Duration of daylight

length between sunrise and sunset, and it varies ac-

on October 1.

cording to location on the earth and time of year. Sun-

The times of sunrise and sunset at Kenai for the

rise and sunset are conventionally considered to oc-

year 2000 are shown in Table 9, which was generated

cur when the upper edge of the sun is coincident with

from a web-based routine that is available at the U.S.

an unobstructed horizon, under average atmospheric

Naval Observatory website (USNO 2000). The times

conditions for an observer at sea level. However, at

shown are accurate to within a few minutes for any

northern latitudes, twilight, when the sun is not far

other year of interest. A random comparison of dates

below the horizon, adds significant time of usable light

throughout the 21st century showed that the predicted

to the period of continuous sunlight. The duration of

times of sunrise and sunset agree to within three min-

continuous sunlight plus twilight for a band of north-

utes of those shown for the year 2000. At Kenai the

ern latitudes is shown in Figure 28. Several represen-

duration of sunlight ranges from about 5.7 hours

tative Alaskan locations are shown for reference. The

around December 21 to 19.1 hours around June 21.

Figure 28. Total length of daylight (combined sunlight and twilight) for Alaska locations. (From Selkregg

et al. 1972)

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