What Are Decans?
Ten degrees of ecliptic longitude, two rulership traditions that disagree three times out of four, and an honest account of what the distinction is worth.
A decan is a third of a zodiac sign: ten degrees of ecliptic longitude, not a mood. Every sign spans 30°, so each divides cleanly into decan 1 (0°00′–9°59′), decan 2 (10°00′–19°59′) and decan 3 (20°00′–29°59′). Whichever of those three your Sun's degree lands in is your decan — and the same cut applies to every other body in the chart, not just the Sun. The boundaries are exact by definition; there is nothing to interpret about them. Everything else about decans — which planet rules which third, and what that rulership is meant to add — is inherited tradition, and there are two surviving traditions that disagree with each other about three times out of four. This page gives you the degrees, both tables, where the thirty-six came from, and a straight answer about how much a decan is worth standing next to your Moon or your rising sign.
The boundaries are degrees, not dates
A sign is 30° of the tropical zodiac. Ten degrees is a third of it. That is the entire definition, and it is why decan boundaries are the least arguable thing in astrology: 0°, 10° and 20° of every sign, twelve signs, thirty-six decans, 360° with nothing left over.
What is not exact is the calendar. Decans get quoted as ten-day date ranges — Aries I as March 21–30, and so on — because the Sun covers roughly one degree a day. Roughly. The tropical year is not a whole number of days, so the Sun's degree at a given clock time drifts by up to about a day between years, and the leap-year correction snaps it back. A date table is a good approximation and a bad authority.
Finding your decan, in three steps
- Get the degree, not just the sign. Any chart that reports a planet's position to the degree gives you this for free. The free Zodiux chart prints sun, moon, rising and every planet by sign and degree, computed in your browser.
- Divide by ten and take the whole number. 0–9° is the first decan, 10–19° the second, 20–29° the third. A Sun at 17°42′ Virgo is second-decan Virgo. There is no rounding rule to remember, because 9°59′ is still the first decan and 10°00′ is already the second.
- Repeat for anything else you care about. The Moon has a decan. So does the Ascendant, Mercury, Mars. Most people only ever look up the Sun's, which is a habit rather than a rule.
If you were born within about a day of a sign boundary the decan question is downstream of a bigger one — which sign you are in at all. That is what cusps are about, and the answer is the same either way: run the actual chart rather than trusting a date table.
Thirty-six star-groups, and the hours of the night
Decans did not start as slices of the zodiac. They started as stars. Ancient Egyptian astronomers tracked a set of thirty-six star-groups whose successive first appearances at dawn fell roughly ten days apart, and used them to mark the hours of the night — the tables survive painted on Middle Kingdom coffin lids, laid out as a grid of stars against nights of the year. Thirty-six groups, ten days each, a 360-day scheme with five days left over at the end of the year.
The arithmetic is why they survived. When Hellenistic astrology inherited the Babylonian twelve-sign zodiac, thirty-six decans and 360 degrees mapped onto each other exactly: three decans per sign, ten degrees each. The star-groups themselves were dropped in the process. What crossed over was the number and the division — which is worth knowing, because it means the modern decan has no observational content at all. Nothing rises at 10° Taurus. The boundary is a convention that outlived the sky-watching that produced it.
The Greek word behind it is dekanos, from the ten. Later traditions attached long chains of imagery to the thirty-six — Egyptian, Indian and medieval European sources all built different pictures onto the same skeleton — and none of those chains agree either. The two schemes below are the ones that stayed in general use, and the only two Zodiux publishes.
Two rulership systems, both still in use
Almost every site that mentions decans assigns each one a ruling planet. Very few say which of the two schemes they are using, and the two schemes give different answers for twenty-seven of the thirty-six decans. Zodiux prints both, labelled, on every birthday decan page, for exactly that reason.
The Chaldean faces
The older scheme, usually called the faces. Take the seven classical planets in descending order of apparent speed — Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon — and deal them out one per decan, starting at Mars for the first ten degrees of Aries. Then keep dealing. The sequence runs straight through all thirty-six decans without pausing at sign boundaries, which is the defining feature of the system rather than a flaw in it.
The order is the same one behind the planetary hours and the days of the week, so the faces are not an isolated invention — they are the classical speed sequence applied to a different unit.
The triplicity decanates
The newer and far more commonly quoted scheme. Each sign's first decan takes that sign's own traditional ruler; the second and third take the rulers of the other two signs of the same element, in zodiac order, wrapping around. So Aries — a fire sign — runs Mars (Aries), Sun (Leo), Jupiter (Sagittarius). Leo starts one step further along the same cycle: Sun, Jupiter, Mars.
