It is the one fact everybody remembers about the equinox, and it is wrong everywhere on Earth — always in the same direction, and by more than you would guess. On the March equinox the day runs to 12h 07m at the equator and 12h 13m at Tromsø. Never 12h 00m, anywhere.
Sunrise is the first sliver, not the middle. The moment is defined by the sun’s upper limb touching the horizon, and the disc is about half a degree across. You get roughly 16 arcminutes of daylight at each end before the centre arrives.
The atmosphere lifts the image. Refraction near the horizon raises the sun by about 34 arcminutes — slightly more than its own diameter. When you watch the sun touch the horizon, it is already geometrically below it. The sunset you are looking at has technically already happened.
Together those put the event at a zenith angle of 90.833° rather than 90, which is the constant this tool uses. Both effects hand out free daylight at both ends of every day, so the error never cancels — it accumulates.
The day when light and dark really are equal has its own name: the equilux. In London it falls on about 18 March, two days before the equinox, and again in late September. Nearer the equator it drifts further away — Singapore never has one at all, because every day of its year is longer than 12 hours.
The same effect shows up if you add London’s longest and shortest days together: 16h 39m plus 7h 50m is 24h 28m, not 24 hours. The refraction bonus is paid on both days, so you count it twice.
Sun times use the NOAA approximation — Spencer’s series for declination and the equation of time, then one hour-angle solve. That is accurate to about a minute at mid-latitudes and degrades towards the poles, where the sun approaches the horizon so shallowly that a small error in altitude becomes a large one in time. Near the Arctic Circle around the solstices, treat the times as indicative.
The moon is a mean model. Phase comes from the mean synodic month of 29.530589 days counted from a reference new moon. The real orbit is elliptical, so true new moon can fall up to about 14 hours either side of this. That is fine for naming a phase or drawing the disc, and nowhere near good enough to predict an eclipse.
There is no moonrise or moonset here. The moon shifts about 13° a day against the stars, so its rising needs an iterative solve rather than the single step the sun gets. Adding it badly would be worse than leaving it out.
Refraction is the standard 34 arcminutes at sea level; real refraction varies with pressure and temperature and can move the horizon by minutes of time. Altitude is ignored — from a hill you see the sun earlier. No nutation, aberration, parallax or delta-T. Everything is checked against sunmoon_model.py, which holds London’s midsummer and midwinter day lengths to within a few minutes of the published figures.
Sunrise and sunset depend on just three things: your latitude, the date (which sets how the Earth is tilted toward the sun), and your longitude (which sets your clock relative to the sun). This tool computes the sun's declination and the hour angle at which it crosses the horizon, then converts to your city's local time — daylight saving included. No internet lookup, no data feed; it's pure astronomy.
On the June solstice, London gets about 16 hours 39 minutes of daylight — sunrise near 4:43 am, sunset near 9:21 pm. Six months later at the December solstice it's under 8 hours. The equator barely changes: about 12 hours all year.
The higher your latitude, the more extreme the swing. Past the Arctic or Antarctic Circle it becomes total — the sun never sets in midsummer, and never rises in midwinter.
The period when the sun is just below the horizon (up to 6° down) — dim but still light enough to see outside without artificial lighting. It's why it's bright well before sunrise and after sunset.
Because land and sea take weeks to warm up. Peak daylight is the solstice, but peak temperature lags by a month or two — the same reason the hottest part of the day is after noon, not at it.
Within about a minute for a flat sea-level horizon. Mountains, tall buildings, and altitude can shift the real moment you see the sun by several minutes either way.
Clocks run at a steady pace; the sun doesn't, quite — the Earth's tilt and elliptical orbit make solar noon wander by up to ~16 minutes over the year (the "equation of time"), plus an offset for your longitude within your time zone.
Each one is a single hour of the day at your chosen reference latitude — 24 in total, one per hour. A ray is only drawn if the sun is genuinely above the horizon at that hour; in deep summer at a high latitude, all 24 are lit at once, which is exactly what "24 hours of daylight" means in practice — not a metaphor, an actual count of how many of the day's 24 hours have the sun up.
It's a real, inhabited city at 69.6°N — genuinely far enough north to get true 24-hour daylight around the June solstice and 24-hour night around the December one, but still a place people actually live, rather than an abstract line on a map. The Arctic Circle, Svalbard, and the North Pole itself are there for comparison at increasing extremity.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.