What is the midnight sun?

What is the midnight sun?

The midnight sun is the period during which the Sun remains continuously above the horizon for more than twenty-four hours. Wikidata, the structured-data sister of Wikipedia, defines it as the "natural phenomenon when daylight lasts for more than 24 hours, occurring only inside or close to the polar circles."⁸ Encyclopædia Britannica gives the same definition in observer's terms: the Sun "as seen in the Arctic or Antarctic, where the tilt of the Earth's axis ... produces at least one 24-hour period of daylight."¹

The phenomenon is geographically restricted. It happens only inside the Arctic and Antarctic Circles, the two parallels of latitude at 66°33′ north and south. North of the Arctic Circle, the midnight sun runs from sometime in late spring to sometime in mid-to-late summer, with the exact dates set by latitude; south of the Antarctic Circle, it runs from the corresponding part of the southern summer (November through January). The June solstice is the centre of the Arctic midnight-sun season and the date on which the Arctic Circle itself just barely meets the definition; the December solstice plays the same role in Antarctica.

The companion phenomenon is the polar night: the period during which the Sun fails to rise at all, and the day stays at twilight or full darkness. Wikidata defines it as the natural phenomenon in which "the night lasts for more than 24 hours," occurring only inside the polar circles.⁹ Polar night runs through the local winter half of the year at the same latitudes, centred on the December solstice in the Arctic and the June solstice in the Antarctic. Both phenomena come from the same geometry, and both grow toward the poles.⁵

The phrase "midnight sun" is descriptive — at the local midnight, the Sun is visible somewhere above the horizon rather than below it. The Norwegian term midnattssol translates directly. In the polar regions of Russia, the equivalent polyarnyy den' ("polar day") is used in scientific and administrative writing; polyarnaya noch' ("polar night") is the corresponding winter term. The Norwegian term mørketid ("dark time") is the colloquial name used along the north Norwegian coast for what scientific writing calls polar night.

Why does it happen?

Earth's rotation axis is tilted relative to a perpendicular drawn through the plane of its orbit around the Sun. NASA gives the current value of the tilt — known to astronomers as the obliquity of the ecliptic — as 23.4° with respect to the orbital plane, conventionally written to two decimals as 23.44° or in arcminute notation as 23°26′.⁶ The tilt direction stays fixed in space as the planet moves through its orbit, so at any given moment one hemisphere is leaning toward the Sun and the other away — the geometric basis for the seasons, for the tropics, and for the polar circles all at once.

The consequence at high latitudes is that the Sun's daily arc across the sky no longer crosses the horizon. To see why, imagine standing exactly at the North Pole. The local zenith — the point directly overhead — is the celestial north pole, the imaginary point in the sky around which all the stars appear to rotate. The Sun's apparent path through the day is a circle at a fixed altitude equal to its declination on the date in question. On the June solstice that declination is +23.44°, so the Sun circles the zenith at 23.44° above the horizon, never rising and never setting. On the December solstice it circles at −23.44° below the horizon, never rising at all.

The same geometry, in milder form, applies to any latitude inside the polar circles. The polar circles themselves sit at 90° minus the obliquity — 90° − 23.44° = 66.56°, or 66°33′. That is the latitude at which the Sun, on the June solstice, just grazes the horizon at midnight in the north and just grazes it at noon in the south; one degree further from the equator and the grazing turns into a clean miss. The further poleward of 66°33′ a location sits, the more dates each year on which the miss occurs and the longer the resulting stretch of permanent day or permanent night.¹

A reverse derivation makes the same point: the obliquity sets the latitude of the tropics — 23.44° north (Tropic of Cancer) and south (Tropic of Capricorn) — where the Sun stands directly overhead on the respective solstice. The tropics and the polar circles are mirror parallels around the equator, both at distances of 23.44° from their respective extremes (equator and pole). If Earth's axial tilt were larger, the polar circles would slide equator-ward and the tropics would slide pole-ward; the bands of permanent day and night would widen. If the tilt were zero, the seasons would disappear and the polar circles with them.

How long does it last?

Duration depends on latitude, and the relationship is steep. At the polar circle itself, the midnight sun is a one-day event coincident with the local summer solstice. A few degrees of latitude later, the event has stretched into a months-long stretch around the solstice. At the poles, the Sun is up for half the year and down for the other half — though twilight softens both transitions.

