March 26, 2019--The Three Twilights

In search of distractions I stumbled on this--the three twilights. Did you know there are three? Three each day? If not, see below. (No metaphoric meaning necessarily intended.)

Civil Twilight

Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During civil twilight, the geometric center of the Sun's disk is at most 6 degrees below the horizon. In the morning, this twilight phase ends at sunrise; in the evening it begins at sunset. Sunrise and sunset are the moments when the Sun's upper edge touches the horizon.

As the Earth's atmosphere scatters and reflects much of the Sun's rays, coloring the sky bright yellow and orange, artificial lighting is generally not required in clear weather conditions to carry out most outdoor activities. Only the brightest stars and planets, like Venus and Jupiter, can be seen with the naked eye.

Nautical Twilight

Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During nautical twilight, the geometric center of the Sun's disk is between 6 and 12 degrees below the horizon.

In clear weather conditions, the horizon is faintly visible during this twilight phase. Many of the brighter stars can also be seen, making it possible to use the position of the stars in relation to the horizon to navigate at sea. This is why it is called nautical twilight.

Astronomical Twilight

Each twilight phase is defined by the solar elevation angle, which is the position of the Sun in relation to the horizon. During astronomical twilight, the geometric center of the Sun's disk is between 12 and 18 degrees below the horizon.

To the naked eye, and especially in areas with light pollution, it may be difficult to distinguish astronomical twilight from night time. Most stars and other celestial objects can be seen during this phase.

However, astronomers may be unable to observe some of the fainter stars and galaxies as long as the Sun is less than 18 degrees below the horizon – hence the name of this twilight phase.