When is it lightest and darkest during the year?
It is lightest around the summer solstice on 21 June and darkest around the winter solstice on 21–22 December in the Northern Hemisphere. In London, day length swings from about 16 hours 39 minutes at midsummer to roughly 7 hours 50 minutes in midwinter. Below you can see when – during the year and during the day – it is lightest and darkest, and why.
When is it lightest during the year?
The year peaks for light at the longest day of the year around 21 June. The sun is highest in the sky at midday and there are the most hours of daylight. In the weeks around the summer solstice the difference from day to day is tiny – the days «stand almost still» – before slowly shortening again towards autumn.
Note that the earliest sunrise and the latest sunset do not fall exactly on the solstice. The earliest sunrise comes a few days before 21 June, and the latest sunset a few days after. This is due to Earth's elliptical orbit and axial tilt.
When is it darkest during the year?
It is darkest around the shortest day of the year at the winter solstice. There are the fewest hours of daylight and the sun stays low. After the winter solstice the days start getting longer again – slowly at first, then faster towards the spring equinox, when day length grows the most (up to 4–5 minutes per day).
When is it lightest and darkest during the day?
On a daily basis it is lightest at solar noon, when the sun is highest. It is darkest in the middle of the night, as far as possible from solar noon. Between day and night lies twilight: in summer at high latitudes it never gets fully dark, because the sun does not drop far enough below the horizon.
To see exact figures for your location, use the calculator on the front page, check the solar calendar, or view the whole year in the year chart. You can also look up sunrise and sunset for your city.
Light is not the same as warmth
The lightest part of the year and the warmest part of the year are about six weeks apart, and the gap catches people out every summer. The longest day falls in late June, yet late July and early August are reliably hotter. In the other direction, the shortest day is in December while the coldest weeks are usually late January and February.
The effect is called seasonal lag, and it happens because temperature depends on accumulated energy rather than instantaneous input. Around the solstice the ground and — much more importantly — the ocean are still absorbing more heat each day than they radiate away, so the stored total keeps climbing even as the days begin to shorten. The peak arrives only when the daily gain and loss finally balance.
How long the lag lasts depends on how much water is nearby. Maritime climates like Britain, Ireland and Denmark lag by six to eight weeks because the sea is slow to warm and slow to cool; continental interiors lag by three or four. It is the same physics that makes coastal winters mild and inland winters severe.
Clock noon is not solar noon
The brightest moment of any day is solar noon — the instant the sun crosses your meridian and reaches its highest point. It is almost never twelve o'clock.
Two things push it around. The first is longitude within your timezone: a timezone is typically 15° of longitude wide but keeps a single clock, so a place at the western edge sees solar noon up to an hour later on the clock than a place at the eastern edge. The second is daylight saving, which adds another hour on top for much of the year. In western Britain in June, solar noon lands well after 13:00; in western Spain, which keeps central European time, it can be close to 14:00.
This matters practically because the sun's height, not the clock, is what drives UV and heat. The advice to avoid the sun "between noon and three" is a rough translation of "around solar noon", and in some places it is off by a good hour.
The equation of time
There is a further wrinkle: solar noon does not even stay put relative to the clock from day to day. It drifts back and forth across the year by up to about sixteen minutes, following a curve called the equation of time.
Two effects combine to produce it. The Earth's orbit is an ellipse, so the planet moves faster when nearer the sun in January and slower in July; and the Earth's axis is tilted, so the sun's apparent motion is not parallel to the celestial equator for most of the year. Neither has anything to do with clocks being inaccurate — a clock ticks uniformly, and the sun does not.
This is the reason the earliest sunrise and the latest sunset do not fall on the longest day, but a few days either side of it. Around the June solstice the whole solar day is shifting slightly later, so the earliest sunrise arrives before the solstice and the latest sunset after it. In December the same effect is larger, and the earliest sunset can precede the shortest day by more than a week — which is why the evenings start drawing out well before Christmas even though the mornings keep getting darker into January.
How high does the sun actually get
The maximum altitude the sun reaches at your location on any date follows a simple rule: roughly 90° minus your latitude, plus or minus the 23.4° axial tilt depending on the season.
- London (51.5°N): about 62° at the June solstice, about 15° in December.
- Copenhagen (55.7°N): about 58° in June, about 11° in December.
- Tromsø (69.7°N): about 44° in June, and the sun does not rise at all around the December solstice.
- Singapore (1.3°N): within a few degrees of straight overhead twice a year.
That difference is why northern winter light feels so distinctive. A sun 11° above the horizon casts shadows five times the height of whatever is casting them, never rises above the rooftops in a city street, and delivers so little energy per square metre that the UV index stays near zero for months. It is also why northern summer evenings are so long: a sun that rises high must also take a long, shallow route back down. See when it gets dark for that side of it, and what day length is for the underlying geometry.
Frequently asked questions
When is it lightest during the year?
It is lightest around the summer solstice on 21 June, the longest day of the year, when there are the most hours of daylight.
When is it darkest during the year?
It is darkest around the winter solstice on 21–22 December, the shortest day of the year, when there are the fewest hours of daylight.
Why doesn't the earliest sunrise fall on the longest day?
The earliest sunrise comes a few days before the summer solstice, and the latest sunset a few days after. This is due to Earth's elliptical orbit around the sun and its axial tilt, which shift solar noon slightly through the year.