Can Two People Have the Same Birth Chart?

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Yes, but only in a narrow sense, and the answer depends on what "the same" covers.

• Same instant, same place, same method: the same calculated chart. A birth chart is a calculation from a date, a clock time and a place. Two births at exactly the same moment in the same place, run through the same software and settings, give the same calculated chart, degrees, angles and houses included.
• Same instant, different place: same planets, different frame. The site calculates planet positions as seen from the centre of the Earth, so two births at the same instant anywhere in the world share every planet's sign and degree. The Ascendant, Midheaven and house cusps depend on the place, so they can differ.
• Minutes apart, same place: usually the same planet signs, not an identical chart. A few minutes rarely change any planet's sign, but the exact degrees move, and the angles and houses move fastest of all. Sharing the same ten planet signs is therefore not the same thing as having a literally identical chart.

This page measures one thing only: how long the ten standard planet signs stay the same. Those ten are the Sun, Moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Exact degrees, aspects, the Ascendant and the houses were not measured. Whether a lineup ever comes back in a later year is a separate question, answered on does your birth chart repeat.

The results in brief:

• A ten-sign lineup typically lasts about two days. Over 140 years of computed sky, the median unchanged stretch was 49.7 hours; one in ten lasted under 10.6 hours, and the longest complete one 61.1 hours.
• The Moon is almost always the reason it ends. It caused 76.3% of all lineup changes. With the Moon left out, the remaining nine signs held for a median of 6.3 days.
• Births 5 minutes apart shared all ten signs for 99.80% of the measured time, 15 minutes apart 99.40%, one hour apart 97.61%. One day apart it was 48.82%, and one week apart 0%.
• On 48.7% of London calendar dates, all ten signs stayed the same from midnight to midnight. On the rest, at least one planet changed sign during the day.

These are shares of modelled time in the sky, not probabilities about people, twins or births.

Calculator note: the free birth chart lists the sign of each planet for the moment you enter. It does not tell you how long that lineup lasted or who else shares it. Ask the Astrologist does not compute these base rates either.

Four layers of "the same chart"

It helps to separate what a chart contains, from the slowest-changing layer to the fastest.

1. Planet signs. Which of the twelve signs each planet occupies. This is the layer measured here.
2. Planet degrees. The exact position inside the sign. The Moon moves about half a degree an hour, the Sun about one degree a day, so two births even minutes apart differ slightly here.
3. Aspects. The angles between planets, which depend on the degrees and the orb used.
4. Angles and houses. The Ascendant, Midheaven and house cusps depend on the clock time and the birthplace. On average the Ascendant moves through the whole zodiac in a day, so this layer can change within minutes. How sensitive it is, sign by sign, is covered in how accurate your birth time needs to be, and the angles themselves in chart angles explained.

Two charts can match on the first layer and differ on all the others. No Ascendant was calculated for this page.

How long one ten-sign lineup lasts

A lineup here is the list of ten signs, Sun through Pluto, at one instant. Every time any of the ten planets crosses into a new sign, in either direction, the lineup changes. The measured window ran from 1 January 1920 to 1 January 2060 (UTC). Inside it the lineup changed 29,422 times, so the window splits into 29,423 unchanged stretches, or runs. The first and last runs are cut off by the edges of the window, so they are left out of the figures below.

The ten-sign figures cluster just above two days because the Moon spends about two and a half days in each sign, and other planets cut many of those stays short. About 1% of complete ten-sign runs lasted less than an hour, which happens when two planets change sign close together. The nine-sign view is shown only to make the Moon's role visible; it is not a second definition of a birth chart.

Which planet ends a lineup

The single shared row is a moment on 19 December 1964 when the Moon and Venus changed sign within a second of each other, too close to separate in this calculation, so it is counted once as a joint change rather than given to either planet. Mercury, Venus and the slower planets include their backward crossings during retrograde periods; the guide to how long each planet stays in a sign explains why their stays vary.

Two births a set time apart

Pick a moment at random from the window, add a fixed gap, and compare the ten signs at the two moments. The table gives the share of the measured time for which they match.

A week is always long enough for the Moon to move on by more than one sign, so no pair of moments a week apart shared all ten. These figures describe the sky, not a population: they do not say how often real people born minutes apart share a lineup, only how much of the time two such moments would.

