Where the sun is,
and what the day will do.

What is worked out, and how

Everything on this page is arithmetic done in your browser; the sun’s figures make no network request at all. The sun’s declination and the equation of time come from the NOAA Global Monitoring Laboratory’s General Solar Position Calculations, the equations behind NOAA’s own solar calculator, lifted out of this site’s Clockwork page into a shared file, assets/js/sun-core.js. Each crossing is iterated: the sun’s position is recomputed at the moment of the event itself rather than at noon, which matters most near the poles.

The equinoxes and solstices are Meeus, Astronomical Algorithms, chapter 27, with its twenty-four periodic terms. The eclipses come from moon-events.js, on the Meeus lunar and solar theories the moon page is drawn from; the moon’s height over a place adds sidereal time and the moon’s parallax. The hundred cities are Clockwork’s own register, coordinates to a hundredth of a degree; time zones are your browser’s copy of the IANA database.

The lines, defined

Sunrise and sunset: the sun’s centre 0.833° below the horizon — half a degree of refraction and a quarter-degree of its own radius, so the upper edge is just touching a sea horizon. Civil, nautical and astronomical twilight end at 6°, 12° and 18° below. Golden hour and blue hour have no official definition; this page uses the photographers’ commonest: golden while the sun is between 6° up and 4° down, blue between 4° and 6° down. The day length is sunset less sunrise; a day with no sunset is counted as twenty-four hours, one with no sunrise as none.

Checked against the Naval Observatory

Before it went in, the core was run against the US Naval Observatory’s published figures for twelve places — London, New York, Sydney, Singapore, Reykjavik, Tromsø, Honolulu, Anchorage, Quito, Cape Town, Tokyo and McMurdo — on seven dates each. Of 385 times (sunrises, sunsets, solar noons and civil twilights), 371 print the same minute as the Observatory’s and 14 are one minute apart; the largest difference underneath the rounding is about half a minute. The polar days and nights are called the same way, and 36 equinoxes and solstices from 1950 to 2099 land within a minute and a half. Run over 2026–2028 the eclipse finder returns all six solar and eight lunar eclipses in the published record, each of the right kind, and invents none.

What it does not know

Your height above the sea, the hills on your horizon and the air on the day. A mountain in the east makes sunrise later; a high balcony makes it earlier; unusual refraction moves it by a minute or more either way. Near the poles, where the sun skims the horizon, all of those grow, and so does the arithmetic’s own error. Days before 1901 or after 2099 are refused rather than guessed. A typed latitude and longitude has no time zone of its own here, so it is given the nominal one for its longitude — fifteen degrees to the hour — and the page says so; the zone menu offers this device’s zone and UTC instead.

Your location

“Use my location” asks the browser once, and only when you press it. The answer is rounded to a hundredth of a degree — about a kilometre — and kept in this browser’s own storage so the page opens on it next time. It is never sent anywhere: nothing on this page asks a server about the sun. The site’s shared furniture — the menu’s search, the assistant, the analytics every labs.llc page carries — loads as it does everywhere else and is never given your position. Your browser’s network panel will show it. Questions and corrections: labs@labs.llc.