Representative numbers across the inhabited Arctic, from Wikipedia's compilation cross-checked against the basic geometry:²

• Arctic Circle (66°33′ N). Approximately one day of midnight sun, on or near the June solstice. Atmospheric refraction stretches this into a few days in practice — see the next section.
• Bodø, Norway (67.30° N). About 35 days, 4 June to 8 July — only just inside the Arctic Circle, but the southernmost Norwegian mainland city with continuous summer daylight.³
• Murmansk, Russia (68.97° N). About 62 days, roughly 22 May to 22 July. Russia's largest Arctic city; the local term is polyarnyy den'.²
• Tromsø, Norway (69.65° N). About 64 days, 20 May to 22 July — the hub of Norwegian Arctic tourism.³
• Hammerfest, Norway (70.66° N). About 73 days, 16 May to 27 July. Nordkapp, the European mainland's northernmost cape, gets a slightly longer 77-day run from 14 May to 29 July.³
• Longyearbyen, Svalbard (78.22° N). About 125 days, 20 April to 22 August — the deepest inhabited polar-night and polar-day case.³
• North Pole (90° N). Roughly six months — about 18 March to 24 September — modified by the long polar twilight around the September equinox.¹²

The southern hemisphere counterpart is sparsely populated. Cities and towns inside the Antarctic Circle are limited to research stations — McMurdo (77.85° S) and Vostok (78.46° S) both experience months-long midnight-sun seasons centred on the December solstice. Tourist-accessible parts of the Antarctic Peninsula sit north of 66.5° and miss the boundary by a few degrees, so they see white nights at midsummer rather than continuous daylight.

Polar night runs the corresponding cycle but is slightly shorter for the same latitude than midnight sun, because atmospheric refraction biases the Sun's apparent altitude upward at the horizon. Wikipedia notes the asymmetry: "polar day is longer than polar night, and the area that experiences polar night is slightly smaller than the area that experiences polar day."⁵ Murmansk's polar night runs about 40 days, centred on the December solstice; Longyearbyen's about 80 days; the North Pole's about 179 days, slightly less than its 186-day polar day.

What about latitudes right at the Arctic Circle?

The 66°33′ figure is a geometric idealisation, computed under the assumption that the atmosphere does not bend light. The real atmosphere does, and the bend is substantial near the horizon. The same atmospheric refraction that lets a setting Sun be visible after its geometric centre has dropped below the horizon shifts the apparent latitude band of the midnight sun a little equator-ward of the geometric polar circles.

The standard refraction allowance used for sunrise and sunset times is 34 arcminutes — the average bend the atmosphere imparts to a ray of light grazing the horizon. Adding 16 arcminutes for the apparent radius of the Sun's disc (an observer sees the rising Sun's upper limb, not its centre) gives the 50-arcminute total used by the U.S. Naval Observatory and most modern almanacs. The full convention places the Sun's geometric centre 50 arcminutes — about 0.833° — below the horizon at the moments of sunrise and sunset.⁴

The consequence at the polar circle is that an observer there sees more than a single day of midnight sun. The refraction-corrected horizon-crossing condition is easier to satisfy than the pure geometric one, and the band shifts equator-ward by something of the order of half a degree of latitude. Wikipedia quantifies the size of the shift: "midnight sun may be experienced at latitudes slightly south of the Arctic Circle or north of the Antarctic Circle, though not exceeding one degree."² Grímsey, the Icelandic island whose centre sits almost exactly on the Arctic Circle at 66.55° N, sees several days of midnight sun around the June solstice on the strength of refraction alone.

The reverse asymmetry applies at the polar-night end of the year. Refraction lifts the Sun's apparent altitude, so a location whose geometry would just put it inside the polar-night band sees the Sun briefly rise on what should be a sunless day. The polar-night band is therefore smaller than the polar-day band by the same shift, which is why Wikipedia's quoted polar-night durations for any given latitude run a little shorter than the corresponding polar-day durations.⁵

What is "polar twilight"?

Inside the polar circles, the winter days that are not full daylight are not necessarily full dark either. The Sun spends some part of the day below the horizon, but at high latitudes it may be only a few degrees below — close enough that scattered sunlight in the atmosphere keeps the sky visibly lit. The site's twilight page covers the three-stage convention — civil, nautical, astronomical — by which observers carve up the gradient between sunset and full darkness; the same convention applies through the polar winter, just with the Sun parked below the horizon for the full 24 hours rather than dipping into twilight twice a day.