A whole calendar day

Each London calendar date from 1 January 1920 to 31 December 2059 (51,135 dates) was followed from local midnight to the next local midnight, keeping the 23-hour and 25-hour days when the clocks change. A date counts as having one lineup only if no planet changed sign at any point during it; checking only the first and last minute would miss a planet that changed and came back.

• 24,927 dates (48.7%) kept all ten signs the whole day.
• 44,485 dates (87.0%) kept the nine signs other than the Moon the whole day.

Most of the difference is the Moon. Figures for the Moon on its own, including how often it changes sign during a date, are on how often the Moon is in your Sun sign; they were not measured again here. If your birth time is unknown, the guide to a birth chart without a birth time explains what still holds.

Twins and births close together

Taken mechanically, the tables say this: two births in the same place, five or fifteen minutes apart, nearly always share all ten planet signs, while their exact degrees differ slightly and their Ascendant and house cusps may differ more. Births a day apart share all ten signs less than half the time. That is the whole of what these numbers say about twins. They say nothing about personality, health, relationships or how lives turn out, and they do not support or count against astrology.

Ruben as the worked example

Ruben is the fictional chart from how to find aspect patterns: London, 5 January 1993, 12:00 GMT. He was fixed as this page's example before any figure here was calculated, and his signs were recomputed with code version 476c723.

• His ten-sign run began when the Moon entered Gemini at 01:42 GMT on 5 January 1993 and ended when the Moon entered Cancer at 06:10 GMT on 7 January. It lasted 52 hours 28 minutes, a little longer than the median. In this calculation, anyone born at any moment of that stretch, anywhere, has the same ten signs, but not his degrees, angles or houses unless born at his exact moment and place.
• His nine-sign run (Moon left out) began when Venus entered Pisces at 23:53 GMT on 3 January and ended when the Sun entered Aquarius at 01:20 GMT on 20 January, a stretch of 16 days 1 hour.

Neither length says anything about him; they describe the sky around his birth.

How it was measured

This is one offline run of the routines behind the free chart, code version 476c723. For each of the ten planets, every moment its calculated longitude reached a multiple of 30° was found, in both directions, including the backward crossings of retrograde periods, and refined to well under a second. The sign boundaries come from the full-precision longitudes, not from the rounded figures the chart displays. The timeline was then cut at all 29,423 planet sign changes, and every piece was checked at its midpoint against a fresh calculation of all ten signs, as were 200,000 further moments picked at random, with no mismatches.

The measured window is 1 January 1920 to 1 January 2060. All results are shares of elapsed time. Dates after publication are computed positions, not forecasts. Chiron, the Nodes, the angles and the houses play no part.

One caveat on precision: compared offline with the Swiss Ephemeris, individual sign changes agree to within about a minute and a half for the Moon, Mercury and Venus, within about 15 minutes for the Sun and Mars, and within about 40 minutes for the slow outer planets, whose sign changes are gradual. Shifts of that size leave the figures above essentially unchanged.

A second check: noon dates

As a coarser cross-check, the free chart's own routine was run for London at 12:00 local clock time on all 51,135 dates. On 51,058 of them the chart's displayed signs matched the exact calculation for all ten planets; the 77 others each had a planet sitting exactly on a sign line in the chart's 0.01° display. Consecutive noons shared all ten signs 48.9% of the time and the nine non-Moon signs 87.0%, close to the one-day figures above. The exact results control everything on this page.

Reading it sensibly

A lineup that lasts two days is not ordinary in a meaningful way, and one that lasts an hour is not special. Sharing the ten planet signs with someone does not mean sharing a personality or a fate, and none of these figures says how many people have any lineup.

Doing the check yourself

1. Open the free birth chart for each person and read the sign of each of the ten planets from the planet table.
2. Compare the two lists sign by sign. Matching all ten means the same planet-sign lineup, not the same chart.
3. Compare the Ascendant and house cusps separately; they need an accurate birth time and depend on the birthplace.
4. If a planet sits within a fraction of a degree of a sign line, a small time difference can flip it.

Related articles

• Does Your Birth Chart Repeat? When the Same Planet-Sign Lineup Comes Back
• What Is a Birth Chart? A Simple Explanation
• How Accurate Does Your Birth Time Need to Be?
• Chart Angles Explained
• How Long Does Each Planet Stay in a Sign?
• How Often Is the Moon in the Same Sign as Your Sun?
• Birth Charts