Did you know that there are three twilights?
• Civil twilight
• Nautical twilight
• Astronomical twilight
Well, four if you count the name given to yacht races that start in the late afternoon and finish before dark.
In fact, it should be remembered that twilight occurs before sunrise as well as after sunset.
The definitions depend on where the centre of the Sun is in relation to the horizon.
Civil twilight is after sunset when the centre of the sun is 6 degrees below the horizon and before sunrise when it’s 6 degrees below the horizon.
Similarly, nautical twilight in the evening occurs when the centre of the sun is 12 degrees below the horizon. In the morning it’s when the centre of the sun is 12 degrees below the horizon.
Finally, in the evening astronomical twilight is when the centre of the sun is 18 degrees below the horizon, i.e. after nautical twilight. In the morning it is when the centre of the Sun is 18 degrees below the horizon, i.e. before nautical twilight. After astronomical twilight at night and before it in the morning the Sun provides no illumination to the sky.
Surprisingly to the navigator, nautical twilight is when the horizon becomes indeterminable. It’s during civil twilight that a sextant can be used to take shots when there is sufficient light to see the horizon and sufficient dark to determine the useful navigational stars and planets.