Year chart

The year chart draws day length for every single day of the year as one continuous curve. Instead of looking up individual dates, you see the whole shape at once: how steeply the days grow through March, how flat the curve lies around midsummer, and how fast the light drains away again in October. The solstices and equinoxes are marked, so the turning points are easy to place.

Why the curve is steepest at the equinoxes

The curve looks like a wave, and that is no accident. Day length follows the Earth's 23.4° tilt relative to its orbit around the Sun, and the day-to-day change is largest where the curve crosses the middle — around the equinoxes. London gains about 4 minutes of daylight per day around the spring equinox, Edinburgh nearly 5. That is a difference you notice from one week to the next.

At the solstices the opposite happens. The curve flattens out completely and the day-to-day change falls close to zero — which is where the name comes from: the sun "stands still". In the days around 21 June and 21 December, day length shifts by seconds rather than minutes. That is why late June feels like one long unchanging stretch of light, while March and September move quickly.

The curve is not quite symmetrical

Look closely and sunrise and sunset are not mirrored about the middle. The cause is the equation of time — the combined effect of the Earth's elliptical orbit and its axial tilt. It puts solar time up to 16 minutes ahead of or behind clock time over the year. That is why December's earliest sunset falls well before the shortest day, and June's earliest sunrise well before the longest one.

How tall the curve stands depends on where you are. The further from the equator, the wider the swing: on 21 June London gets about 16 hours 38 minutes of daylight while Edinburgh, 530 km north, gets about 17 hours 37 minutes. At the equator the curve would be nearly flat all year.

The same shape everywhere, at different heights

Plot day length against date for anywhere on Earth and you get a curve close to a sine wave: a peak at the June solstice, a trough in December, and two steep crossings in between. What changes from place to place is not the shape but the amplitude — how tall the wave is.

• Near the equator the curve is almost a flat line at twelve hours. Quito varies by around ten minutes across the entire year.
• Mid-latitudes give the familiar shape. London runs from about 7h 50m to about 16h 38m — a swing of nearly nine hours.
• High latitudes exaggerate it further. Copenhagen swings by more than ten hours, Stockholm by twelve.
• Inside the polar circles the curve stops being a wave at all. It flattens against ceilings of 24 hours and floors of zero, and the further poleward you go the longer it spends pinned to each. At the pole itself the "curve" is essentially a square wave: six months of light, six of dark.

The southern hemisphere gives the same curve inverted. Switching between a northern and a southern city on the comparison page shows the two waves in antiphase, crossing at the equinoxes.

Reading the gradient, not the value

The height of the curve tells you how long today is. The slope tells you something people generally find more interesting: how fast that is changing, and therefore how much difference next week will make.

The steepest part of the curve is at the equinoxes, where at British or Danish latitudes the day is gaining or losing three to four minutes every twenty-four hours. That is around 25 minutes a week — enough that a Saturday walk in late March is noticeably lighter at the end than the previous Saturday's. The flattest parts are the solstices, where a fortnight can pass with almost no change at all.

This is why late December feels stagnant even though the days are technically lengthening, and why the light seems to arrive suddenly in February and March. Nothing sudden has happened; you have simply moved from the flat part of the curve onto the steep part. Our page on when the days start getting longer works through the numbers.

The milestones on the curve

The chart marks the points worth knowing, and they do not all fall where you would expect:

• The summer and winter solstices, at the peak and the trough.
• The two equinoxes, at the steepest crossings — where the curve passes through roughly twelve hours, though as the equinox page explains, not exactly.
• The earliest sunrise and latest sunset, which are not at the June solstice, and the earliest sunset and latest sunrise, which are not at the December one. Those are displaced by the equation of time, sometimes by well over a week.

That last group is the most common source of confusion about the whole subject, and it is the clearest argument for looking at a chart rather than a single date: the extremes of day length and the extremes of clock times are different things, and the chart shows both.

Where to go next

For a specific month day by day, use the solar calendar. For how the pattern varies geographically, the daylight map or the 3D globe. For the physical explanation of why the curve exists at all, what day length is; and for how the numbers are computed, how we calculate the times.

Frequently asked questions

When does day length change fastest?

Around the equinoxes in March and September. In the UK day length then shifts by roughly 4 to 5 minutes per day, while around the June and December solstices it barely changes at all.

Why is the curve flat around the solstices?

Because a solstice is the turning point of the curve. The Sun's highest or lowest path changes only very slightly from day to day near the turning point, so day length shifts by seconds rather than minutes.

Why is the year chart not quite symmetrical?

Because of the equation of time. The Earth's orbit is an ellipse and its axis is tilted, so solar time runs up to 16 minutes ahead of or behind clock time. That shifts sunrise and sunset relative to each other over the course of the year.