Birthday to zodiac sign: the twelve ranges, and the seventeen dates a table cannot settle
For most birthdays a printed table is right and one step is enough. The sun spends about a month in each sign, so any date lying two or more days clear of a boundary returns the same answer in every table and in every year. The exceptions can be counted: across the 101 years from 1930 to 2030, seventeen calendar dates change sign at least once. On those, a lookup needs the year of birth before it needs anything else.
The twelve ranges as they fall this year
For 2026: Aries 3.21 – 4.19, Taurus 4.20 – 5.20, Gemini 5.21 – 6.20, Cancer 6.21 – 7.22, Leo 7.23 – 8.22, Virgo 8.23 – 9.22, Libra 9.23 – 10.22, Scorpio 10.23 – 11.21, Sagittarius 11.22 – 12.21, Capricorn 12.22 – 1.19, Aquarius 1.20 – 2.18, Pisces 2.19 – 3.20.
That is one year's slice rather than a permanent table. The same list on /zodiac-dates is recomputed for whichever year it is read in, which is the only way a date range can be true of a particular reader rather than true on average. Its convention is stated in the footnote and every figure below rests on the same probe: the sun's ecliptic longitude at noon UTC on each date, divided by 30°.
Two entries above are worth marking. In 2026 Aquarius begins on 20 January and Sagittarius on 22 November, which happen to be the two least stable dates in the calendar.
The seventeen dates that will not hold still
Run all 101 years and the calendar dates that change sign are these: 20 January, 18 and 19 February, 20 and 21 March, 20 April, 21 May, 21 June, 22 and 23 July, 23 August, 22 and 23 September, 23 October, 22 November, and 21 and 22 December. The remaining three hundred and forty-eight dates never moved across the whole span, and a birthday among them is settled by the date alone.
The seventeen are not equally unstable. 22 September falls to Virgo in one hundred of the 101 years and to Libra in exactly one, 2028, so a table will almost never mislead anyone born on it. At the other end, 20 January and 22 November are close to a coin toss, each splitting 53 years to 48. The year-by-year counts for all seventeen are on /zodiac-dates, and why the boundaries drift at all is worked out at /blog/zodiac-dates-shift; only the consequence is needed here.
On those dates the year decides before the hour does
An even split is not a random one. 20 January fell to Capricorn in roughly three years out of four from 1930 through the 1960s; the two signs divided the 1970s, 1980s and 1990s almost equally; and since 2000 the date has returned to Capricorn only in leap years. A birth on 20 January 1955 is Capricorn and a birth on the same date in 2010 is Aquarius. Both answers are correct, and the year is the whole difference. 22 November follows the same path, weighted to Scorpio early and to Sagittarius lately.
Other dates are cut cleanly by the leap cycle, and there the exceptions can simply be named. 21 December falls to Capricorn in only five of the 101 years — 2012, 2016, 2020, 2024 and 2028 — and to Sagittarius in the other ninety-six, so a child born on 21 December 2020 has a Capricorn sun while nearly every printed table answers Sagittarius. 18 February belongs to Pisces in four years only: 2017, 2021, 2025 and 2029.
So the second step of a lookup is short. Take the date back to the table with the year attached, and for most of the seventeen the answer stops there.
What the hour settles, and what the birthplace does not
Noon UTC is a stated convention rather than a fact about any birth. Since an ingress is an instant rather than a date, a birth before dawn or late at night on a changeover date can fall on the far side of it, which is why the pages that take an hour and a place use the birth instant itself. The sun covers about one degree a day, two and a half arcminutes an hour, so beyond a day from a boundary the date decides on its own and closer than that the clock does; the tolerance on the underlying figure is at /method.
The birthplace does no astronomical work here at all. The sun's longitude does not depend on where it is seen from, and a place is asked for only to turn a clock reading into a universal instant, using the offset actually in force there that year rather than the offset in force now. In that respect a sun sign is the exact opposite of an ascendant, which cannot be computed without a location and is the subject of /blog/rising-sign. The figure to enter is the figure written on the certificate.
After the sign is found
The sun is one position of three, and much the most forgiving of the hour. The ascendant needs an hour and a city, the moon a date and an approximate hour; both are free, at /rising-sign and /moon-sign, and /blog/three-placements sets out what each of the three is taken to describe.
A sign name is an astronomical position rather than a verdict on a life. What this site computes can be laid beside any other ephemeris and checked; what is written beneath it belongs to a tradition, offered as a vocabulary for reflection and composed by an AI working in that tradition.