Two consequences fall out of that construction. The first decan of every sign is always ruled by the sign's own ruler, so it never adds anything the sign did not already say. And because the scheme uses the traditional seven-planet rulers, Aquarius decans run Saturn, Mercury, Venus rather than starting from Uranus — the same traditional-versus-modern split covered in what a chart ruler is.
| Decan | Degrees of Aries | Usual dates | Chaldean face | Triplicity ruler |
|---|---|---|---|---|
| First | 0°00′ – 9°59′ | Mar 21 – Mar 30 | Mars | Mars — Aries' own ruler |
| Second | 10°00′ – 19°59′ | Mar 31 – Apr 9 | Sun | Sun — from Leo |
| Third | 20°00′ – 29°59′ | Apr 10 – Apr 19 | Venus | Jupiter — from Sagittarius |
Aries is deliberately the friendliest possible example: it is where the Chaldean sequence is anchored, so two of its three decans happen to come out the same under both systems. That is not typical. Gemini is the other extreme — Jupiter, Mars and Sun by face, against Mercury, Venus and Saturn by triplicity — where the two schemes agree about nothing at all.
Why the two systems drift apart
The disagreement is arithmetic, not doctrine. Thirty-six decans divided among seven planets does not come out even — 36 is not a multiple of 7 — so the Chaldean sequence cannot stay in step with a twelve-sign, three-decan-per-sign structure. Each sign advances the face sequence three places in a seven-place cycle, so the faces only return to their starting planet every seven signs — and there are twelve. The two schemes drift in and out of phase across the circle and never lock.
There is even a visible seam. Run the faces to the end and the last decan of Pisces lands on Mars, immediately before the first decan of Aries, which is also Mars. Two adjacent Mars decans across 0° Aries, because the sequence restarts at the start of the zodiac rather than continuing its count. Anyone who tells you the faces are a smooth unbroken cycle has not run it to the end.
Counted across all thirty-six decans, the two systems name the same planet nine times — exactly a quarter — and disagree the other twenty-seven. The table below shows the first decan of each sign, where the pattern is easiest to see: the triplicity column is just the sign's own ruler copied over, every time, while the Chaldean column marches on regardless.
| Sign | Traditional sign ruler | Chaldean face, decan 1 | Triplicity ruler, decan 1 |
|---|---|---|---|
| Aries | Mars | Mars | Mars |
| Taurus | Venus | Mercury | Venus |
| Gemini | Mercury | Jupiter | Mercury |
| Cancer | Moon | Venus | Moon |
| Leo | Sun | Saturn | Sun |
| Virgo | Mercury | Sun | Mercury |
| Libra | Venus | Moon | Venus |
| Scorpio | Mars | Mars | Mars |
| Sagittarius | Jupiter | Mercury | Jupiter |
| Capricorn | Saturn | Jupiter | Saturn |
| Aquarius | Saturn | Venus | Saturn |
| Pisces | Jupiter | Saturn | Jupiter |
Only Aries and Scorpio match in that column — the two signs Mars traditionally rules, which is the anchor of the Chaldean sequence showing through. Everywhere else the first decan of a sign is described by two different planets depending on which book you opened.
How much weight a decan actually carries
A decan is a modifier inside a sign, not a thirteenth sign and not a second sun sign. Everything true of your sign is still true; the decan shifts emphasis within it.
The honest way to size it is by how much of the population shares it. Your sun sign is a label on about a twelfth of everyone; your sun decan narrows that to about a thirty-sixth. That is a real refinement, and it is still a very large group of people. The placements that actually separate you from someone born the same week are the fast ones — and that is a fact about orbital speed, not a matter of interpretive taste.
| What moves | Typical speed | Time to cross one decan |
|---|---|---|
| Ascendant | about 1° every 4 minutes | about 40 minutes |
| Moon | 12–15° a day | 16 to 20 hours |
| Sun | about 1° a day | about 10 days |
| Mars | 0.52° a day on average | about 19 days |
| Jupiter | 0.083° a day on average | about 4 months |
| Saturn | 0.033° a day on average | about 10 months |
Read down that column and the ranking sorts itself out. Everyone born in your ten-day window shares your sun decan. Your moon sign changes every two and a half days and your moon decan every sixteen to twenty hours. Your rising decan changes every forty minutes, which is why it is the most individual thing on the list — and also the most fragile. An hour of error in a recorded birth time moves the Ascendant by roughly half a sign, so it can cost you the rising sign outright; it costs you the rising decan every time.
So: the sun decan is a genuine refinement of a coarse label, and the coarse label is not where the information is. If you have an accurate birth time, the Moon and the Ascendant are worth more attention than the third of Leo you happen to occupy.
What Zodiux publishes, and what it does not
Every one of the thirty-six decans has its own page under birthday decans, reached by birth date. Each of those pages states the exact degree range, the sign's own ruler, the Chaldean face and the triplicity decanate — all three labelled, and with a line saying whether the two schemes agree on that particular decan or not. The three decans of a sign link to each other, so the neighbouring thirds are one click away rather than something to hunt for.
The degrees on those pages are computed, not transcribed: both rulership tables are generated from the rules described above rather than copied from a source that might have propagated someone else's typo. The date ranges are the conventional ones, and the pages say plainly that dates shift by up to a day between years while degrees do not.
What is not there: house placement. The free chart reports every body by sign and degree but does not draw house cusps, so it takes no position on Placidus versus Whole Sign versus Koch — and no decan reading here depends on one. There is also no attempt at the medieval decan imagery, which varies so much between sources that publishing one version as the version would be a claim Zodiux cannot support.
