About
About SkyAlmanac
An independent astronomical almanac covering 848 cities in 7 countries, in three languages. Every number on the site is either computed from first principles or attributed to a named public data source.
SkyAlmanac exists to answer practical sky questions — when the light turns golden, whether tonight is dark enough for the Milky Way, what Kp the aurora needs at your latitude — with numbers computed for your exact coordinates rather than a regional average.
It is a small, independent project with no editorial staff between you and the data: the same formulas run for every one of our cities, and this page documents exactly what those formulas are.
How the numbers are calculated
Sun times (sunrise, sunset, twilights, golden and blue hour) are computed with a PHP port of the SunCalc algorithm, which implements Jean Meeus's "Astronomical Algorithms". Accuracy is typically within a minute or two of observatory tables; refraction is assumed standard.
Moon phase, illumination and rise/set times use the same Meeus-based lunar theory. Full and new moon instants are found by locating the exact phase crossings, then shown in each city's local time.
Light-pollution (Bortle) classes are estimates, not measurements: we start from each city's population and metro context — which correlates strongly with VIIRS satellite radiance — and hand-correct places where that heuristic fails (dark-sky towns, cities inside larger metros). Each stargazing page says this explicitly.
Aurora thresholds come from geomagnetic geometry: we convert each city's coordinates to geomagnetic latitude with a centred-dipole model (IGRF ~2020 pole) and apply the empirical rule that the auroral oval's equatorward edge sits near magnetic latitude 66.5° − 2×Kp. Live activity is NOAA SWPC's Kp forecast and OVATION model, refreshed every few minutes.
Data sources
- Meeus / SunCalc astronomical algorithms — sun and moon positions, rise/set times, twilights, phases — computed locally on our servers, no external service involved.
- NOAA Space Weather Prediction Center — planetary Kp index forecast and the OVATION aurora nowcast. Public, free, refreshed continuously.
- N2YO — ISS visual-pass predictions per city, cached server-side and refreshed throughout the day.
- Open-Meteo — hourly cloud-cover forecasts shown alongside tonight's conditions; fetched by your browser directly.
- Bortle-class estimate (population-based) — our own estimate as described above — treat it as a starting point, not a sky-quality meter reading.
Accuracy & honest limits
Computed times (sun, moon) are reliable to a minute or two. Forecasts (Kp, clouds, ISS passes) are exactly that — forecasts from the named sources, and they change; each block shows when its data was last refreshed.
Bortle classes describe the city centre and get pessimistic fast as you leave town. If a number on this site looks wrong for your location, please tell us — corrections are the fastest way this project improves.
Corrections & contact
Spotted an error, or want your city added? Write to us via the contact page: Contact.