Sunrise Sunset and Twilight

Wg Cdr Rajagopal

Sunrise & Sunset

Index

Sunrise and Sunset

  • The tilt of spin axis of the earth with the orbital plane results in seasonal variation in duration of daylight
  • The time of sunrise and sunset are affected by certain factors
  • A combination of Sun’s Declination and observer’s latitude affects the duration of daylight
  • Semi-diameter or radius of the sun also affects sunrise and sunset
  • Refraction of light in the atmosphere increases daylight time

Effect of Sun’s Declination on Sunrise and Sunset

  • Declination of the sun affects the time of sunrise and sunset
  • Consider the sun’s position in summer during northerly declination of sun
  • Sun’s northerly declination results in longer daylight in northern hemisphere
  • Sun would be above horizon before sun reaches observer’s meridian
  • Sun remains above horizon after sun crosses from observer’s meridian
  • Sunrise is earlier than 0600 LMT and sunset is later than 1800 LMT
  • Declination effect would reverse in the case of southerly declination of the sun

Effect of Latitude on Sunrise and Sunset

  • Latitude of the observer affects the time of sunrise and sunset
  • Consider the sun’s position in summer when sun’s declination is 23.5 North
  • An observer at the Equator experiences equal periods of day and night
  • An observer above 66.5° North experiences constant daylight
  • An observer above 66.5° South experiences constant night time
  • Latitude effect would reverse in the case of southerly declination of the sun

Semi-Diameter of Sun

  • Semi-diameter of sun and atmospheric refraction affects sunrise and sunset
  • Semi diameter of the sun is 16 minutes of arc
  • Atmospheric refraction of 34 minutes of arc

Sunrise and Visual Horizon

  • Sensible horizon is the horizon which is tangential to the earth surface while the visual horizon is 34 minutes below the sensible horizon
  • Sun light is visible when the sun is below sensible horizon due to bending of light
  • Centre of sun at sunrise is 16 minutes of arc below visual horizon
  • Centre of sun at sunrise is 50 minutes below sensible horizon
  • At sunrise and sunset times the sun is three minutes in terms of time below the horizon

Civil Twilight

  • Twilight is the period of partial light before sunrise and after sunset
  • Night flying starts from end of ECT to beginning of MCT
  • Regulations prescribe half hour after sunset to half hour before sunrise

Morning and Evening Civil Twilight

  • Evening civil twilight is the period of sufficient light after sunset
  • ECT extends from sunset to sun at 6 degrees below sensible horizon
  • Morning civil twilight is the period of sufficient light before sunrise
  • MCT extends from sun at 6 degrees below sensible horizon to sunrise

Nautical and Astronomical Twilight

  • Nautical twilight is the time when sky is dark but stars and planets are visible
  • Centre of sun is between 6 to 12 degrees below sensible horizon
  • Astronomical twilight is actually total darkness and only used by astronomers
  • Centre of sun is between 12 to 18 deg below sensible horizon

Duration of Twilight

  • Duration of twilight varies due a combination of the Declination of the sun and Latitude of the observer
  • Altitude of the observer and Meteorological conditions also affect the twilight duration

Value of Civil Twilight Duration

  • Civil twilight is at minimum value if observer and sun are at the equator
  • Minimum time of civil twilight is 21 minutes at an angular distance of 5 degrees 10 minutes

Effect of Declination on Twilight

  • Duration of twilight increases with increase in declination of sun
  • Sun spends more than 21 minutes in the twilight zone at solstice

Effect of Latitude on Twilight

  • Duration of twilight increases with increase in observer’s latitude
  • Sun spends more than 21 minutes in the twilight zone
  • Maximum duration is seen above Arctic and Antarctic circles
  • Summer sun never sets or civil twilight extends until mid-night
  • Winter sun never rises or civil twilight does not start until mid-noon

Effect of Altitude on Twilight

  • Sunrise, sunset and twilight published in air-almanac are for mean sea levels
  • Increase in altitude sun would rise early and set later due to increase in Visual horizon with altitude
  • Therefore, increase in altitude results in a decrease in the duration of civil twilight
  • Reduction in atmosphere at higher levels reduces refraction and hence duration of twilight

Effect of Scattering on Twilight

  • Volcanic eruptions increase the duration of twilight due to increase in volcanic ash pollution
  • Polluted atmosphere increases refraction due to scattering of light

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