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What is Twilight , its types in detail?

twilight
Written by Amit Sharma

Twilight is the period of incomplete darkness before sunrise and after the sunset. So the morning twilight ends at sunrise, when the upper limb of the sun just appears above the visible horizon, and evening twilight starts at sunset, when the upper limb of the sun just disappears below the visible horizon.

The period of twilight is very important to the navigator, because in this period the visible horizon can still be seen and the celestial bodies on the sky are bright enough to be observed by the sextant.

There are three stages of twilight:

types of twilight

  1. Civil twilight is the interval of time between visible sunrise or sunset and the time when the centre of the sun is 6° below the rational horizon.
In other words we can say:

Civil twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 6 degrees below the horizon.

  1. Nautical twilight is the interval of time between visible sunrise or sunset and the time when the centre of the sun is 12° below the rational horizon.
In other words we can say:

Nautical twilight is defined to begin in the morning, and to end in the evening, when the center of the sun is geometrically 12 degrees below the horizon.

During nautical twilight the illumination level is such that the horizon is still visible even on a Moonless night allowing mariners to take reliable star sights for navigational purposes, hence the name.

  1. Astronomical twilight is the interval of time between visible sunrise or sunset and the time when the centre of the sun is 18° below rational horizon.
In other words we can say:

Astronomical twilight is defined to begin in the morning, and to end in the evening when the center of the Sun is geometrically 18 degrees below the horizon.

Before the beginning of astronomical twilight in the morning and after the end of astronomical twilight in the evening, scattered light from the Sun is less than that from starlight and other natural sources.

For a considerable interval after the beginning of morning twilight and before the end of evening twilight, sky illumination is so faint that it is practically imperceptible.

About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.

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