CREPUSCULUM, twilight, the time from the first dawn or appearance of the morning to the rising of the sun ; and again, between the setting of the sun and the last remains of day.
The crepusculum, or twilight, it is sup posed, usually begins and ends when the sun is about 18 degrees below the hori zon ; for then the stars of the 6th magni tude disappear in the morning, and ap pear in the evening. It is of longer du ration in the solstices than in the equi noxes, and longer in an oblique sphere tan in a right one ; because, in those cases, the sun, by the obliquity of his path, is longer in ascending through 18 degrees of latitude.
Twilight is occasioned by the sun's rays refracted in our atmosphere, and reflected from the particles of it to the eye. Kepler indeed assigned a different cause of the crepusculum, -viz. the lumi nous matter about the sun. This may lengthen the duration of the twilight, by illuminating the air, when the sun is too low to reach it with. his own light, but is not the principal cause of it : which is unquestionably the refraction of the at mosphere.
The depth of the sun below the hori zon, at the beginning of the morning, or end of the evening twilight, is determin ed in the same manner as the arch of vi sion; viz. by observing the moment when the air first begins to shine in the morn ing, or ceases to shine in the evening ; then finding the sun's place for that mo ment, and thence the time till his rising in the horizon, or from his setting in it in the evening. It is now generally agreed that this depth is about 18 degrees upon an average. Alhazen found it to be 10°; Tycho, Rothman, 24°; Stevenius, 18° ; Cassini, 15° ; Riccioli, in the equi nox in the morning 16°, in the evening 20° 30'; in the summer solstice in the morning- 21° 25', in the winter solstice in the morning 17° 25'.
This difference among the determina tions of astronomers is not to be wonder ed at, the cause of the crepusculum be ing inconstant ; for, if the exhalations in the atmosphere be either more copious or higher than ordinary, the morning twilight will begin sooner, and the even ing hold longer, than ordinary; for the more copious the exhalations are, the more rays will they reflect, consequently the more will they shine ; and the higher they are, the sooner will they be illumi nated by the sun. On this account, too,
the evening twilight is longer than the morning, at the same time of the year, in the same place. To this it may be added, that in a denser air the refraction is greater; and that not only the bright ness of the atmosphere is variable, but also its height from the earth : and there fore the twilight is longer in hot weather than in cold, in summer than in winter, and also in hot countries than in cold, other circumstances being the same. But the chief differences are owing to the different situations of places upon the earth, or to the difference of the sun's place in the heavens. Thus, the twilight is longest in a parallel sphere, and short est in a right sphere, and longer to places in an oblique sphere, in proportion as they are nearer to one of the poles ; a circumstance which affords relief to the inhabitants of the more northern coun tries, in their long winter nights. And the twilights are longest in all places of north latitude, when the sun is in the tro / pic of cancer ; and to those in soutl lati tudes, when he is in thetropic °feat) worn The time of the shortest twiligh is also different in different latitudes : in Eng land, it is about the beginning of Octo ber and of March, when the sun is in the signs Libra and Pisces.