Star Party Planner Calculator

This calculator estimates star-party planning conditions from three inputs -- Month, Latitude, and Bortle Class -- using simplified seasonal approximations rather than a precise ephemeris. Est. Sunset (24h) is a smoothed seasonal model (a sine-wave approximation of day length by month and latitude) -- verified across the full declared grid (every month at latitude -60 to 70), its own reachable range is roughly 16:03 to 19:57 local time, so treat it as a rough seasonal estimate, not a precise sunset time; it does not know your longitude, time zone, or elevation, and it cannot represent real high-latitude effects like a much later summer sunset or, above the Arctic/Antarctic Circle, no sunset at all. Moon Illumination has no year input to anchor a real calendar date, so instead of chasing an unreachable "real date" it spreads the 12 calendar months evenly across one illustrative 29.53-day lunar cycle -- deliberately covering the full range of possible conditions across a year (some months near-new/dark, some near-full/bright), not the actual moon phase on any specific real date; check a real lunar calendar before planning around a new moon.

Visible Planets reports a fixed, typical estimate rather than a month-keyed table, because naked-eye planet visibility is set by each planet's own synodic period (116 to 780 days) -- none commensurate with the calendar year -- so there is no real month-to-count relationship to source. Observable DSOs (deep-sky objects) scales a sky-darkness-based catalog count down sharply as Bortle Class rises (worse light pollution hides fainter objects), then scales that by the real geometric fraction of the celestial sphere visible from your Latitude over a full year, (1 + cos(latitude)) / 2 -- highest at the equator, lowest near the poles. This is a geometric visibility estimate, not one tuned to any specific catalog's declination distribution -- the widely-used Messier catalog was itself compiled largely from Northern-hemisphere (49°N) observations, so it skews toward northern declinations in a way this model does not correct for, and it treats Northern and Southern latitudes of the same magnitude identically. Month has no effect on Observable DSOs in this model, since Month only feeds the sunset, moon-phase, and visible-planets estimates.