One in 95 birthdays gets the wrong zodiac sign

What we found

• On March 20, 65% of babies were born after the Sun had entered Aries, so most of them are Aries, not the Pisces the table gives them.
• On December 21, 43% were Capricorn rather than Sagittarius. On July 22, 43% were Leo rather than Cancer.
• The share follows the leap-year cycle: 1.5%–1.7% of births in leap years, and as little as 0.6% two years later.
• Tables disagree with each other too. Britannica's dates put 1 in 72 births in the wrong sign, and Wikipedia's 1 in 30.

Astrology is a symbolic language, not a science, and this study doesn't test it. It is about the calendar: whatever a Sun sign means to you, the dates a table prints are not the dates the Sun keeps.

The dates the table gets most wrong

The share of babies born on each date, 2000–2014 together, whose Sun sign differs from the common table. The twelve dates with the largest share.

Wrong-sign births by year

The share of each year's US births that the common table puts in the wrong sign. The four-year rise and fall is the leap-year cycle.

Why the dates move

A Sun sign is the twelfth of the zodiac the Sun was in when you were born. The Sun crosses from one sign into the next at a precise moment, and that moment is different every year.

The calendar year is about six hours shorter than the Sun's cycle, so each crossing comes about six hours later than the year before, until a leap day pulls it back by almost a day. Over a four-year cycle the same crossing wanders across two or three calendar dates.

Time zones split it again. The Sun entered Aries at 03:06 UTC on March 20, 2024: still the evening of March 19 in New York, already the early hours of March 20 in London.

Which table, and how much the assumptions matter

The headline uses the most common table, printed identically by astrology.com, horoscope.com, cafeastrology.com, astrostyle.com and zodiacsign.com (Aries from March 21, Taurus April 20, Gemini May 21, Cancer June 21, Leo July 23, Virgo August 23, Libra September 23, Scorpio October 23, Sagittarius November 22, Capricorn December 22, Aquarius January 20, Pisces February 19). Two other widely read tables, and two assumptions that could move the result:

Each cell is the share of births in the wrong sign, and how many births that is one in. The two assumptions move the common table's result between 1.05% and 1.07%: the finding doesn't depend on them.

Method

Births: the Social Security Administration's US births by date of birth, 2000–2014 (62,187,024 births), as published by FiveThirtyEight (CC BY 4.0).

Time of day: births on each date are spread over the day by the CDC's hourly distribution of US births (NCHS Data Brief 200, 2013 data for 41 states and the District of Columbia, 90% of births), weekday or weekend by the date's day of the week, and evenly within each hour.

Time zones: births are split across the seven US zones by each zone's share of births (Census county birth counts, 2000–2014), using each zone's daylight-saving rules as they were, including Indiana's year-round Eastern Standard Time before 2006 (IANA time-zone database).

The real sign: the moment the Sun's ecliptic longitude crossed each sign boundary, from our engine. For every year from 1920 to 2035 it agrees with Swiss Ephemeris within a minute, and with the U.S. Naval Observatory's equinoxes and solstices within their printed minute.

The check: every number on this page was computed twice. A second, independent program (Swiss Ephemeris instants, Python's own time-zone database, births sampled minute by minute) agreed with the first within 0.004% on the headline and within 144 births on every variant.

Limitations

• Birth times are modelled, not observed: births on a given date follow the 2013 hourly curve only on average. Spreading them evenly over the day instead gives 1.07%.
• The SSA counts run about 2% above the CDC's for the same years, probably because they include births in US territories. Those births are placed in the 50 states' time zones here.
• The counts are expected numbers of births, not a list of people. Holidays are treated as weekdays.
• This is the tropical zodiac most Western astrology uses. Precession and the constellations (the "Ophiuchus" story) are a different question.

Data

Download the data (CSV): every date with wrong-sign births, 2000–2014

Births: FiveThirtyEight, US births 2000–2014 from the Social Security Administration, CC BY 4.0. Hourly distribution: CDC/NCHS. Our derived data is CC BY 4.0: credit Moon and Rising (moonandrising.com) and link to this page.

Find your own birthday

The cusp-date pages give the exact moment for every year from 1925 to 2030, in your time zone:

• March 20
• December 21
• July 22
• All zodiac sign dates

Know your birth time and place? Your birth chart gives the exact answer.