Sunsets

Sunset or sundown is the daily disappearance of the Sun below the horizon as a result of Earth's rotation. The Sun will set exactly due west at the equator on the spring and fall equinoxes, each of which occurs only once a year.

The time of sunset is defined in astronomy as the moment when the trailing edge of the Sun's disk disappears below the horizon. Near to the horizon, atmospheric refraction causes the ray path of light from the Sun to be distorted to such an extent that geometrically the Sun's disk is already about one diameter below the horizon when sunset is observed.

Sunset is distinct from twilight, which has three phases, the first being 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; and the third 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. Night occurs when the Sun reaches 18 degrees below the horizon and no longer illuminates the sky.

Locations north of the Arctic Circle and south of the Antarctic Circle experience no sunset or sunrise on at least one day of the year, when the polar day or the polar night persists continuously for 24 hours.

Sunset creates unique atmospheric conditions such as the often intense orange and red colors of the Sun and the surrounding sky.

The time of sunset varies throughout the year, and is determined by the viewer's position on Earth, specified by longitude and latitude, and elevation. Small daily changes and noticeable semi-annual changes in the timing of sunsets are driven by the axial tilt of Earth, daily rotation of the Earth, the planet's movement in its annual elliptical orbit around the Sun, and the Earth and Moon's paired revolutions around each other. During winter and spring, the days get longer and sunsets occur later every day until the day of the latest sunset, which occurs after the summer solstice. In the Northern Hemisphere, the latest sunset occurs late in June or in early July, but not on the summer solstice of June 21. This date depends on the viewer's latitude (connected with the Earth's slower movement around the aphelion around July 4). Likewise, the earliest sunset does not occur on the winter solstice, but rather about two weeks earlier, again depending on the viewer's latitude. In the Northern Hemisphere, it occurs in early December or late November (influenced by the Earth's faster movement near its perihelion, which occurs around January 3).

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Wikipedia