Zero degrees: where the zodiac closes and restarts
This is the only boundary that is also the origin of the coordinate system. Zero degrees of ecliptic longitude is the March equinox point, the place the tropical zodiac is measured from, so the Sun does not merely leave Pisces here; it completes the full circle and the count begins again at zero.
In practice the crossing lands on 19 or 20 March. Every other sign boundary is defined by reference to this one, which means that if this point ever moved, the whole zodiac would move with it. It cannot, because it is the definition.
The equinox itself, not an arbitrary date
The instant is the moment the Sun crosses the celestial equator heading north, when day and night are close to equal everywhere on Earth. Astronomers publish it to the minute for every year, and astrology's Aries ingress is that same event under another name.
It falls on the 19th or the 20th because the Sun needs about six hours more than three hundred and sixty five days to return to the equator, an excess the leap day removes every fourth year. That is the whole of the variation, and it is the reason a spread of nearly a day and a half exists.
Jupiter's water to Mars's fire, ending to beginning
No other changeover involves a sign ending the zodiac and a sign starting it. Mutable water becomes cardinal fire, and the most dissolved of the signs is followed immediately by the most initiating, with the greater benefic handing over to the lesser malefic.
The contrast is stark enough that a hybrid type has been invented for it. The chart offers no such thing: the Sun sits at one longitude and that longitude is either under three hundred and sixty degrees or at zero of the next circuit.
Cusps, and what they actually divide
A cusp in the technical sense separates two houses. Houses are constructed from the horizon and the meridian at the birthplace, they depend on latitude and on the minute of birth, and they have nothing in common with the equal signs except that both are drawn on the same wheel.
Reading a birth on 19 or 20 March
Late Pisces holds Venus's exaltation at twenty seven degrees and closes under Mars in the Chaldean decans; early Aries opens under Mars as well, so this is the one boundary where the same decan ruler stands on both sides of it. The Sun's exaltation follows at nineteen degrees of Aries, a fortnight into the new season.
To settle a particular birth, take the equinox instant for that year, convert it to the local clock at the birthplace and compare. The answer is one sign, and if it is the unexpected one, only the Sun's placement changes.
Questions people ask about this
Am I a Pisces Aries cusp?
No, because that placement does not exist. The Sun was either short of zero degrees at your birth or past it, which makes you Pisces or Aries.
What date does the Sun move from Pisces to Aries?
19 or 20 March, depending on the year and on the time zone the birth is dated in. The instant is the same worldwide; only the local calendar date differs.
Do I have traits of both signs?
Popular astrology says so and sells the blend as a type. The chart does not show it: one Sun, one longitude, one sign. Other placements can of course sit in either sign.
What is a cusp in real astrology?
A house boundary. Houses are divided using the horizon and the meridian of the birthplace, and their cusps are genuine dividing lines within a chart.
How close to the boundary do I need to be to bother checking?
About forty eight hours either side. Inside that window the year and the birth time decide the answer; outside it, any date range gives the same result.