What is twilight?

What is twilight?

Twilight is the soft, indirect light that fills the sky when the Sun is just below the horizon. The atmosphere scatters sunlight that would otherwise pass overhead unseen, so an observer on the ground keeps receiving illumination after sunset and starts receiving it again before sunrise. Wikidata, the structured-data sister of Wikipedia, summarises it concisely as the "illumination of the Earth's lower atmosphere when the Sun itself is not directly visible because it is below the horizon."⁵

The morning phase, between the start of twilight and sunrise, is dawn. The evening phase, between sunset and the end of twilight, is dusk. The two are mirror images: the Sun follows the same arc geometrically in reverse, and the same three-stage scheme of civil, nautical, and astronomical thresholds is used for both. Almanacs and computational tools therefore quote up to six twilight times for any given day at any given place — begin morning civil twilight through end evening astronomical twilight.

Twilight is not a separate astronomical event so much as a gradient. The transition from sunset to night is continuous; the three phases are conventional thresholds chosen because each marks a real change in what a person on the ground can do — read by natural light, find the horizon at sea, observe stars without a glow in the western sky. Different observers, different thresholds, same falling Sun.

What are civil, nautical, and astronomical twilight?

The three phases are defined by the angle of the Sun's centre below the horizon, ignoring atmospheric refraction (refraction is a separate correction baked into the rise/set angles, not part of the twilight definition). The U.S. Naval Observatory states each definition tersely:¹

• Civil twilight ends in the evening — and begins in the morning — when the centre of the Sun is geometrically 6° below the horizon. After it ends, "artificial illumination is normally required to carry on ordinary outdoor activities."
• Nautical twilight ends when the Sun is 12° below the horizon. During it, "the illumination level is such that the horizon is still visible even on a Moonless night allowing mariners to take reliable star sights for navigational purposes before sunrise or after sunset."
• Astronomical twilight ends when the Sun is 18° below the horizon. Past that point, "scattered light from the Sun is less than that from starlight and other natural sources" — what an astronomer would call dark night.

The 6°/12°/18° convention is hardcoded into the major astronomical references. The glossary published by the U.S. National Oceanic and Atmospheric Administration alongside its solar calculator gives the same three angles in the same form, expressed equivalently as solar zenith angles of 96°, 102°, and 108°.² Every major sun-position library, almanac, and operating-system astronomy package follows the same convention.

The angles are evenly spaced — six degrees between each phase — because the Sun's apparent brightness in the sky falls off in roughly equal logarithmic steps with each step in solar depression. The civil/nautical/astronomical scheme captures three of those photometric thresholds at the points the human eye cares about.

Why three different definitions?

Three thresholds exist because three different communities, working in three different professions, found three different points in the falling light meaningful enough to give names to.

The civil threshold is set by what a person on the ground can do without a torch. Around 6° of solar depression, the unilluminated outdoor world stops being legible: roads, faces, ground hazards, the difference between a kerb and a step. Many U.S. statutes that turn on "first light" or "darkness" — when vehicle headlights are required, when burglary becomes a more serious offence — define the threshold against civil twilight rather than against sunrise or sunset. U.S. aviation's regulatory definition of "night" is similarly anchored to the end of evening civil twilight.

The nautical threshold is set by what a navigator at sea can do with a sextant. Star sights — the angle of a known star above the horizon — only work when both the star and the horizon are simultaneously visible. The horizon needs enough light to give a sharp line; the stars need enough darkness to be picked out individually. Nautical twilight is the window during which both conditions hold. Mariners traditionally took fixes during morning nautical twilight (last few stars before they fade) and again during evening nautical twilight (first few stars as they appear). The U.S. military's planning-cycle acronyms BMNT and EENT — begin morning nautical twilight, end evening nautical twilight — descend directly from this maritime usage.

The astronomical threshold is set by what a telescope operator can do. Faint deep-sky objects — galaxies, nebulae, low-magnitude stars — require a sky background dimmer than the residual scattered sunlight produced by the Sun above 18° below the horizon. Inside astronomical twilight the western sky is still measurably brighter than the eastern sky; once the Sun crosses the 18° threshold, sky brightness has dropped to within a few per cent of its true night-time value, and any further dimming is dominated by airglow and Galactic light, not by the Sun.

How long does twilight last?

Twilight duration is set by latitude and time of year. The Sun rises and sets along a path inclined to the horizon by an angle that depends on the observer's latitude. Near the equator, the path is nearly perpendicular to the horizon, so the Sun cuts down through the twilight angles quickly. Near the poles, the path is nearly parallel to the horizon, so the Sun grazes through the same angles slowly.

The numbers are sharper than the description suggests. Near the equator, civil twilight can be as short as 24 minutes. At Greenwich Observatory in London (51.5° N), civil twilight runs 33 to 48 minutes depending on the season, with the longest twilights occurring near the summer solstice. At the poles, where the Sun's path nearly aligns with the horizon, a single civil twilight can last two to three weeks, the Sun creeping up at an angle of barely two degrees per day.³

The same logic applies to the deeper phases. Nautical twilight always lasts longer than civil; astronomical longer still — except where high latitude prevents the Sun from ever reaching the deeper thresholds, in which case the longer phase simply does not exist on that date.

What are white nights?

White nights are the high-latitude phenomenon where one or more twilight phases never actually end overnight. The Sun sets, but it never falls deep enough below the horizon for the sky to reach full darkness. The result is a kind of dim, blue-tinted twilight that persists from sunset through to sunrise.

