the southern-most Amercan cities, has relatively static sunrise and sunset times while Milwaukee, located about halfway between the equator and the North Pole, shows more signficant changes in in the length of the day over the course of the year. In fact, during the summer months, this northern city gets nearly three more hours of daylight than the island paradise.
Things get a little weirder as you go farther north. The relative orientation of the Sun to the surface of the Earth starts to have a greater impact on the amount of daylight each area receives. Anchorage, which is located at about 60 degrees latitude, has six-hour days in the winter and almost 20-hour days in mid-summer. Barrow, located north of the Arctic Circle, starts to experience full darkness during the winter and 24-hours of daylight in the summer months.
Human civilization imposes a rigid structure on this natural daily cycle by setting up concepts like the work day, meal times, play dates and TV schedules. Of course, this structure only works if everyone’s schedule is the same so we’ve also developed things like alarm clocks, wrist watches and other timekeeping devices to keep us in synch. In some areas of the world (like Hawaii), the alignment of natural and societal cycles is fairly good. In other places it imposes some problems.
For example, a typical Milwaukean who wakes up at 6:00 AM and goes to bed at 10:00 PM misses out on almost two hours of daylight during the summer mornings. However, if they adjusted their schedule to wake up at dawn during the month of June, they would be waking up 3-4 hours before the sun rises in December. By shifting the schedule by an hour during the summer months, Daylight Saving Time helps even out the daylight period in relation to the natural cycle.
The energy savings kicks in when you plot the additional amount of day time that some experiences during the summer. Presumably, this extra time is spent with candles unlit.
These charts also explain why Hawaii doesn’t use Daylight Savings time (it does’t experience enough variation in daylight to warrant the shift) and why some very northern locations may actually experience two sunsets in a single day (the time zone doesn’t quite line up with the natural cycle of the Sun).