The final ten degrees
Twenty to thirty degrees of Pisces, the third of the sign's divisions and the end of the zodiacal circle. Its closing degree is the March equinox, after which the count begins again at zero degrees of Aries.
The Sun crosses these degrees in roughly ten days, usually from about 11 March to about 20 March. Both edges move. The tropical year overruns three hundred and sixty five days, so ingresses drift by up to a day and a half between years until a leap day corrects them, and the closing edge here is also the boundary with Aries, which makes birthdays around the twentieth of March among the most often misfiled in the zodiac.
Mars at the close, from either direction
The Chaldean order repeats Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter from the start of Aries through all thirty six decans. Because the cycle of seven divides into thirty six with one left over, the sequence does not finish neatly, and the last decan of Pisces falls on Mars, the planet the whole count began with.
Triplicity reaches Mars a different way. Each decan goes to a sign of its own element in order, and the water signs run Cancer, Scorpio, Pisces, so a Pisces sequence continues Pisces, Cancer, Scorpio. This decan therefore belongs to Scorpio, ruled by Mars in traditional practice and by Pluto in modern practice.
So the schemes agree if the older ruler of Scorpio is used, and part company if the modern one is. That is the usual reason a decan turns up described one way in one article and differently in another, and it is why the method behind a stated ruler is worth asking about.
Scorpio inside Pisces, and what Mars is read as adding
Pisces shaded by Scorpio is read as the sign's receptiveness acquiring a floor. Where the opening decan is described in terms of impressions that pass through without resistance, this one is described in terms of what is held onto: an accurate sense of what people are concealing, a tolerance for material other people avoid, and an unwillingness to be reassured. Put unkindly, the same disposition is suspicion that finds evidence for itself.
Mars is taken to add effort and direction to the mixture. The older material on this face speaks of undertakings pursued to a finish and of situations resolved rather than left. Mars in Pisces has no dignity, so the tradition does not read the combination as straightforwardly forceful; the more usual account is force applied indirectly, in circumstances where the pressure is not visible. What that comes to in a life is left to the rest of the chart, since the decan describes the manner and not the outcome.
Other placements, and what the technique is for
Nothing here is peculiar to the Sun. An Ascendant at twenty six degrees of Pisces sits in this decan, and so does a Moon at twenty three, and traditional practice read the Ascendant's decan at least as attentively as the solar one. The rulers described apply to whichever point occupies the degrees.
Keeping the technique in proportion matters. The decans reached astrology from Egyptian star lists, groups of stars whose rising marked the hours of the night, and the planetary rulerships were laid over them centuries later. That is a real and unusually long lineage. It is still a supporting one. A reading rests on sign, house and aspect, and the decan qualifies the tone. Ten degrees can shade a description; they cannot carry it.
Given that this decan ends at the equinox, the check above earns its place. It computes your actual solar degree from your birth date and reports the decan and both rulers, which a date range cannot do for anyone born in the last days of Pisces.
Questions people ask about this
What dates are the third decan of Pisces?
Roughly 11 to 20 March, with the usual movement of up to a day and a half between years. The closing edge is the equinox and the start of Aries, so births around the twentieth need the degree.
Which planet rules the last decan of Pisces?
Mars under the Chaldean order. Triplicity assigns the decan to Scorpio, which gives Mars again in traditional practice and Pluto in modern practice, so the answer depends on which ruler of Scorpio is used.
Why does Scorpio appear in a Pisces decan?
Because triplicity allots each sign's decans to the signs of its element in order. Continuing from Pisces through the water signs gives Pisces, Cancer, Scorpio, so the final decan falls to Scorpio.
Is this the same as the Pisces to Aries cusp?
They overlap at one end but are not the same thing. The decan is a fixed ten degree band, whereas cusp talk describes a loose window around the sign change. Only the degree decides which sign the Sun is in.
Does Mars having no dignity in Pisces change the reading?
The tradition reads it as force expressed indirectly rather than openly, so descriptions of this face avoid the plain assertiveness attached to Mars elsewhere. It is a qualification on the interpretation, not a verdict on it.
Do decans apply to the Ascendant and Moon?
Yes. Any placement falls in one of the thirty six divisions, and the Ascendant's decan was standard in older practice. The Sun receives the attention now mostly because it is the placement most people already know.