About Cast-a-night

A personal tool for planning the perfect night under the Milky Way.

What is Cast-a-night?

Cast-a-night is an astrophotography planning tool that scores and ranks the best nights to photograph the Milky Way's Galactic Centre at any location on Earth.

Rather than manually checking weather apps, moon calendars, and astronomy tables in separate places, Cast-a-night pulls everything together into a single ranked list.

Each night is scored 0 to 10 and labelled Excellent, Good, Decent, or Poor based on sky darkness, moon interference, Galactic Centre visibility, and cloud cover. When a forecast is available, an hourly timeline shows exactly when conditions align within a given night.

How it works

  • Pick a location: click the map or choose a named site.
  • Light pollution: the Bortle scale (1 to 9) measures how dark a sky is, from pristine wilderness to heavily light-polluted city. Cast-a-night looks up your location's Bortle class and applies a sky-quality coefficient to the score.
  • Galactic geometry: Skyfield calculates when the Galactic Centre rises above 15° elevation, peaks, and sets for every night in range. Shooting the Galactic Centre near the horizon is technically possible but rarely useful: at very low elevations you are looking through far more atmosphere, which scatters and reddens light, dramatically reducing contrast and sharpness.
  • Moon interference: phase, illumination %, and elevation during the dark window are combined into an interference penalty. Even a full moon is not necessarily a showstopper: if it sets before the shooting window opens, or rises after it closes, the sky can still be dark enough to shoot.
  • Cloud forecast: Open-Meteo provides 14-day hourly cloud cover; a ratio over the shooting window drives the final score.

Data sources

  • Open-Meteo: free, open weather API.
  • World Atlas of Artificial Night Sky Brightness 2015: Falchi et al.; used for Bortle class and SQM estimates.
  • VIIRS 2024 trend: annual satellite-based light pollution change applied as a correction to the 2015 atlas.
  • NASA DE421 ephemeris: planetary positions via Skyfield.
  • Astral: solar and lunar event times (sunrise, twilight, moonrise).
  • OpenStreetMap / Esri: base map and satellite imagery tiles.

Tips for best results

  • The Galactic Centre is best visible from the southern hemisphere or tropical/subtropical latitudes in the north (roughly below 55°N). From higher northern latitudes it stays low and the shooting window shrinks considerably.
  • Core season runs roughly February-October in the northern hemisphere (peaking May-July) and October-April in the southern hemisphere (peaking November-February).
  • Use the dark-site hint: if your chosen location has poor Bortle, Cast-a-night will suggest a nearby darker spot.
  • The Astro theme is designed to protect your night vision while you use the app in the field - toggle it before you head out.
  • Download the CSV export to keep a record or plan a multi-night trip.

About the creator

Hi - I'm Pierre, an astrophotography enthusiast. I built Cast-a-night because I kept wishing for a single tool that would tell me, clearly and without guesswork, whether a given night was worth driving two hours to a dark site.

The planning workflow used to mean cross-referencing a weather app, a moon-phase calendar, and an astronomy tool at the same time. Cast-a-night collapses all of that into one ranked view, so I can spend less time planning and more time shooting.

The project is written in Python with a vanilla JS frontend.

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