How the numbers are produced
Methodology
Celest's calendar, sky guides and observing forecasts are calculated rather than copied. This page explains the data sources behind them, how the calculations work and where their limits are.
Last reviewed:
Positions, rise and set times, eclipses
The positions of the Sun, Moon and planets, rise and set times, twilight, Moon phases, eclipses, conjunctions and oppositions are calculated with Astronomy Engine, an open-source library whose results are validated against NASA JPL Horizons and the U.S. Naval Observatory's NOVAS to about one arcminute.
Calculations run for the location you choose. Times are computed in Coordinated Universal Time (UTC) and shown in your local time zone, so daylight-saving changes and the date line are handled automatically.
Meteor showers
Activity windows, peak times and ideal hourly rates follow the annual shower calendar of the International Meteor Organization. Celest adds the radiant's altitude for your location and the Moon's interference on each night.
An ideal hourly rate (ZHR) assumes the radiant overhead and a perfectly dark sky, so the number of meteors you actually count is usually lower.
Comets, satellites and space weather
Comet positions come from NASA JPL ephemerides. Brightness estimates use recent observer reports from the Comet Observation Database (COBS) when they are available, and comet brightness remains uncertain by nature. Satellite passes are computed from CelesTrak orbital elements, and aurora and geomagnetic information comes from NOAA's Space Weather Prediction Center.
Weather and the observing score
Hourly cloud cover, precipitation, visibility, humidity and wind come from the Open-Meteo forecast service. The observing score weighs cloud cover at 36%, darkness at 16%, moonlight at 14%, precipitation at 12%, visibility at 10%, and dew risk and wind at 6% each, and is then capped by the light pollution at your location.
The score is a planning aid rather than a promise: forecasts change, and confidence drops the further ahead you look.
Light pollution
Light pollution is estimated from NASA's VIIRS night-lights imagery (the 2016 annual composite) and converted into an approximate Bortle class. Satellite radiance is a proxy: it does not measure the sky brightness you see from the ground, and local lamps, snow, haze or new development can change real conditions.
What Celest cannot know
Calculations assume an open horizon. Hills, buildings and trees, sudden weather changes and the unpredictable behaviour of comets or meteor outbursts can all change what you actually see. Celest labels estimates as estimates and explains the assumptions behind them.
Articles
Articles take their event dates and visibility from the same calculations. How articles are researched, sourced and reviewed is described in the editorial policy.