How the Dyyna calendar is calculated
Every day in the calendar is calculated for a place and its time zone: the position of the moon, the constellation it stands in front of, its phase, its path and the hours the biodynamic calendar counts as unfavourable. This page sets out the basis for it.
The moon in the sky
The position of the moon follows the series of Jean Meeus (“Astronomical Algorithms”), with nutation and the difference between Universal Time and Terrestrial Time (ΔT). From this come the phase, the distance to Earth and the declination, that is whether the moon is ascending or descending.
The constellations the moon stands in front of
The day type depends on the constellation the moon actually stands in front of. The constellation boundaries are those of the International Astronomical Union, converted to each date. As with Maria Thun, the calendar counts twelve constellations. Because they differ in width across the sky, the day types differ in length too.
Change times to the minute
The calendar calculates the moment the moon enters the next constellation and gives the time from which the next day type applies. It also calculates new moon, the quarters and full moon, perigee and apogee, the lunar nodes and the turning points of the moon’s path.
Unfavourable hours
Lunar nodes: as long as the lunar disc touches the ecliptic, about five hours before and after the node. Perigee: twelve hours before and after. Planetary nodes: twelve hours on each side. Occultation of a planet by the moon: from start to end. Eclipses: for their duration. Plus the time from Good Friday to sunrise on Easter Sunday.
The calendar does not block full or new moon. The day type applies as on any other day.
Planets and constellations
The planets Mercury to Neptune follow the VSOP87 theory of Pierre Bretagnon and Gérard Francou, Pluto a calculation of its own. From this come trines, oppositions and conjunctions, the felling and sowing times for trees and the force trines that make a day especially favourable.
Cross-checked
The calculation is checked against astronomy-engine, a library based on the ephemerides of the Jet Propulsion Laboratory. From October 2026 to September 2027 the constellation changes differ by 0.06 minutes on average, the moon phases by 0.5 minutes, the lunar nodes by 0.3 minutes. The day type at noon matches on every day of the period. From 2026 to 2030 the planets differ by at most 20 arcseconds from the JPL ephemerides (Horizons).
Place and time zone
Every day is calculated in the time zone of the place, and times are given in local time. In the southern hemisphere the moon’s path is reversed: when the moon ascends for Europe, it descends for Australia. The calendar app switches this by location or by choice.
Sources
- Jean Meeus: Astronomical Algorithms. 2nd edition, Willmann-Bell 1998
- Eugène Delporte: Délimitation scientifique des constellations. International Astronomical Union, Cambridge 1930
- Pierre Bretagnon, Gérard Francou: Planetary theories in rectangular and spherical variables. VSOP87 solutions. Astronomy and Astrophysics 202 (1988)
- astronomy-engine (Don Cross, MIT licence), for cross-checking
- JPL Horizons, NASA Jet Propulsion Laboratory, for cross-checking the planets
- Maria Thun: Aussaattage (annual), for assigning the constellations to the day types
- Pierre Masson: A Biodynamic Manual, for the rules on nodes, perigee and the moon’s path
Frequently asked questions about the calculation
Does Dyyna calculate by constellations or by zodiac signs?
By constellations. The day type depends on the constellation the moon actually stands in front of, with the boundaries of the International Astronomical Union.
How accurate are the times?
The constellation changes differ on average by less than a minute from the JPL-based values of astronomy-engine, and so do the moon phases.
Does the calendar work outside Europe?
Yes. Every day is calculated in the time zone of the place. In the southern hemisphere the moon’s path is reversed, and the calendar app switches accordingly.