Twilight is the time after sunset, and before sunrise, when the sky remains bright, providing an ambient deep-blue illumination.
It is caused by the scattering of sunlight around the Earth's day-night boundary by particles in the Earth's upper atmosphere, the same process which gives rise to the blue appearance of the sky in daytime.
Different types of twilight
The apparent brightness of the sky during the hours of twilight depends on many factors, including atmospheric conditions and altitude, but most importantly on the distance of the Sun beneath the horizon. For this reason, the degree of twilight is customarily defined in terms of the angular distance of the Sun below the horizon.
Three classes of twilight are defined: civil twilight is that when the Sun is less than 6° below the horizon; nautical twilight is that when the Sun is between 6° and 12° below the horizon; and astronomical twilight is that when the Sun is between 12° and 18° below the horizon. When the Sun is more than 18° below the horizon, there is said to be astronomical darkness.
The names of the various classes of twilight gives some indication of their history. During civil twilight, the sky remains quite obviously bright, even to a casual observer. Only a few of the brightest stars may be visible to the naked eye. The beginning and end of civil twilight approximates the times that a lay person might describe as dawn and dusk.
During nautical twilight, many of the night sky's brightest stars are visible, though an observer at a dark location, for example at sea, will still be able to make out a residual background glow, especially in the direction facing where the Sun is below the horizon. Observations of the Moon and bright planets are possible, though most deep sky objects will be lost in the glow of the sky.
During astronomical twilight, the sky is not perceptibly bright to the naked eye, even from a dark site, but the residual glow of the sky is sufficient to affect the faintest objects that can be seen through a telescope. Bright and non-diffuse deep sky objects – in particular bright open clusters – will typically be observable, but faint diffuse objects such as galaxies may be more difficult to make out.
The times for twilight shown on the homepage of this website refer to astronomical twilight.