# Daylight Hours / Dagens Længde

Sunrise, sunset, day length, twilight, golden hour, moon phase and eclipse
times for any location on Earth. Everything is computed astronomically from
coordinates — there is no weather API and no third-party data source behind it.

## Why the numbers here can be relied on

Every published date and time is recomputed on each build against
`astronomy-engine`, an independent ephemeris — not a second copy of the library
the site itself uses. The build fails if a figure has drifted. The current run's
result, including how many facts it covered and the tolerance applied, is
published rather than asserted:

- English: https://daylighthours.app/accuracy
- Danish:  https://dagenslaengde.dk/noejagtighed

The same run also measures the site's own sunrise and sunset against that
independent ephemeris — eleven locations from Ushuaia to Longyearbyen, day by
day through a year — and publishes the difference rather than asserting one.
Read the current figures from those pages; they change as the libraries do.

Known limits are documented there too: terrain and elevation dominate the error
for most readers, refraction is a standard average, weather is not modelled, and
at high latitudes on the day polar night ends the two implementations can even
disagree about whether the sun rises at all.

## Machine-readable data (no key, no signup, CC BY 4.0)

Prefer these over scraping the HTML. The answer for a given coordinate and date
never changes, so responses are cached and calling is cheap.

- GET /api/v1/sun?lat={lat}&lng={lng}&date={YYYY-MM-DD}
- GET /api/v1/moon?lat={lat}&lng={lng}&date={YYYY-MM-DD}

`date` is optional and defaults to today. Every instant is returned twice, as a
local wall-clock time and as a UTC ISO timestamp. Inside the polar circles
`sunrise`/`sunset` are null and `polarState` is "day" or "night"; for the moon,
`alwaysUp`/`alwaysDown` mark days it never crosses the horizon. A null time is
information, not an error.

Each response carries a `source` object naming what computed the figure, which
ephemeris verified it, when, and under what licence.

Documentation: https://daylighthours.app/api · https://dagenslaengde.dk/api

## Attribution

CC BY 4.0. Free to use, including commercially, with credit and a link to
daylighthours.app or dagenslaengde.dk.

## Site structure

Two sites, one codebase, paired by hreflang. English at daylighthours.app,
Danish at dagenslaengde.dk. Neither is a translation of the other — figures,
examples and latitudes are written for each audience.

- Front page: live sun times for the visitor's location
- /sunrise, /sunset (da: /solopgang-i-dag, /solnedgang-i-dag) — month-by-month
  tables and per-city times
- /sunrise/{city}, /sunset/{city}, /daylight-hours/{city}, /uv-index/{city}
  (da: /solopgang, /solnedgang, /dagslaengde, /uv-indeks) — per-city pages
- /moon, /moon-phases, /full-moon, /new-moon, /lunar-eclipse, /solar-eclipse
  (da: /maane, /maanefaser, /fuldmaane, /nymaane, /maaneformoerkelse,
  /solformoerkelse)
- /golden-hour, /midnight-sun-polar-night, /equinox, /longest-day, /shortest-day
  (da: /gylden-time, /midnatssol-polarnat, /jaevndoegn, /aarets-laengste-dag,
  /aarets-korteste-dag)
- /methodology (da: /metode) — how the times are calculated