Different Types of Twilight
There are three types of twilight, namely: Astronomical Twilight, Civil Twilight and Nautical Twilight. They are categorized on the basis of the solar depression angle. This is the angle which forms between the sea level horizon and the center of the Earth and the center of the solar disc.
Civil twilight occurs when solar zenith angle is 96° or solar elevation angle is -6°. That is the time in the morning or evening in which the center of the solar disc is 6° below the horizon. Civil twilight is appropriate to watch the brightest stars, provided the sky is clear and a well defined horizon is one of its characteristics. Usually artificial light is required to carry out outdoor activities in the time before civil twilight in morning and after civil twilight in the evening.
As soon as solar depression angle is more than 6° nautical twilight starts. The presence of light when the Sun is positioned at an angle, which is between 6°and 12° below the horizon, is called nautical twilight. It is signified by a solar zenith angle of 102°or a solar elevation angle of -12°. In the absence of artificial light it is possible to see shadows of objects on the ground, but it requires proper lighting to identify the object. Also, it is very difficult to identify the horizon as it is blurred. It lasts till the solar depression angel reaches 12°, now astronomical twilight starts.
At the time of sunrise or sunset, when the position of the Sun is at an angle between 12° and 18°below the horizon, astronomical twilight occurs. Alternatively, astronomical twilight is the phenomenon which is experienced when solar zenith angle is at 108° or solar elevation angle is at -18°. Even though the sky is dark during this time, it is difficult to identify galaxies and dimmer stars. But, once astronomical twilight is over, the sky is pitch dark and one can study all stellar objects within view. Astronomers consider this time as the most beneficial to study astronomical objects as the scattering of light by the upper atmosphere is at a bare minimum.
This figure shows how Civil Twilight, Nautical Twilight and Astronomical Twilight are measured. (Image courtesy of the Canadian Space Agency.)