Method. Decan boundaries are definitional: each sign spans 30° of the tropical zodiac and each decan is the 10° third, so the cuts fall at 0°, 10° and 20° of every sign. Degrees come from the same pipeline as the rest of the site — the birth time converted from local wall clock to UTC through the birth city's IANA time zone, then Astronomy Engine for geocentric apparent ecliptic longitude, corrected for light-travel time and aberration, expressed in twelve equal 30° tropical signs measured from 0° Aries. Both rulership tables in this article are generated from their rules, not transcribed: the Chaldean faces by dealing Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon in that order across the thirty-six decans starting at Mars for Aries I, and the triplicity decanates by taking the traditional ruler of each sign of an element in zodiac order, offset so that a sign's first decan takes its own ruler. The count of nine agreements out of thirty-six is the number of decans where those two generated tables name the same planet. The decan-crossing times are 10° divided by each body's mean daily motion — for the Ascendant, the standard figure of roughly one degree every four minutes of clock time — and are averages that ignore retrograde loops and the Moon's varying speed.
Limits. Decan rulerships are inherited symbolic assignments, not measurements: nothing observable changes at 10° of a sign, and the ancient star-groups the thirty-six are named after were dropped when the scheme was mapped onto the zodiac. The two systems presented here disagree for twenty-seven of the thirty-six decans and there is no evidence that could settle between them, so this page publishes both rather than choosing. The calendar date ranges are approximations good to about a day; only the degree is exact, and the degree requires a real chart. The Ascendant's decan additionally requires an accurate birth time — with no birth time Zodiux assumes local noon, says so, and omits the rising sign entirely rather than guessing, so there is no rising decan to report. Nothing on this page is a claim about anyone's health, relationships, finances or future; it is for reflection and entertainment.
Sources.
- Astronomy Engine — Open-source implementation of the VSOP87 and NOVAS algorithms, bundled into this site and used identically in the browser and at build time. Supplies the ecliptic longitude any decan is read from.
- Zodiux decan rulership tables — Both schemes are computed in the site's own page builder from the rules stated above — Chaldean speed order anchored at Mars for Aries I, and triplicity decanates anchored on each sign's traditional ruler — and the same generated values appear on all thirty-six /birthday/ pages. The nine-of-thirty-six agreement count is taken from those generated tables.
- Egyptian decanal star tables — The thirty-six decans originate as star-groups used to mark the hours of the night, attested in the diagonal star tables painted inside Middle Kingdom coffin lids. The mapping onto ten-degree thirds of the twelve-sign zodiac is a later Hellenistic development; the original star-groups play no part in it.
- Traditional seven-planet rulership — The triplicity decanates use the classical rulers — Mars for Aries and Scorpio, Venus for Taurus and Libra, Mercury for Gemini and Virgo, Moon for Cancer, Sun for Leo, Jupiter for Sagittarius and Pisces, Saturn for Capricorn and Aquarius — which is why the modern rulers Uranus, Neptune and Pluto do not appear in either table.
Written and reviewed by the Zodiux editorial team · Last reviewed · Editorial standards and corrections · Astrology is for reflection and entertainment, not advice.
Get the degree, then read the decan
The free Zodiux chart reports your sun, moon, rising and every planet by sign and degree, so finding a decan is just dividing by ten. It runs entirely in your browser: your birth details are never sent to us and never written into the URL. Leave the birth time blank and it assumes local noon, says so, and leaves the rising sign — and therefore the rising decan — blank rather than guessing.
How do I know which decan I am?
Look at the degree of your Sun, not the date. 0–9° of your sign is the first decan, 10–19° the second, 20–29° the third. A Sun at 24°08′ Scorpio is third-decan Scorpio. Date tables are accurate to about a day, so if your birthday falls on the edge of a published range, only the computed degree can settle it.
Which decan ruler is correct, the Chaldean face or the triplicity ruler?
Neither is verifiable, and they disagree for twenty-seven of the thirty-six decans. The Chaldean faces are older and deal the seven classical planets in speed order across all thirty-six decans without regard to sign boundaries. The triplicity decanates are what most popular astrology quotes, usually without naming the scheme, and give each sign's first decan to its own ruler. Zodiux prints both and marks the nine decans where they agree.
Is a decan the same thing as a cusp?
No, and they are near-opposites. A cusp is the boundary between two signs — the point where one ends and the next begins. A decan is a ten-degree third of the space inside a single sign. A third-decan Aries is at the end of Aries and still entirely Aries; being at 29° of a sign does not put anyone partly in the next one.
Does my decan change my sun sign?
No. The decan is a subdivision of the sign, so everything true of the sign is still true. It shifts emphasis inside the sign by adding a second traditional ruler — under one scheme or the other — and that is the whole of its job.
Does everything in my chart have a decan, or only the Sun?
Everything. The decan is a property of a degree, so the Moon, the Ascendant and every planet each fall in one. The Moon crosses a decan in sixteen to twenty hours and the Ascendant in about forty minutes, which makes those the most individual decans in a chart — and the ones most easily lost to a birth time rounded to the nearest hour.