Sunrise, Sunset and Twilight Explained

Time & Astronomy

Sunrise looks like the simplest event in astronomy: the Sun comes up, and someone writes down the time. In fact the Sun is still entirely below the horizon at the moment your table calls sunrise, the length of twilight afterwards depends on where you are standing and what month it is, and whether the whole thing happens at 06:00 or at 10:00 is decided less by geography than by which time zone your government picked.

Key takeaways

Sunrise is the moment the upper edge of the Sun appears, not its centre.
Refraction lifts the image of the Sun by roughly its own width, so you see it rise a few minutes before it geometrically arrives.
Twilight comes in three grades — civil, nautical and astronomical — defined by how far the Sun is below the horizon.
Twilight is brief at the equator and can last all night in summer above about 48.5 degrees of latitude.
The earliest sunrise does not fall on the longest day, because clock time and solar time disagree by up to 16 minutes.
Time zone borders move sunrise by hours: the same solar event reads as 06:00 on one side of a line and 10:00 on the other.

What Sunrise Actually Means

The official definition is narrow and worth knowing: sunrise is the instant when the upper limb of the Sun first appears on an ideal, flat, sea-level horizon, and sunset is the instant when it finally disappears. Two corrections separate that from the naive picture of the centre of the Sun crossing the line.

The disc is about half a degree wide, so its upper edge arrives roughly 16 arcminutes before its centre would.
The atmosphere refracts light passing through it at a shallow angle, raising the apparent position of the Sun by about 34 arcminutes at the horizon.

Add them and the geometric centre of the Sun sits about 50 arcminutes below the horizon at the moment of official sunrise. In other words the Sun you see resting on the horizon is not there at all: the entire disc is already below it, and you are looking at an image bent over the curve of the Earth. The effect adds several minutes of daylight at each end of the day — more at high latitudes, where the Sun approaches the horizon at a shallow angle — and is the reason an equinox does not deliver exactly twelve hours of light, as explained in Solstices and Equinoxes Explained.

Published tables also assume a flat horizon at sea level. A mountain to the east delays sunrise; standing on a hill brings it forward, by about a minute for the first 1.5 kilometres of altitude and more thereafter. For anywhere with real terrain, treat the printed time as a good approximation rather than a measurement.

The Three Twilights

Twilight is graded by the depth of the Sun below the horizon, and each band has a practical definition rather than an aesthetic one:

Stage	Sun below horizon	What it means on the ground
Civil	0° to 6°	Bright enough to read and to work outdoors; the brightest stars appear. Most lighting-up laws use this boundary.
Nautical	6° to 12°	The horizon at sea is still visible against the sky, so a sextant can be used against known stars.
Astronomical	12° to 18°	Almost dark; faint objects are still washed out. Below 18 degrees, the sky is as dark as it will get.

Photographers work to a different pair of names for the same geometry. The golden hour is the low-Sun period just after sunrise and before sunset, when light travels a long path through the atmosphere and turns warm; the blue hour is the civil twilight after that, when the sky holds a deep, even blue that balances well against artificial light.

Why Twilight Is Quick at the Equator and Endless in the North

The Sun sets along a path perpendicular to the horizon at the equator and at an increasingly shallow angle as you move towards the poles. Since twilight lasts until the Sun has dropped 18 degrees, the shallower the angle the longer the journey:

Where	Roughly	Civil twilight lasts
Equator	Quito, Nairobi, Singapore	About 20 minutes, all year
40°N	New York, Madrid	25 to 35 minutes
52°N	London, Warsaw	35 to 50 minutes, longest in summer
60°N	Saint Petersburg	Effectively all night around midsummer

Beyond certain latitudes the Sun never gets deep enough for the next stage of darkness to begin at all. The thresholds around the June solstice are worth remembering:

Above about 48.5° — no astronomical darkness. Paris sits just past this line, so its summer sky never becomes fully dark.
Above about 54.5° — no nautical darkness. This covers most of Scotland, Denmark and the Baltic.
Above about 60.5° — no true night at all: civil twilight from dusk to dawn. These are the famous white nights of Saint Petersburg.
Above 66.6° — the polar circles, where the Sun does not set at all around midsummer, as in Tromso, and does not rise around midwinter.

The winter version is just as extreme. Fairbanks gets under four hours of daylight in late December, and Reykjavik a little over four, with the Sun barely scraping above the rooftops. Push all the way to the poles and the cycle degenerates into a single sunrise and a single sunset each year, which is where clock time stops meaning anything at all.