The latitude bands at which each phase persists all night, around the local summer solstice, are approximately:³

• Astronomical twilight (or its absence — sometimes called grey nights) above about 48°34′ N or S. Paris (48.9° N) just qualifies; Vienna (48.2° N) just doesn't. From here northwards, the Sun never falls 18° below the horizon at midsummer.
• Nautical twilight above about 54°34′. Hamburg, Edinburgh, Copenhagen — and most of southern Sweden, Norway, Estonia, Latvia. From here northwards, the Sun never falls 12° below the horizon at midsummer.
• Civil twilight above about 60°34′. Saint Petersburg, Helsinki, Stockholm, the southern half of Norway. From here northwards, the Sun never falls 6° below the horizon at midsummer; the sky stays bright enough for outdoor activity throughout the night. This is the latitude band Russian and Scandinavian writers traditionally have in mind when they speak of white nights.

Above about 81°25′ — well inside the Arctic and Antarctic circles — twilight persists for the full 24 hours through a months-long stretch around the solstice: the Sun is either above the horizon (midnight sun) or in the twilight zone, but never deep enough below for true night.³ Locations such as Tromsø, Murmansk, and Longyearbyen experience this every summer.

The mirror situation occurs at the winter solstice, when the same latitude bands experience polar night — the Sun never rises high enough to give civil daylight, with only twilight or full darkness for weeks at a time.

What can you see during twilight?

Twilight has a small catalogue of distinctive sky features that go almost unnoticed during full daylight or full night. They share a cause: the lit portion of the atmosphere is in geometric tension with the unlit portion, and the boundary between them shows up to a careful observer.

Earth's shadow. Look east at sunset on a clear evening with a low horizon. A few minutes after the Sun has dropped below the western horizon, a dark blue-grey band rises out of the eastern horizon — the planet's own shadow, projected onto the lower atmosphere on the opposite side of the sky. As the Sun continues to fall, the shadow rises further and spreads to span up to 180° of the eastern horizon. The same happens in reverse at sunrise: the shadow descends in the western sky. Atmospheric scientists sometimes call this band the twilight wedge or dark segment.⁴

The Belt of Venus. Capping Earth's shadow on its upper edge is a soft pink band, typically a few degrees wide. The pink colour comes from red sunlight backscattered by atmospheric particles still illuminated above the shadow line. There is no sharp boundary — the pink fades into the dark blue below and into the regular blue sky above — but on a clear evening with low horizon dust, the Belt of Venus is straightforwardly visible to the naked eye opposite the Sun for several minutes around the end of civil twilight.⁴

The blue hour. Around the end of civil twilight, the dominant tint of the western sky shifts from the warm reds and oranges of sunset to a deep saturated blue. This happens because the longer-wavelength red light is scattered out of the line of sight first, leaving the shorter-wavelength blue to reach the observer through the increasingly long atmospheric path. The colour is striking enough that photographers and painters have given it its own name — blue hour, or l'heure bleue in the original French.³

The brightest stars and planets. Venus, when well placed in the sky, is visible during civil twilight; its brightness exceeds the residual sky brightness even before the Sun has fallen 6° below the horizon. The brightest stars (first-magnitude objects: Sirius, Canopus, Arcturus, Vega, Rigel) emerge during civil twilight as well. Most navigational stars become usable for sextant fixes during nautical twilight. Faint stars — fifth and sixth magnitude, the dimmest the unaided eye can see — appear only after astronomical twilight has ended.

How is twilight used today?

Civil twilight is the operational threshold for several legal and regulatory definitions of "night". U.S. aviation regulations define night with reference to evening civil twilight, citing the figures published in the American Air Almanac; the same threshold appears in many U.S. state statutes that govern when vehicle headlights are required, when "nighttime burglary" applies, and when hunting becomes legal or illegal. The U.S. military uses begin-morning-civil-twilight (BMCT) and end-evening-civil-twilight (EECT) as planning milestones around dawn and dusk operations.

Nautical twilight retains its place in maritime practice. Watchkeeping schedules on commercial and naval vessels still treat morning and evening nautical twilight as the reliable windows for celestial fixes when GPS is unavailable or unreliable. The military planning acronyms BMNT and EENT — begin morning nautical twilight, end evening nautical twilight — show up in field manuals as the cue for heightened readiness around dawn and dusk.

Astronomical twilight is the threshold professional and amateur astronomers use to define when an observing session can begin or must end. Most observing schedules quote astronomical twilight times for the location and date; deep-sky imaging programmes typically wait for the end of evening astronomical twilight before starting and stop at the beginning of morning astronomical twilight. The Sun crossing the 18° threshold is, for an astronomer, the operational definition of night.

How does twilight relate to other sun events?

Twilight bookends two events the site covers in their own right. Solar noon — the Sun's highest point in the sky — sits halfway between morning and evening civil twilight on most dates. The equation of time shifts the actual clock-time of solar noon by up to 16 minutes through the year, but the symmetry of twilight around it holds.

The lit complement of twilight is golden hour: the period when the Sun is above the horizon but within roughly 6° of it, casting the warm low-angle light that photographers prize. Civil twilight begins as the Sun crosses 0°; golden hour ends just before. The two phenomena share a cause — long atmospheric path lengths preferentially scattering blue light — but differ in whether the Sun itself is visible.

The full picture of how civil time, sunrise, sunset, twilight, and solar noon relate at any given location for any given date is what the site's twilight calculator and city sun pages exist to make accessible — pick a city, see the six twilight times alongside sunrise, sunset, solar noon, and day length for the date.

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