The Jonas Method
In the 1950s a Slovak psychiatrist claimed astrology could tell a woman when she was fertile: accurately enough to be used as contraception. It is one of the most consequential claims anyone has made for astrology, and it is worth knowing exactly what it was, and what came of it.
The dates your Sun–Moon angle comes back round. Free, runs in your browser.
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What Eugen Jonas proposed
Eugen Jonas was a psychiatrist working in Czechoslovakia in the late 1950s. Abortion had recently been legalised there, and he was looking for a method of family planning that needed no drugs and no surgery. What he arrived at came from astrology.
His central claim was this: alongside the ovulation cycle that medicine already knew about, a woman has a second fertile window each month, and it falls when the Sun and the Moon return to the exact angle they held at the moment of her own birth. Because the Moon returns to a given phase roughly every 29.5 days, that angle recurs about once a month: close to, but not the same as, her ovulation.
He made two further claims on top of it. That the sign the Moon occupied at conception determined the sex of the child: masculine signs a boy, feminine signs a girl. And that the planetary configurations at conception governed whether a pregnancy would hold and whether the child would be born healthy.
Jonas put the reliability of the contraceptive use at 98–99%, and the sex-selection figure at around 97–98%. For a few years in the 1960s he ran a research centre in Nitra with state funding, and other physicians worked on it with him.
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What the evidence shows
This is the part that matters, and it needs saying carefully, because it is easy to overstate in either direction.
Jonas’s own figures were never published in a peer-reviewed journal. He said he had extensive proof; it did not appear anywhere it could be examined by people who were not already convinced. His centre was closed at the end of the 1960s, and the work afterwards was carried on commercially, not clinically.
His specific claim has never been independently replicated. Not "tested and disproved". Something weaker and, in a way, worse: nobody outside his circle has produced a result supporting it, and it has not been taken up in reproductive medicine anywhere.
The nearest thing to a direct test is the broader question of whether the lunar cycle influences human birth at all, and that has been examined properly with large samples:
- Benski and Gerin (1991) looked at the birthdays of 4,256 babies born at a French clinic and found them evenly distributed across the synodic lunar cycle.
- Periti and Biagiotti (1994) examined 7,842 spontaneous deliveries over five years at a clinic in Florence and found no relationship between the phase of the Moon and the number of births.
Both of those test whether the Moon’s phase affects when babies are born, which is related to Jonas’s claim without being identical to it: he was talking about conception, not delivery. We would rather point that out than let two good studies be used to settle a question they did not quite ask. What they do establish is that the most obvious lunar effect on human reproduction, the one that would have to exist for the rest of the picture to hold together, is not there in samples of thousands.
What Umbriel does with it
The Sun–Moon angle at your birth is a real quantity, and this app already knows it: it is your natal moon phase. The Jonas returns list every date that angle comes round again, twelve or thirteen a year, worked out from the real positions of the Sun and Moon.
Umbriel computes the angle and nothing beyond it. If you did not give a birth time, the angle is a range, and the returns page says so. It is not medical advice, and any decision you take from it is your own; the Terms set out the same position in full.