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

• INTERVAL

interval during which the See also:

• ATMOSPHERE (Gr. fir µ6r, vapour; orkaipa, a sphere)

atmosphere is illuminated after the setting of the See also:

• SUN
• SUN (0. Eng. sonne, Ger. sonne. Fr. soleil, Lat. sol, Gr. ijXior, from which comes helio- in various English compounds)

sun . The analogous phenomenon in the See also:

• MORNING

morning, i.e. the interval between the first See also:

• APPEARANCE (from Lat. apparere, to appear)

appearance of See also:

• LIGHT

light and the rising of the sun, is known as the See also:

• DAWN (the 16th-century form of the earlier " Jawing " or " dawning," from an old verb " daw," O. Eng. dagian, to become day; cf. Dutch dagen, and Ger. tagen)

dawn . These phenomena are due to the light of the sun after See also:

• REFRACTION
• REFRACTION (Lat. refringere, to break open or apart)

refraction by the atmosphere being reflected to the observer by the clouds, dust, and other See also:

• ADVENTITIOUS (from Lat. adventicius, coming from abroad)

adventitious See also:

• MATTER

matter See also:

• PRESENT

present in the atmosphere . Even in the See also:

• EARLY
• EARLY, JUBAL ANDERSON (1816-1894)

early See also:

• INFANCY

infancy of See also:

• ASTRONOMY (from Gr. &arpov, a star, and v ,usiv, to classify or arrange)

astronomy, the duration of twilight was associated with the position of the sun below the See also:

• HORIZON (Gr. 6pfTwv, dividing)

horizon, and measurements were made to determine the maximum See also:

• VERTICAL (from Lat. vertex, highest point)

vertical depression of the sun which admitted the phenomena . This was found by See also:

• ALHAZEN (ABU ALI AL-HASAN IBN ALHASAN)

Alhazen, Tycho See also:

• BRAHE, PER, COUNT (1602-1680)
• BRAHE, TYCHO (1546-1601)

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 See also:

• TIME (0. Eng. Lima, cf. Icel. timi, Swed. timme, hour, Dan. time; from the root also seen in " tide," properly the time of between the flow and ebb of the sea, cf. O. Eng. getidan, to happen, " even-tide," &c.; it is not directly related to Lat. tempus)
• TIME, MEASUREMENT OF
• TIME, STANDARD

time in which the sun traverses an arc of r8° of vertical depression, and primarily depends on the See also:

• LATITUDE (Lat. latitudo, latus, broad)

latitude of the observer and the See also:

• DECLINATION (from Lat. declinare, to decline)

declination of the sun . It is subject to several See also:

• MINOR
• MINOR (Lat. for smaller, lesser)
• MINOR, ROBERT CRANNELL (1839-1904)

minor See also:

• VARIATIONS
• VARIATIONS, CALCULUS
• VARIATIONS, CALCULUS OF

variations, occasioned by the variable amount of dust, clouds, &c. suspended in the See also:

• AIR (from an Indo-European root meaning " breathe," " blow ")
• AIR, or ASBEN

air; and also on the temperature, which alters the See also:

• ALTITUDE (Lat. altitudo, from altus, high)

altitude of the reflecting particles; thus at the same See also:

• PLACE (through Fr. from Lat. platea, street; Gr. IrAar6s, wide)

place and on the same See also:

• DAY (O. Eng. dreg, Ger. Tag; according to the New English Dictionary, " in no way related to the Lat. dies")
• DAY, JOHN (1574-1640?)
• DAY, THOMAS (1748-1789)

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 .