Sunset, also known as sundown, is the daily disappearance of the Sun below
the horizon due to Earth's rotation.
As viewed from everywhere on Earth
(except the North and South poles), the equinox Sun sets due west at the
moment of both the Spring and Autumn equinox.
As viewed from the Northern
Hemisphere, the sun sets to the northwest (or not at all) in the Northern
hemisphere's spring and summer, and to the southwest in the autumn and
winter; these seasons are reversed for the Southern Hemisphere.
The time of sunset is defined in astronomy as the moment when the upper
limb of the Sun disappears below the horizon.
Near the horizon,
atmospheric refraction causes sunlight rays to be distorted to such an
extent that geometrically the solar disk is already about one diameter
below the horizon when a sunset is observed.
Sunset is distinct from twilight, which is divided into three stages.
The
first one is civil twilight, which begins once the Sun has disappeared
below the horizon, and continues until it descends to 6 degrees below the
horizon.
The second phase is nautical twilight, between 6 and 12 degrees
below the horizon.
As for the third one, it is astronomical twilight,
which is the period when the Sun is between 12 and 18 degrees below the
horizon.
Dusk is at the very end of astronomical twilight, and is the
darkest moment of twilight just before night.
Finally, night occurs
when the Sun reaches 18 degrees below the horizon and no longer
illuminates the sky.
This information is from Wikipedia
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