Agricultural sun & moon calendar — pure Elixir, zero dependencies.

From a geolocation, a date (or a date range) and optionally an elevation, GreenCal computes everything a gardening / market-farming calendar needs:

• sunrise, solar noon, sunset, day length, civil dawn/dusk
• moonrise, moonset, phase, illuminated fraction, distance
• the three independent lunar cycles used by agricultural calendars: waxing/waning (illumination), ascending/descending (declination — the one biodynamic sowing calendars care about), and perigee/apogee (distance)
• lunar node crossings (β = 0), the days biodynamic calendars mark as unfavorable
• the sidereal constellation the Moon stands in, its element and the plant organ traditionally associated with it

Quick start

day = GreenCal.day({48.8566, 2.3522}, ~D[2026-06-21])
day.sun.rise            #=> ~U[2026-06-21 03:46:57Z]
day.moon.phase          #=> :first_quarter
day.moon.trend          #=> :descending

GreenCal.calendar({48.8566, 2.3522}, Date.range(~D[2026-07-01], ~D[2026-07-31]))

Options

Accepted by day/3 and calendar/3:

• :elevation — meters above sea level (default 0.0). Deepens the apparent horizon dip: about one minute of earlier sunrise per 100 m. Applies to sun and moon rise/set only — twilights are defined by the Sun's geometric altitude and are deliberately unaffected (a higher vantage point sees the Sun sooner, but the sky's illumination geometry does not change).
• :time_zone — IANA zone name (e.g. "Europe/Paris"). The civil day then runs from local midnight to local midnight and every DateTime is returned in that zone. Requires a configured time zone database (e.g. tzdata); the default "Etc/UTC" needs none.
• :twilight — altitude used for dawn/dusk: :civil (default), :nautical, :astronomical, or degrees.
• :boundaries — constellation convention: :equal_sidereal (default) or :iau (see constellation_of/3).
• :delta_t — override ΔT in seconds (see GreenCal.Astro.Time).

A warning about the interpretive layer

Rise/set times, phases and declination trends are astronomy: they are computed here to the minute and validated against published references. Elements, organs and "sowing days" are tradition, not science — GreenCal computes the underlying astronomy faithfully and labels the traditional mapping for what it is.

Note on constellations: the default is the sidereal zodiac with a Lahiri ayanamsa and equal 30° sectors. Printed biodynamic calendars (Maria Thun et al.) use the unequal IAU constellation boundaries instead — pass boundaries: :iau to match them (13 sectors, Ophiuchus included).