ADB , the
See also:plane of the
See also:horizon; EDF, the Z path of the
See also:sun . Let the circles ADB and FDE intersect in the points D and D,; then these points correspond to the rising and setting of the sun . Now
See also:twilight prevails from sunrise or sunset until the sun is depressed through 18°; hence if we draw arcs ZC and ZC, equal to 108°, and terminating on the circle FDE at C and C,, then the arcs DC and D,C, represent the distance traversed by the sun during the twilight . Also it may be observed that C,EC represents the path of the sun during the
See also:night, and DFD1 during the
See also:day . The arc CD is readily determined by spherical trigonometry . For, join CP by an arc of a
See also:great circle; then in the triangle ZPC we know ZP (the colatitude of O) ; PC (the sun's polar distance) and ZC (=Io8° by construction) . Hence the
See also:angle ZPC, the sun's
See also:hour angle, may be found; this gives the
See also:time before or after
See also:noon when the sun passes C . The times of sunrise and sunset being known, then the arcs DC and DIC, (and the duration of
See also:dawn and twilight) are determined . So far we have considered the case when the sun does attain a depression of 18°, but it is equally possible for this depression not to be attained . To investigate this, take ZG equal to Io8° . Now if G lies beyond B and E (the maximum depression of the sun), E being also below B, then the sun will rise and set, but never descend so low as to occasion true night, and the entire
See also:interval between sunrise and sunset will be twilight . If E be not below B but above it, the sun will never descend below the horizon, and will neither rise nor set, and we are presented with the phenomenon known as the midnight sun .
Since PE=90° — sun'sdeclination, and PG = latitude of observer + 18°, then it follows that for there to be no night the latitude of the observer together with the declination of the sun must lie between 90° and 72° . The maximum declination of the sun is about 23° 30', and hence in latitude 48° 3o' there will be one day without a true night; in higher latitudes there will be an increasing number of such days; and in
See also:lower latitudes none . In England there is no real night from about the 22nd of May till the 22nd of
See also:July . The phenomenon known as the after-glow, or second twilight, has been referred to a second reflection of the solar rays in the atmosphere .
ADAPTIVE TYPES OF LIMBS AND FEET
ADDA (anc. Addua)
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