Why two lunar calendars show different constellations

Two lunar calendars lie on the table. For the same day one says leaf day, the other says flower day. Not a misprint — the two calculate with different skies.

The question reaches us every spring, when sowing is pressing and the calendars contradict each other: which one is right? A practical decision hangs on it — whether lettuce or kohlrabi goes into the ground today. The answer lies not in biodynamics but in astronomy, and it is pleasingly clear. By the end of this text you will know where the difference comes from and how to tell within seconds which system you have in front of you.

Two systems, one sky

Anyone talking about the position of the Moon means one of two things. In everyday use they get mixed up.

The signs of the zodiac. A computational grid: twelve sections of exactly 30 degrees, laid out along the ecliptic and anchored at the vernal equinox. Neat, even, convenient for calendars. The price: the sections carry the names of constellations but no longer coincide with them.

The constellations. The actual groups of stars in the sky. They differ in width, because stars do not keep to grid lines. Along the ecliptic they span between twelve and forty degrees. The Moon takes 27.32 days for one circuit — it crosses Libra in just over a day, Virgo takes it three and a half.

A calendar that uses the first system and a calendar that uses the second arrive at different answers on many days of the year. Both calculate correctly. They answer different questions.

Precession: why the two drifted apart

About two thousand years ago, signs and constellations still largely lay on top of each other. That was when the grid was set. Since then the Earth's axis has kept wobbling — precession, with a cycle of about 25,920 years. It needs roughly 2,160 years for one sign's width.

The result: the vernal equinox, to which the grid of signs is tied, no longer lies in the constellation Aries today but at the beginning of the constellation Pisces. Between sign and constellation there is now almost a full sign's width.

For practice, converting into days is more tangible than degrees: the Moon moves on by about thirteen degrees a day. A shift of just under thirty degrees therefore corresponds to about two calendar days. The root day that a sign calendar shows for Friday lies closer to Wednesday in a constellation calendar.

What Maria Thun calculated

For biodynamic practice the question of the system is settled. Maria Thun (1922–2012) chose the real sky explicitly. Her sowing days refer neither to the grid of signs nor to the sidereal zodiac of Indian astrology, but to the visible constellations in their real, unequal extent. Her basis for calculation was astronomical ephemerides — the same data every observatory works with.

The way there led through the vegetable bed. From the mid-1950s Thun laid out replicated plot trials on her land near Marburg. With radishes she found four recurring growth types, which she called root, leaf, flower and fruit or seed type. The plants switched at intervals of two to three days. Comparing this with the astronomical positions, she found a rhythm that matched: the Moon's passage from one constellation into the next. Her first sowing calendar appeared in 1963; today it is available in about twenty languages.

The four trigons

Thun assigns an element to each group of three constellations and a part of the plant to each element. This assignment is the core of the system:

• Earth → root day. Taurus, Virgo, Capricorn. For carrot, beetroot, potato, radish.
• Water → leaf day. Cancer, Scorpio, Pisces. For lettuce, cabbage, spinach, leafy herbs.
• Light → flower day. Gemini, Libra, Aquarius. For flowers, broccoli, cauliflower.
• Warmth → fruit day. Aries, Leo, Sagittarius. For tomato, bean, squash, grain, fruit.

Because the constellations are unequal in width, the plant days are unequal in length. A fruit day of three and a half days and a flower day of just over one stand side by side in the same month. That is not a blemish but the basic statement of the system: it follows the sky, not the ruler.

Why two calendars name different days

Three causes, in this order:

First, the choice of system. The best-known example of calculating with signs is the German bestseller "Vom richtigen Zeitpunkt" by Johanna Paungger and Thomas Poppe — it is based on the rigid thirty-degree division. Leafing through, the difference hardly shows, because both types of calendar use the same symbols and the same terms. The difference lies in the basis of calculation, not in the presentation.

Second, the boundaries. In the middle of a wide constellation both systems often land in the same element, because sign and constellation overlap. At the transitions it tips over. If you are puzzled by a contradiction, you will almost always find it on a changeover day — that is the first place to look.

