Glossary

Most of the words below have an exact meaning that popular usage has blurred. Golden hour is not an hour, twilight is not one thing, and sunset is defined by the top edge of the sun rather than its centre.

Where a definition rests on a specific number, that number is here, because it is usually the number that explains why two websites disagree with each other.

Altitude

The angle of an object above the horizon, measured in degrees. Zero is the horizon, +90 is straight overhead, negative values are below the horizon.

Nearly every term on this site is defined by an altitude of the sun: sunset at −0.833, civil twilight at −6, golden hour between +6 and −4. Altitude is the variable; everything else is a threshold on it.

Analemma

The figure-of-eight shape traced by the sun if you photograph it from the same place at the same clock time throughout a year.

It exists because clock time and solar time drift apart — see the equation of time — and because the sun's declination changes with the seasons. The vertical extent of the figure is the declination swing; the horizontal width is the equation of time.

Astronomical twilight

The period when the sun is between 12 and 18 degrees below the horizon.

The faintest of the three twilight bands. At its end the sky is as dark as it will get and the faintest stars and the Milky Way become visible. For most practical purposes it is already night throughout — the distinction matters to astronomers and to almost nobody else. Set out in full in twilight explained.

Atmospheric refraction

The bending of light as it passes through the atmosphere, which lifts the apparent position of the sun above its true one.

Near the horizon the effect is about 34 arcminutes, slightly more than the sun's own diameter. The practical consequence is that when you see the sun touching the horizon, it is geometrically already below it. Every sunset time on every site includes an allowance for this, and the standard allowance is 34 arcminutes at a standard atmosphere.

Refraction is also why the Arctic Circle is not quite where the textbook puts it: it lifts the sun into view about a degree of latitude further north than pure geometry allows.

Azimuth

A compass bearing measured clockwise from true north in degrees. North is 0, east 90, south 180, west 270.

The sunset azimuth is where on the horizon the sun goes down, and it moves through the year — by 79 degrees in London, by less than 50 at the equator. This is the number that decides whether a given viewpoint works on a given date, and it is the subject of sunset direction through the year.

Blue hour

The period when the sun is between 4 and 6 degrees below the horizon, when the sky takes on a deep even blue and artificial lights balance against the remaining daylight.

Shorter than golden hour — typically 20 to 35 minutes at mid-latitudes — and, like it, not an hour. It falls inside civil twilight, beginning where golden hour ends in the evening. See what is blue hour.

Civil twilight

The period when the sun is between 0 and 6 degrees below the horizon.

The brightest twilight band, and the one with legal force in many jurisdictions: lighting-up times, hunting hours and aviation rules are often written against it. At its end there is still just enough light to make out shapes outdoors without artificial light, which makes the end of evening civil twilight the closest thing there is to an official answer to when it gets dark.

Culmination

The moment an object reaches its highest altitude of the day, which for the sun is solar noon.

Day length

The interval between sunrise and sunset, using the same −0.833 degree definition at both ends.

At the equator it is close to 12 hours all year. In London it swings by nearly nine hours between the solstices; above the Arctic Circle it reaches both 0 and 24. The shortest and longest day has the figures by city.

Daylight saving time

The practice of moving clocks forward by an hour for part of the year, so that a given clock time falls earlier in the solar day.

Nothing about the sun changes. What changes is the label attached to the moment, which makes sunset appear to jump an hour overnight. Roughly a third of countries observe it, almost none of them near the equator. See what daylight saving does to sunset.

Declination

The sun's angular distance north or south of the celestial equator, ranging from +23.44 degrees at the June solstice to −23.44 at the December solstice.

It is the single quantity that drives the seasons. Everything seasonal on this site — day length, sunset bearing, golden hour duration — is declination filtered through your latitude.

Elevation

Two different things share this word, which is a persistent source of confusion.

Elevation as an angle: the height above the horizon in degrees, used interchangeably with altitude.

Elevation as a height: metres above sea level. Height above the surrounding terrain makes sunset later, because you can see over the geometric horizon — roughly a minute for the first 100 metres, and less per metre after that. Times on this site are computed at sea level, matching the convention used by national observatories, because the correction depends on the terrain around you and not only on your own height.

Equation of time

The difference between apparent solar time and mean clock time, ranging from about −14 minutes in February to +16 minutes in early November.

It has two causes: the Earth's orbit is elliptical, so the planet moves faster near perihelion, and the axis is tilted, so the sun's apparent motion along the ecliptic projects unevenly onto the equator.

This is the quantity that makes the earliest sunset fall before the shortest day, and it is the horizontal width of the analemma.

Equinox

The two moments each year when the sun's declination is zero, around 20 March and 22 September.

On those dates the sun rises very nearly due east and sets very nearly due west everywhere on Earth, and day and night are close to equal — though not exactly, because refraction and the sun's diameter add several minutes of daylight at both ends. The date of genuinely equal day and night, the equilux, falls a few days off the equinox and depends on latitude.

Golden hour

The period when the sun is between 6 degrees above and 4 degrees below the horizon, during which sunlight is warm, low and soft.

Not an hour. It lasts about 40 minutes at the equator, an hour or so in London, and can run all night above about 60 degrees of latitude in midsummer. Defining it by angle rather than by a fixed offset from sunset is what makes it vary, and the variation is the point. See what is golden hour.

Latitude

Angular distance north or south of the equator, from 0 at the equator to 90 at the poles.

