Two people can be born on the same day, in the same place, hours apart, and the tradition reads their parents through two different pairs of planets. The hinge is not the date. It is whether the Sun was above the horizon or below it at the moment of birth.
Sun and Saturn for the father, Moon and Venus for the mother
Ptolemy's Tetrabiblos states the general association plainly: "the Sun and Saturn are by nature associated with the person of the father and the Moon and Venus with that of the mother" (III.4, "Of Parents"). That pairing holds in every chart, day or night. What is not fixed is which planet in each pair carries the weight.
Sect picks the leader
Ptolemy answers that question with a single sentence: "we must observe by day particularly the Sun and Venus, and by night Saturn and the Moon." In a day chart, the Sun leads for the father and Venus leads for the mother. In a night chart, Saturn leads for the father and the Moon leads for the mother. Astrologers call this quality of a chart its sect: whether it was cast for a birth while the Sun sat above the horizon (day) or below it (night).
This line closes Ptolemy's discussion of the parents' circumstances rather than standing alone as a universal law, so it is fairest to call it Ptolemy's rule rather than flattening it into "the tradition says." Vettius Valens, writing in the same Hellenistic period, gives a different test in his Anthologies: watch whichever of the Sun or Saturn the Moon is actually configured with, by degree, regardless of day or night. Venus and the Moon take the Mother's Place by the same procedure. The two authors genuinely disagree about the mechanism, even while agreeing on the underlying pair of planets.
What a real chart shows
Take one birth, one place, two possible times. For a birth at local noon in New York, NY on October 19, 2026, the Sun sits at 26°17' Libra in the 10th house, and the chart is a day chart. Venus sits at 3°25' Scorpio, also in the 10th. By Ptolemy's rule, the Sun leads for the father and Venus leads for the mother; Saturn (10°07' Aries, 3rd house) and the Moon (7°09' Aquarius, 2nd house) support rather than lead.
Move the same birth to local midnight, same place and date, and the chart flips to a night chart. The Sun is now just below the horizon at 25°47' Libra in the 4th house; Saturn has moved into the 9th at 10°09' Aries. By Ptolemy's rule, Saturn now leads for the father and the Moon (1°09' Aquarius, 7th house) leads for the mother; the Sun and Venus (3°42' Scorpio, 4th house) support. Every position above was computed by the Celestium engine, not estimated, and the interactive version of this example is at Sect and your parents, where you can toggle day and night yourself.
The fourth place
Sect is not the only door the tradition reads for the parents. Vettius Valens names the fourth place outright, "the Place of Parents," in his discussion of the twelve places. Dorotheus of Sidon adds, in the Carmen Astrologicum, that the ruler of that fourth place, by sign, is itself an indicator of the parents' matter. A chart that shows a difficult or well-placed fourth-house ruler is read through that door independent of which planet leads by sect.
What this is, and is not
None of this is a diagnosis of anyone's actual mother or father. It is a description of a symbolic pair of significators and their condition at a given moment, read through named historical rules rather than a vague "the tradition says." What it says about your own parents is worth sitting with as a question, not accepting as a verdict.
Celestium computes sect, the 4th place and its ruler, and the Lots of the Father and Mother from your own birth data, on your device. You can download it free on the App Store for iPhone, iPad, and Mac.
Want to see day and night side by side on the same chart? Try the interactive version: Sect and your parents.