Third, the time of day. The change of constellation falls on a moment, not a day. Whether a calendar assigns a change at 9 p.m. to the current day or already to the next is an editorial decision. With calendars sold internationally, the time zone comes on top.

How to tell in thirty seconds which system you are looking at

It is rarely written on the cover. These four checks are enough:

1. Are the constellation sections unequal in length? In the calendar pages for one month, count how many days fall on each sign. If there are blocks of one day and blocks of three to four days, the calendar calculates with constellations. If all blocks are the same length, the thirty-degree grid is behind it.
2. Does the Moon change at stated times? Constellation calendars usually give the change with the hour, because it does not fall at midnight. Sign calendars more often work day by day.
3. Is a source named in the imprint? Phrases like "based on astronomical ephemerides" point to the constellation method.
4. The cross-check in the sky. The most reliable check takes a minute: look up in a constellation display where the Moon really stands today and compare it with the calendar. If they differ by about two days, you are holding a sign calendar.

A note on wording that causes confusion: the division into "astronomical" and "astrological" calendars is often sold as if that were the decisive difference. It falls short. Both types of calendar assume that the Moon and the zodiac have an effect on the plant — they differ in the basis of calculation, not in the underlying assumption. The term "astronomical" therefore describes the basis of calculation, nothing more.

Ophiuchus: the constellation no rulebook knows

This is where it becomes uncomfortable. According to the constellation boundaries of the International Astronomical Union in force today, the ecliptic passes through not twelve but thirteen constellations. The thirteenth is Ophiuchus, the Serpent Bearer, and it lies between Scorpio and Sagittarius.

The Moon deviates from the ecliptic by at most about five degrees. It therefore passes through this constellation regularly — several times a year, for about a day each time.

It does not appear in the classic rulebooks. The reason is historical: the Babylonian order was brought into twelve sections, matching the twelve lunar cycles of the year, and Ophiuchus fell out in the process. Thun's calendar does not list it either. Systems with twelve constellations close the gap by extending the neighbours Scorpio and Sagittarius to their common boundary.

On these days there is therefore no traditional assignment, only a convention. If you want to know where the Moon actually stands on a given day and how far the two systems are apart right now, the Himmelsuhr offers a display that lays both rings on top of each other (in German) — outside the IAU constellation boundaries, inside the tropical thirty-degree fields. Ophiuchus is visible there instead of being silently divided up.

What this means for work with the preparations

Less than the debate suggests. The question of constellations mainly concerns sowing and care, because there it decides which part of the plant is addressed.

When the preparations are applied, other criteria come first: time of day, weather, soil temperature and soil moisture, together with the common practice of applying horn manure (500) in the afternoon in a descending Moon and horn silica (501) in the early morning. These criteria are untouched by the question of signs versus constellations. Taking the constellation into account as well gains you fine-tuning — not a criterion for exclusion. A good application date in suitable weather beats a bad date in the right trigon.

In brief

• Signs of the zodiac are a grid of twelve equal sections, anchored at the vernal equinox.
• Constellations are the actual groups of stars, twelve to forty degrees wide, crossed by the Moon in one to three and a half days.
• Precession has pushed the two apart by almost a sign's width — for the Moon that corresponds to about two calendar days.
• Thun's sowing days follow the constellations. Sign calendars such as "Vom richtigen Zeitpunkt" follow the grid.
• Ophiuchus lies on the ecliptic but is not listed by twelve-part systems.
• More important than the choice of system is to stay with one system and write down your own results.

Further work

• Our planting calendar — plant days, windows for the preparations and the work of the month, calculated for your location.
• Himmelsuhr (in German) — Moon phase and constellation for every day by IAU boundaries, plus Full and New Moon to the minute, apogee, perigee, lunar nodes and eclipses. Useful as a cross-check when two calendars contradict each other.
• Maria Thun's sowing calendar — the calendar itself, published anew every year and available in English as "The Maria Thun Biodynamic Calendar". The source to which constellation work refers.

And the next time two calendars claim different things: check whether the day is a changeover day. In most cases that explains it.