Daylight Hours

No matter where in the world you live, do you get the same number of daylight hours over the course of a year?

No. The equator actually gets fewer hours of daylight than most other latitudes.

Any given place would be in daylight exactly 50 percent of a year’s time if Earth moved in a circular orbit, the Sun were a point source, and we had no atmosphere. But owing to atmospheric refraction and the fact that the Sun has a disk, the top of the Sun is visible when its center is substantially below the horizon, increasing the yearly daylight beyond 50 percent. The effect is least significant at the equator, where the Sun rises and sets at the steepest possible angle. It is most significant near the Arctic and Antarctic Circles, where the Sun grazes the horizon for the longest time.

Also, Earth is farthest from the Sun, and moving most slowly in its orbit, in early July. That’s why the Northern Hemisphere’s spring and summer contain about eight more days than the Southern Hemisphere’s spring and summer (check a calendar). This preponderance of longish days gives the Northern Hemisphere more cumulative daylight than the Southern.

To plot the unusual curve showing these effects, I adapted Jean Meeus’s program SUNSHINE.BAS, downloadable from SkyandTelescope.com/resources/software.

— Tony Flanders