How Long the Sun Takes to Set

The disc itself takes about two minutes to cross the horizon at the equator, a little under three at 40 degrees, and five or more above 60 degrees, where it slides sideways rather than dropping. Two curiosities belong to this moment:

The green flash, a brief green rim on the last visible sliver, produced when the atmosphere disperses sunlight like a weak prism. It needs a clean, distant horizon, which is why it is a sailor’s sight.
The flattened Sun. Refraction is stronger nearer the horizon, so the lower limb is lifted more than the upper one and the disc is visibly squashed into an oval as it sets.

The Longest Day Does Not Have the Earliest Sunrise

Daylight is longest on the solstice, but the earliest sunrise arrives days before it and the latest sunset days after. At 50 degrees north the gap is around a week at midsummer and a fortnight at midwinter.

The reason is the equation of time. A clock ticks out identical 24-hour days, while the real Sun runs early or late by up to about 16 minutes over the year, because the Earth moves faster along its orbit in January than in July and because its axis is tilted. Sunrise and sunset are shifted together by that amount, so the symmetrical curve of day length sits on top of an asymmetrical offset. Plot the position of the Sun at clock noon over a year and it traces a figure of eight, the analemma, which is that offset made visible.

This mismatch is also why civil time abandoned the Sun. Mean time — an averaged, evenly ticking day — replaced local apparent time precisely because sundials and clocks could not be reconciled, a change traced in The History of Time Zones.

Your Time Zone Decides When the Sun Comes Up

Sunrise happens when the rotation of the Earth brings you into the light. What number your clock shows at that moment is a political decision, and the effect dwarfs anything astronomy does:

Spain keeps Central European Time despite lying at the longitude of Britain, so Vigo in the far west sees the Sun set well after 22:00 in midsummer, and Madrid runs an hour behind the Sun all year.
China uses a single zone across a country five zones wide. In Kashgar, in the far west, official sunrise in midwinter falls close to 10:00, which is why much of Xinjiang informally keeps a second clock two hours behind Beijing and why Urumqi keeps offices on later hours.
Daylight saving time moves the whole pattern by an hour on a fixed date, without touching the Sun at all — the entire purpose being to shift the daylight to the other end of the working day. See What Is Daylight Saving Time?

The general rule is that a place on the western edge of its time zone gets late sunrises and late sunsets, while one on the eastern edge gets early ones, with up to two hours of difference across a wide zone before daylight saving is even considered. The mechanics are in How Time Zones Work Around the World, and the most extreme cases in The Strangest Time Zones in the World.

Sunrise and sunset are local events: unlike a solstice or an eclipse of the Moon, they happen at a different instant for every point on Earth, so they are always published in local time rather than converted from UTC. The distinction matters when you are reading an astronomical timetable, and is set out in How Astronomical Events Are Converted to Local Time. Current sunrise and sunset for any city are on its clock page.

Frequently Asked Questions

What exactly counts as sunrise?

The moment the upper edge of the Sun first appears above an ideal sea-level horizon. Because the atmosphere bends light, the whole disc is actually still below the horizon at that instant, and you are seeing a refracted image of it.

What is the difference between civil, nautical and astronomical twilight?

They are defined by how far the Sun has dropped below the horizon: 0 to 6 degrees for civil twilight, 6 to 12 for nautical and 12 to 18 for astronomical. Civil twilight is bright enough to work in, nautical twilight still shows a sea horizon, and after astronomical twilight ends the sky is fully dark.

Why does twilight last so long in summer in northern Europe?

Because the Sun sets at a shallow angle at high latitudes and therefore takes far longer to reach 18 degrees below the horizon. Above roughly 48.5 degrees it never gets that far around midsummer, so the sky never becomes astronomically dark.

Why is the earliest sunset not on the shortest day?

Because clock time and solar time drift apart by up to 16 minutes over the year, an effect called the equation of time. It shifts sunrise and sunset together, so the earliest sunset falls before the December solstice and the latest sunrise after it.

Why does the Sun rise so late in western China and Spain?

Because both use a clock well ahead of their longitude. China runs a single time zone across a span that would naturally hold five, and Spain keeps Central European Time despite sitting at the longitude of the United Kingdom, so official sunrise arrives an hour or two later than the Sun would suggest.