A year of light.

The equinoxes and solstices are the instants the sun's apparent longitude reaches 0°, 90°, 180° and 270°, computed by Meeus's method (Astronomical Algorithms, chapter 27: the mean instant of each turning point for the year, corrected by twenty-four periodic terms), with the same ΔT the moon instrument carries, then read in the clock time of this place's zone from the zone tables the browser itself holds — the clock changes included. The method is good to about a minute across this century; the published tables agree.

The calendar seasons are not Sol's to define, so the page cites the authority that describes them and names it first where it is the local convention: the Bureau of Meteorology, the Met Office and the National Weather Service. The Bureau of Meteorology keeps the seasons by whole months — summer December to February, autumn March to May, winter June to August, spring September to November — and notes that the astronomical dates are the other convention.. In both conventions the hemisphere decides the names — the September turning point begins autumn north of the equator and spring south of it — and neither is a forecast: a season's first day says nothing about the weather that day, and the copy here never implies it does.

Day length is sunrise to sunset, the sun's upper edge on the horizon (its centre at −0.833°), computed for every day of the year at this place by the same engine the daylight pages use; the longest and shortest days are read from that table, which is why they can sit a day from the solstice itself. New York, London and Melbourne are baked nightly so a search engine can read the answer; the same engine runs in your browser for any place, any year, any day of it. The year's turning points are published beside each city page as JSON, CC BY 4.0.