Data sources and limitations
Weather forecasts and astronomical calculations answer different questions. We keep their sources and update times separate.
Weather in the United States
Supported US cities use NOAA / National Weather Service forecast grids. Nearby cities may share a grid. We cache results and recheck location mappings periodically.
International weather
International cities and towns use MET Norway Locationforecast. Based on data from MET Norway, used under CC BY 4.0. We convert units and summarize samples for the selected observing interval. This processing is ours and does not imply endorsement by MET Norway.
Outside the provider's priority regions, model resolution is approximately 9 km. Missing fields are shown as unavailable. Later forecast samples may be six hours apart; we retain those sample times and withhold detailed observing recommendations when the data is too coarse or incomplete.
Astronomy
Sun, Moon and planet geometry is calculated locally using the bundled Astronomy PHP library. Times describe a modelled horizon and are approximate. Terrain, refraction, buildings and local lighting can change the actual view. A planet above a useful altitude may still be hidden by cloud.
The sunrise and sunset pages use the NOAA Solar Calculator equations (after Jean Meeus). The Moon pages use Paul Schlyter's lunar orbital elements with their main perturbation terms. Both are calculated on our server for a sea-level observer with a flat horizon and standard refraction. Before release, they were checked against the bundled library for nine cities across five dates, from Honolulu to northern Scotland and Alaska. Sunrise and sunset agreed within 10 seconds, and moonrise and moonset within 20 seconds. Principal Moon phase instants agreed within about ten minutes.
Backup forecasts
When the National Weather Service or MET Norway cannot be reached, or their forecast for a place is too old, we use an hourly forecast from Open-Meteo.com. It is licensed CC BY 4.0, and forecast pages show its name whenever it is in use. Requests are capped well below Open-Meteo's free non-commercial limits.
Climate averages
Monthly averages come from the NASA POWER project (NASA Langley Research Center), using its long-term climatology for a grid area of about 50 km. Average highs and lows are estimated from the monthly mean temperature and the average daily range. If NASA POWER is unavailable, a few areas a day use five years of ERA5 reanalysis data from the Open-Meteo.com historical archive instead, and the page says so. The “clear, dark hours” estimate multiplies mid-month astronomical darkness by the average share of clear sky. It is a planning guide, not a forecast.
Space weather and aurora
The planetary Kp index and its three-day forecast come from the NOAA Space Weather Prediction Center. If it is unavailable, observed Kp from GFZ Potsdam (CC BY 4.0) is shown, without a forecast. Aurora outlooks use geomagnetic latitude from a centred-dipole model (IGRF-14 pole for 2025) and a common rule of thumb: overhead near 66.5° − 2.05° × Kp, and low on the horizon about 6° further from the pole. Real displays vary with the solar wind, so treat them as a guide.
Eclipses
Eclipse dates, central paths and local circumstances are calculated on our server with the bundled Astronomy PHP library, for sea-level observers with a flat horizon. Times for each place are prepared in the background as places are published. Lunar eclipse visibility checks whether the Moon is above the horizon at each stage. Always use certified solar filters for solar eclipses.
International Space Station
Pass predictions use the standard SGP4 orbit model (Vallado et al., 2006), which we checked against the published verification case. Orbital elements come from CelesTrak, falling back to the public TLE API mirror if needed. They refresh every six hours, and predictions are withheld when the elements are more than 10 days old. A pass counts as visible when the station is at least 10° high and sunlit while the Sun is at least 6° below your horizon. Brightness uses a standard magnitude of −1.8 at 1,000 km. Orbit-raising burns can shift times, so check again on the day.
Object reference positions and resources
Deep-sky reference centres come from SIMBAD, operated at CDS in Strasbourg. We keep a local extract and use the astronomy library to calculate approximate directions. The sky view samples one moment every 30 minutes and uses an ideal horizon. It is a broad orientation aid, not a detailed finder chart or a prediction of visible detail.
Object and observing guides link to their references. The event calendar uses checked NASA, US Naval Observatory, American Meteor Society and Royal Observatory Greenwich material. Exact instants, date-only windows and local visibility are kept separate. Calendar downloads are snapshots. Place records use official park or operator sources and separate access notes from any designation.
Forecast refresh policy
The original city forecasts retain their established coverage. New locations prepare in the background when requested. Recently used and subscribed locations receive priority within a site-wide work budget. Old or incomplete data remains labelled, and detailed scores are withheld when the selected interval lacks usable coverage.
Locations
City and town references use a local extract of GeoNames data under CC BY 4.0. We select populated places and screen their coordinates and time zones. Source-checked references can be public while astronomy and weather prepare. Pages link to their location source; approximate city coordinates do not identify an observing site or establish access. Coverage remains a selected catalogue.
Update and outage behaviour
Scheduled workers refresh forecasts according to provider caching rules and prepare astronomy. Public pages read stored data. A failed refresh retains the last successful forecast, with a warning. Once data is too old, weather-based scores and alerts stop while valid astronomy remains available.
Forecast retrieval time, provider issue time when available, and location review date are different timestamps. A recently retrieved forecast can still have an older model issue time. Check the forecast again before travelling.