6.18 Twilight

6.18.2 Define:

(a) sunrise;

The moment when the upper limb of the Sun first appears above the horizon at a given location, marking the beginning of direct sunlight.

(b) sunset;

The moment when the upper limb of the Sun disappears below the horizon at a given location, marking the end of direct sunlight. 

(c) daylight;

The period of time between the beginning of Morning Civil Twilight (MCT) and the end of Evening Civil Twilight (ECT). 

(d) twilight;

The period of diffused natural light that occurs when the Sun is below the horizon, before sunrise (morning twilight) and after sunset (evening twilight). 

(e) morning civil twilight (MCT);

The period in the morning when the centre of the Sun is 6° below the horizon until the moment of sunrise.

(f) evening civil twilight (ECT).

The period in the evening from the moment of sunset until the centre of the Sun is 6° below the horizon.

6.18.4 Describe the factors that affect the times of sunrise and sunset (daylight). 

Latitude

The tilt of the Earth’s axis (approximately 23.5°) relative to its orbit around the Sun means that different latitudes experience varying amounts of daylight throughout the year.