Wikipedia's polar-night article tabulates the latitude bands at which each twilight tier persists through the local midwinter solar noon, the deepest point of the polar-night day:⁵

• Civil polar twilight (Sun between 0° and 6° below the horizon at solar noon): roughly 67°24′ to 72°34′. The world is dim but legible; outdoor activity is feasible without artificial light at midday. Tromsø sits inside this band and runs about two months of polar-night civil twilight from mid-November to mid-January.
• Nautical polar twilight (6° to 12° below): roughly 72°34′ to 78°34′. The horizon is still visible from the sea, but the sky is darker than its sub-polar equivalent. Longyearbyen sits inside this band and runs nautical polar twilight from around 11 November to 30 January.
• Astronomical polar twilight (12° to 18° below): roughly 78°34′ to 84°34′. Stars become visible, but a slight glow on the horizon persists at solar noon. Ny-Ålesund on Svalbard, the world's northernmost civilian research settlement, experiences this for several weeks each winter.
• True polar night (deeper than 18° below): above 84°34′. No twilight at all at solar noon — the depth of polar night looks the same as the depth of mid-latitude night. The North Pole and the South Pole both spend the centre of their long polar-night season in this regime.

The same geometry runs in the summer half of the year, but inside the midnight-sun band there is no need for the twilight scheme — the Sun is above the horizon at solar midnight and the question of how deep it dips at solar noon does not arise. The stacking matters only for the dark half of the polar year.

What are white nights?

White nights are the sub-polar phenomenon that the midnight-sun band's southern boundary blurs into. Saint Petersburg (59.9° N), Stockholm (59.3° N), Helsinki (60.2° N), and the southern half of Norway all sit south of the Arctic Circle — the Sun does set there each summer night — but they sit far enough north that the Sun never falls more than 6° below the horizon at solar midnight around the local summer solstice. Civil twilight persists all night, and the sky stays bright enough to read by.

The lower latitude boundary of the white-nights band sits at about 60°33′ in either hemisphere — 90° minus the obliquity plus 6° for the civil-twilight threshold.⁷ Below that latitude, the Sun does cross 6° below the horizon at midsummer midnight and civil twilight does end for at least a little while; above that latitude, civil twilight runs uninterrupted from sunset to sunrise. The two deeper twilight phases have their own thresholds — nautical twilight persists all summer night above about 54°34′, astronomical twilight above about 48°34′ — but the civil threshold is the one folk usage cares about because it is the threshold below which artificial light is normally needed.⁷

White nights are the basis of Saint Petersburg's annual cultural festival, which runs roughly from late May to mid-July under the same name. The white-nights band is geographically much larger than the midnight-sun band — it covers Edinburgh and Hamburg in nautical form, most of Scotland and southern Scandinavia and central Russia in civil form — so for most readers of this page, the white-nights variant is closer to home.

The same description applies in the southern hemisphere at the corresponding latitudes — about 60.5° S and pole-ward — but very little land lies there: only the southern tip of South America, a small slice of New Zealand's South Island, and the sub-Antarctic islands. The phenomenon exists symmetrically but is rarely observed in inhabited surroundings.

How is the boundary calculated?

The signature of midnight sun and polar night falls cleanly out of the standard sunrise-and-sunset formula. NOAA's solar calculator — the canonical reference implementation, derived from Jean Meeus's textbook Astronomical Algorithms and used in effectively the same form by most modern sun-position software — computes the hour angle of sunrise H₀ from the location's latitude φ and the Sun's declination δ on the date in question:¹⁰

cos H₀ = cos 90.833° / (cos φ · cos δ) − tan φ · tan δ

The 90.833° term is the same horizon-correction angle that defines sunrise and sunset elsewhere on the site — 90° plus the 50-arcminute (0.833°) refraction-and-disc allowance. The result H₀ is the hour angle at which the Sun's centre crosses the horizon-correction line; sunrise occurs H₀ hours of clock time before solar noon, sunset H₀ hours after, and day length is 2 H₀.

Polar day and polar night announce themselves when the right-hand side falls outside the valid range of the cosine function. If the right side is less than −1, no real H₀ exists, and the Sun never reaches the horizon — the midnight sun, day length 24 hours. If it is greater than 1, again no real H₀ exists, but in the opposite direction — the Sun never rises, the polar night, day length zero. The first case occurs when φ and δ share a sign at sufficient magnitude (summer pole); the second when they have opposite signs at sufficient magnitude (winter pole).¹⁰

NOAA cautions that the textbook algorithm's accuracy degrades at high latitudes. Its quoted figures: "theoretically accurate to within a minute for locations between +/- 72° latitude, and within 10 minutes outside of those latitudes."¹⁰ The U.S. Naval Observatory makes the same point: "the accuracy of rise and set computations decreases at high latitudes. There, small variations in atmospheric refraction can change the time of rise or set by many minutes, since the Sun and Moon intersect the horizon at a very shallow angle."⁴ Local topography (mountains, fjord walls) and unusual atmospheric conditions (cold air near the horizon, temperature inversions) shift the practical times of first and last sunlight by minutes or more at these latitudes; the published boundary dates of midnight sun and polar night at any specific city are accurate only to within a day or two.

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