Where 19 and 20 April come from
Thirty degrees of ecliptic longitude is the mark. The Sun reaches it on 19 or 20 April at a minute an ephemeris prints and a wall calendar cannot show, and fire gives way to earth, Mars to Venus, in that one instant.
The date is unstable for an arithmetical reason. The Sun's return to any fixed longitude takes roughly six hours more than three hundred and sixty five days, so the crossing creeps later through a four year run and is hauled back when February gains its extra day. End to end the swing is about thirty six hours, which is why some tables print the 19th and others the 20th without either being wrong.
Nothing is half Aries and half Taurus
Longitude is a single number. At twenty nine degrees and fifty nine minutes of Aries the Sun is in Aries; minutes later, at thirty degrees exactly, it is in Taurus. No classical text and no modern method reads the degrees around the mark as a mixture of the two.
Cusp is a real term, but it names a house boundary rather than a sign boundary. Houses are cut from the horizon and meridian of the birthplace, so they shift with where and when someone was born; the equal thirty degree signs do not. Applying the word to this changeover runs two different divisions of the sky together.
The Aries end, and the exaltation degree
Closing degrees of a sign are not a diluted version of it. One detail does mark this stretch out: the Sun's exaltation sits at nineteen degrees of Aries, so a birth in the final days of the season has the Sun already past that point rather than climbing towards it.
In the Chaldean decans the last third of Aries falls to Venus, the only part of the sign with a benefic over it. Texts read that band as drive with an object in view rather than motion for its own sake, which has nothing to do with the sign that follows.
The Taurus end
Past the mark the Sun stands in fixed earth under Venus and the reading is Taurus without qualification from the first degree onward: slow to be shifted, attached to what can be held and counted, unhurried by anyone else's timetable.
The opening decan of Taurus belongs to Mercury, which is why the earliest Taurus births are so often described as unusually talkative for the sign. That is a decan with a stated ruler, not Aries seeping over the line.
Settling a birth on 19 or 20 April
Year, clock time and birthplace are enough. Put them against an ephemeris, or the calculator below, and the solar longitude comes back to a fraction of a degree, on one side of thirty or the other.
Two practical notes. Within about two days of the mark a printed range is not evidence, because those ranges were rounded to whole dates and the crossing never falls on one. And should the answer contradict the sign someone grew up with, the correction touches the Sun alone; the Moon, the Ascendant and every other placement stay where they were.
Questions people ask about this
Am I a Aries Taurus cusp?
No, because that placement does not exist. The Sun was either short of thirty degrees at your birth or past it, which makes you Aries or Taurus.
What date does the Sun move from Aries to Taurus?
19 or 20 April, 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.