XVIII.—Sunset and Twilight Curves, and Related Phenomena.

§ 1.THE objects of the present paper are, first, to describe certain curves which approximate to the graphs of the time of sunset (or sunrise) and end of twilight (or daybreak) all the year round for various latitudes; second, to tabulate the yearly phenomena of light and darkness for different latitudes under various conditions. The time of sunset depends upon the latitude and the sun's declination, and the extent of its variations depends upon the obliquity of the ecliptic. The extent of the variations of twilight depends also upon a certain angle a, which is the maximum depression of the sun's centre below the horizon for which the light reflected by the upper strata of the atmosphere is sensible.