TWILIGHT  , formerly known as Crepusculum (a Latin word meaning dusky or obscure), properly the

interval during which the atmosphere is illuminated after the setting of the sun . The analogous phenomenon in the morning, i.e. the interval between the first appearance of light and the rising of the sun, is known as the dawn . These phenomena are due to the light of the sun after refraction by the atmosphere being reflected to the observer by the clouds, dust, and other adventitious matter present in the atmosphere . Even in the early infancy of astronomy, the duration of twilight was associated with the position of the sun below the horizon, and measurements were made to determine the maximum vertical depression of the sun which admitted the phenomena . This was found by Alhazen, Tycho Brahe and others, to be about 18°, and although other observers obtained somewhat different values, yet this value is now generally admitted . The duration of twilight is therefore measured by the time in which the sun traverses an arc of r8° of vertical depression, and primarily depends on the latitude of the observer and the declination of the sun . It is subject to several minor variations, occasioned by the variable amount of dust, clouds, &c. suspended in the air; and also on the temperature, which alters the altitude of the reflecting particles; thus at the same place and on the same day, the morning twilight or dawn is generally shorter than the evening twilight . The duration and possibility of twilight may be geometrically exhibited as follows : Let 0 be the position of the observer (fig .