Editing Polar night (section)

{{Short description|Phenomenon where the Sun remains below the horizon for more than 24 hours}}
{{More citations needed|date=October 2022}}
{{multiple image
| align             = right
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| image1            = Anta0151 (19771223522).jpg
| caption1          = Polar night and [[Aurora|aurora australis]] at the [[South Pole]], [[Antarctica]].
| image2            = Polar-Night Longyearbyen.jpg
| caption2          = Characteristic nautical ([[Blue hour|blue]]) polar twilight in [[Longyearbyen]], [[Svalbard]], [[Norway]].
| image3            = Polar night in Naryan-Mar.jpg
| caption3          = Civil polar twilight in [[Naryan-Mar]], [[Russia]].
}}
'''Polar night''' is a phenomenon that occurs in the [[polar regions of Earth|northernmost and southernmost regions of Earth]] when the [[Sun]] remains below the [[horizon]] for more than 24&nbsp;hours. This only occurs inside the [[polar circle]]s.<ref>{{Cite book |last=Burn |first=Chris |url=http://nwtresearch.com/sites/default/files/the-polar-night.pdf |title=The Polar Night |publisher=The Aurora Research Institute |access-date=28 September 2015}}</ref> The opposite phenomenon, polar day or [[midnight sun]], occurs when the Sun remains above the horizon for more than 24&nbsp;hours. 

There are multiple ways to define twilight, the gradual transition to and from darkness when the Sun is below the horizon.{{sfn|Greene|2003|p=86}} "Civil" twilight occurs when the Sun is between 0 and 6 degrees below the horizon. Nearby planets like [[Venus]] and bright stars like [[Sirius]] are visible during this period.{{sfn|Mason|1933|p=690}} "Nautical" twilight continues until the Sun is 12 degrees below the horizon.{{sfn|Ottewell|2019}} During nautical twilight,  the horizon is visible enough for navigation.{{sfn|Kher|Bikos|n.d.}} "Astronomical" twilight continues until the Sun has sunk 18 degrees below the horizon.{{sfn|Mason|1933|p=690}}{{sfn|Shubinski|2023|loc="In twilight"}} Beyond 18 degrees, refracted sunlight is no longer visible.{{sfn|Shubinski|2023|loc="In twilight"}} True night is defined as the period when the sun is 18 or more degrees below either horizon.{{sfn|Ottewell|2019}}

Since the atmosphere refracts sunlight, polar day is longer than polar night, and the area that experiences polar night is slightly smaller than the area that experiences polar day. The polar circles are located at [[latitude]]s between these two areas, at approximately 66.5°. While it is day in the [[Arctic Circle]], it is night in the [[Antarctic Circle]], and vice versa.

Any planet or moon with a sufficient [[axial tilt]] that [[Rotation period|rotates]] with respect to its star significantly more frequently than it [[Orbital period|orbits]] the star (and with no [[tidal locking]] between the two) will experience the same phenomenon (a nighttime lasting more than one rotation period).