Sunrise, sunset and twilight

Navigation · Chapter 16 · DGCA CPL and ATPL

Why days are long or short, exactly when the Sun rises and sets, what the three twilights are, and how to work out sunrise at another place.

Remember

Around 21 Jun the Sun does not set north of about 66°N; around 21 Dec it does not rise there.
Sunrise/sunset: Sun's centre 50′ below the horizon (34′ + 16′).
Civil 50′–6°, nautical 6°–12°, astronomical 12°–18°.
Shortest civil twilight 21 min, at the equator on an equinox; altitude shortens it.
Same latitude → same sunrise LMT: go through UTC.

Long and short days

The Earth from space • The Sun (left) lights half the Earth • The gold line is where the Sun sits on the horizon: sunset on this side • Watch a town at 50°N ride into the night as the Earth turns
Your day is your circle in sunlight • As the Earth turns, the town goes round its latitude circle (pink dashes) • The part of the circle in sunlight is its day. In March and September: half, 12 h
About 21 June • The Sun is over 23.4°N: the lit half tips towards the north • Most of the town's circle is now in sunlight: 16 h of day
About 21 December • The Sun is over 23.4°S: the lit half tips south • Most of the circle is dark: about 8 h of day
Far north in June • At 70°N the whole circle is in sunlight • The Sun never sets: the midnight sun (north of about 66°N)
At the equator • The gold line always cuts the equator's circle in half • About 12 h of day all year round

Sunset from the ground

An evening at the airfield • You stand at a small airfield, looking west • Watch the Sun go down, the sky change colour and the lights come on
The Sun slants down • At 50°N the Sun sets on a slant: its path leans 90° − 50° = 40° to the horizon • So it takes a while to sink each degree
At the equator: straight down • At the equator the path stands upright: the Sun drops fast and night comes quickly