## Sunrise and sunset times and day length

The sun is said to rise and set when the solar altitude angle is 0. So, the hour angle at sunset, hss, can be found by solving Eq. (2.12) for h when a = 0°:

sin(a) = sin(0) = 0 = sin(L)sin(6) + cos(L)cos(8)cos(hss)

where hss is taken as positive at sunset.

Since the hour angle at local solar noon is 0°, with each 15° of longitude equivalent to 1 h, the sunrise and sunset time in hours from local solar noon is then

The sunrise and sunset hour angles for various latitudes are shown in Figure A3.1 in Appendix 3.

The day length is twice the sunset hour, since the solar noon is at the middle of the sunrise and sunset hours. Therefore, the length of the day in hours is

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