The dominant variable on this site. Two cities at the same latitude have nearly identical day lengths and twilight durations whatever their longitude; two cities at the same longitude and different latitudes have almost nothing in common.

Longitude

Angular distance east or west of the Greenwich meridian.

Longitude decides when things happen — four minutes of clock time per degree — but not how long they last. Its effect on your sunset time is mostly hidden inside your time zone, which is why cities in one zone can differ by well over an hour.

Midnight sun

The condition where the sun does not set for at least 24 hours, occurring above roughly 65.7 degrees of latitude in summer.

The textbook boundary is the Arctic Circle at 66.56 degrees, but refraction and the sun's radius push the real boundary about a degree closer to the equator. Covered in midnight sun and polar night.

Nautical twilight

The period when the sun is between 6 and 12 degrees below the horizon.

Named for the practice of sighting stars against a sea horizon that is still faintly visible, which is possible through this band and not after it. The horizon becomes indistinguishable at its end.

Polar night

The condition where the sun does not rise for at least 24 hours: the winter counterpart of the midnight sun.

Polar night is rarely as dark as the name suggests. For most of the inhabited Arctic the sun stays close enough below the horizon that civil or nautical twilight still occurs around midday, giving several hours of blue daylight.

Rayleigh scattering

The scattering of light by molecules much smaller than its wavelength, which is stronger for short wavelengths in proportion to the inverse fourth power.

It makes the sky blue overhead and the sun red at the horizon through one mechanism: at sunset the light has travelled through a far longer column of air, and nearly all of the blue has been scattered out of the beam before it reaches you. See why sunsets are red.

Solar noon

The moment the sun crosses the local meridian and reaches its highest point of the day.

It is the true midpoint of the day and it is rarely at 12:00. Your position within your time zone, the equation of time and daylight saving all shift it; in Madrid in July it falls at 14:16.

Solstice

The two moments each year when the sun reaches its greatest declination, around 21 June and 21 December.

The word means "sun standing still", which describes the seasonal march rather than the sky: within a fortnight either side of a solstice, day length changes by less than a minute a day.

Sunrise

The moment the upper edge of the sun first appears above the horizon, taken as the moment its centre reaches an altitude of −0.833 degrees.

That figure is the sum of two corrections: 34 arcminutes of refraction and the 16 arcminutes of the sun's apparent radius. Sunrise refers to the upper limb, not the centre, which is why the radius is in there.

Sunset

The moment the upper edge of the sun disappears below the horizon, using the same −0.833 degree threshold as sunrise.

Subtracting the sun's radius a second time — an error present in a widely copied piece of open-source code — moves the answer by about 90 seconds, which is why some sites disagree with each other.

Sunset quality score

The 0–100 figure shown in the conditions box on each city page, estimating how likely tonight's sky is to produce colour. It is our own model, not a standard, so the formula is set out here in full.

It starts from a neutral 55 and adjusts on the forecast for the hour of sunset:

• High cloud between 20 and 70 per cent with low cloud under 15 per cent: +20. This is the setup that produces colour — something up high to catch reddened light, and a clear western horizon for that light to travel through.
• Mid cloud between 20 and 60 per cent with low cloud under 25 per cent: +10.
• Visibility above 15 km: +5.
• Low cloud above 50 per cent: −40. Above 25 per cent: −15. Low cloud sits between the sun and everything else.
• Any layer above 90 per cent: −50.
• Visibility below 5 km: −25.
• Precipitation probability above 50 per cent: −20.
• Relative humidity above 85 per cent: −5.
• A completely empty sky, every layer under 5 per cent: −10. Clean light, but nothing for it to land on.

The result is clamped to 0–100 and labelled Poor below 35, Fair to 55, Good to 75, Excellent above that.

Two caveats worth stating. The score reads a forecast, so it inherits every error the forecast has, and it knows nothing about the terrain to your west — a hill or a building will beat any sky. The reasoning behind the model is in do clouds make better sunsets. Forecast data comes from Open-Meteo.

Twilight

The period before sunrise and after sunset when the sky is lit by sunlight scattered from the upper atmosphere while the sun itself is below the horizon.

It is divided into three bands by sun altitude — civil, nautical and astronomical — and its total duration depends strongly on latitude, from about 70 minutes at the equator to all night in high-latitude summer.

Upper limb

The top edge of the sun's disc, as opposed to its centre.

Sunrise and sunset are defined by the upper limb, which is why their threshold carries the sun's 16-arcminute apparent radius. Twilight thresholds, by contrast, are defined by the centre — a distinction that is easy to lose and worth 90 seconds when it is lost.

UTC offset

The difference between a location's clock time and Coordinated Universal Time, expressed as hours and minutes.

Offsets are political rather than astronomical. They can be a whole hour, 30 minutes or 45, and can change with the seasons. Every time on this site is computed as an absolute instant and then converted using the IANA time zone database, which carries the full history of every zone's transitions.

White nights

The informal name for the condition where twilight lasts all night: the sun sets, but never falls far enough below the horizon for full darkness to arrive.

It occurs between roughly 48.5 and 66.5 degrees of latitude in midsummer, and is why St Petersburg, Stockholm and Edinburgh have bright midsummer nights without being anywhere near the Arctic Circle.

Zenith

The point directly overhead, at an altitude of +90 degrees.

The zenith angle is the complement of altitude — 90 degrees minus the altitude — so an object on the horizon has a zenith angle of 90. Between the tropics the sun passes through the zenith exactly twice a year, and on those two dates a vertical object casts no shadow at solar noon.