Day length is the time that passes between sunrise and sunset — how long the sun is above the horizon where you are standing. It sounds like a single fixed number, and at the equator it very nearly is. Everywhere else it is a quantity in constant motion: longer every day for half the year, shorter every day for the other half, and swinging by wider margins the further you travel from the equator.
This page explains what actually drives that motion, why the answer differs so much between two places on the same date, and what the number does and does not include. If you just want today's figure for your own location, the calculator on the front page has it.
The Earth is tilted. Its axis of rotation sits about 23.4° away from vertical relative to the plane in which it orbits the sun, and — crucially — that tilt keeps pointing the same way in space all year round. It does not wobble to follow the sun.
The consequence is that for half of the orbit your hemisphere leans towards the sun, and for the other half it leans away. Leaning towards the sun means the sun traces a higher, longer arc across your sky: it rises further north of east, climbs higher at midday, and sets further north of west. A longer arc takes longer to travel, so the day is longer. Leaning away compresses the same arc into a short, low path, and the day is short.
Two things follow from this that people often find counterintuitive. First, the seasons are not caused by the Earth being nearer the sun in summer — the distance barely changes, and the northern hemisphere is actually slightly further from the sun in July than in January. Second, the two hemispheres are exact opposites: the longest day of the year in Oslo is the shortest in Wellington, on the same date.
Latitude is the single strongest influence on how much your day length varies. The tilt is the same for everyone, but how much of it you feel depends on how far you are from the equator.
You can see the whole shape of this for any location on the year chart, or compare two cities side by side on the comparison page. The pattern is always the same curve; latitude just decides how tall it is.
Day length does not change at a constant rate. It follows something close to a sine wave, which means the fastest change happens in the middle of the climb and the slowest at the top and bottom.
In practice this means the two equinoxes, in late March and late September, are when the light is moving fastest. At the latitude of London or Copenhagen the day grows or shrinks by roughly four minutes every twenty-four hours around then — about half an hour a week, which is fast enough to notice from one weekend to the next. Our page on when the days start getting longer goes through this in detail.
Around the solstices, the opposite. For a couple of weeks either side of the shortest day the change is under a minute a day, and it can be a few seconds. This is why late December feels stuck: the solstice has passed, the days are technically getting longer, and yet nothing seems to be happening. It takes until mid-January for the gain to become obvious.
The word equinox means "equal night", and the definition is real: it is the moment the sun's centre crosses the celestial equator, when the geometric day and night are equal everywhere on Earth. But if you look up the actual sunrise and sunset times on that date, the day comes out longer than twelve hours — by about nine minutes at the latitude of Denmark or Britain, and by more the further north you go.
There are two reasons, and both are about the definition of sunrise rather than about astronomy going wrong.
We measure from the edge, not the centre. Sunrise is counted from the instant the sun's upper edge clears the horizon, not the instant its centre does. Since the disc is about half a degree wide, that gives you a free head start in the morning and an extension in the evening.
The atmosphere bends light. Air refracts light passing through it, and near the horizon the effect is strong enough — a little over half a degree — that you can see the sun while it is still geometrically below the horizon. Sunrise happens before it "should", and sunset after.
Add the two together and the sun is visible for several minutes longer than pure geometry allows, at both ends of the day. The date on which day and night really are equal — sometimes called the equilux — falls a few days before the March equinox and a few days after the September one.
Sunset is not the end of usable light, and this is where the number can mislead. After the sun goes down there is a long, structured decline through three stages of twilight, each defined by how far the sun has sunk below the horizon:
How long this takes depends heavily on latitude, because it depends on the angle at which the sun descends. Near the equator the sun drops almost vertically and dusk is over in about twenty minutes. In northern Europe it slides down at a shallow angle and civil twilight alone can last three quarters of an hour — and in high summer the sun never gets 18° below the horizon at all, which is why Scandinavian nights in June never become properly dark. The when does it get dark page covers this, and when is it lightest covers the other end of the scale.
The seasonal swing in daylight is not merely a curiosity. Light is the primary signal that sets the human circadian clock, and at high latitudes the size of the swing is large enough to have measurable effects — on sleep timing, on mood, and on the seasonal patterns that come with them. It also drives almost everything in the natural world around you: when trees come into leaf, when birds migrate, when animals breed. Plants respond to day length far more reliably than to temperature, which is why a warm February does not fool a tree into budding.
It is also the reason daylight saving time exists — although it is worth being precise about what the clock change does. Moving the clock does not create or destroy a single minute of daylight. It only relabels which hours the light falls in, moving an hour of it from the early morning to the evening.
Every figure on this site is calculated rather than looked up. Given a latitude, a longitude and a date, the standard solar position formulae give the moments at which the sun crosses each defined altitude, and day length is the interval between the sunrise and sunset crossings. The timezone is derived from the coordinates themselves, so the times are shown in local clock time — with daylight saving applied correctly, including for dates in the past.
The times assume a flat, unobstructed horizon at sea level. That assumption is where most real-world discrepancy comes from: a hill or a row of buildings to your west will take the sun several minutes earlier than the calculation says. If you want the full detail — the exact altitude thresholds, how polar day and night are handled, and how accurate any of it is — it is written up on the how we calculate the times page.
As the time between sunrise and sunset, where sunrise is counted from the moment the upper edge of the sun's disc appears above the horizon, and sunset from the moment that same edge disappears.
Because sunrise and sunset are measured at the edge of the sun's disc rather than its centre, and because the atmosphere bends light so the sun is visible while it is still geometrically below the horizon. Together these add roughly nine minutes at the latitude of Denmark or Britain.
No. After sunset comes civil twilight, with the sun up to 6° below the horizon, when it is still light enough to read outdoors. In Copenhagen that lasts about 40 minutes on an August evening; in Singapore, a little over 20.
Around the equinoxes, and the further you are from the equator the faster it goes. At the latitude of London or Copenhagen the day grows or shrinks by roughly four minutes every twenty-four hours in late March and late September. At the equator the change is barely a few seconds a day all year.