Decans in Astrology: The Three Ten-Degree Thirds of Each Sign

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Every zodiac sign covers 30 degrees of the zodiac. Astrologers have long divided each sign into three equal parts of ten degrees, and each part is called a decan. Twelve signs with three decans each gives 36 decans around the whole circle. A planet at 4° Capricorn and a planet at 24° Capricorn are both in Capricorn, but they sit in different decans of it.

This page explains what a decan is, where the idea comes from, the two main ways astrologers assign a ruling planet to each decan, and how much weight a decan carries in a reading. The practical steps for working out your own decans are on how to find your decan, and the full list of rulers for all 36 decans is on decan rulers.

Educational page. Decans are an astrological concept that Astrologist.com explains; they are not a calculator feature. The free birth chart shows the sign and degree of each placement, which is all you need to work out a decan by hand, but it does not calculate, label or display decans or decan rulers. Worked positions on these pages were calculated offline with the same planetary routines the free chart uses. You can ask the Astrologist about decans as a concept or about placements you tell it, but it is not a decan calculator.

What a decan is

A decan is one third of a sign, measured in degrees of zodiacal longitude from the start of that sign. The three decans of every sign follow the same rule:

• First decan: from 0°00' up to, but not including, 10°00' of the sign.
• Second decan: from 10°00' up to, but not including, 20°00'.
• Third decan: from 20°00' up to, but not including, the end of the sign.

There is no position of 30°00' inside a sign. The point where one sign ends is 0°00' of the next sign, which is the start of that sign's first decan. So a planet at 9°59' of a sign is in the first decan, and a planet at exactly 10°00' is in the second. The exact longitude decides, which is why how to find your decan spends some time on positions that sit close to 10° or 20°.

Astrologist.com uses the Tropical zodiac, in which the signs are measured from the March equinox point. Decans on this site are measured the same way: ten-degree thirds of each Tropical sign. They are a subdivision of the sign, not a group of stars in the sky today.

Why there are 36

The number follows from the geometry. Twelve signs of 30° each make 360°, and cutting each sign into thirds gives 36 slices of 10°. The Sun moves along the zodiac at a little under one degree a day, so it spends roughly ten days in each decan. That rough figure applies to the Sun only. The Moon passes through a decan in less than a day, and the slow planets can stay in one decan for months or years.

Because the Sun's timing shifts from year to year, a list of decan dates can only be approximate. The reliable check is the degree in a chart, for the same reasons that sign dates move, as explained on born on the cusp.

A short history

The word decan comes from the Greek for "ten". Its history has two strands that are easy to blur.

In ancient Egypt, the decans were 36 groups of stars that rose in succession through the year. They were used to mark the hours of the night and were tied to the ten-day weeks of the Egyptian calendar. These were star groups in the sky, not divisions of a zodiac.

Later, astrologers writing in Greek in the Hellenistic period divided each zodiac sign into three ten-degree parts and gave each part a ruling planet. These parts were often called faces, and the word decan carried over to them. Historians still discuss how the older Egyptian star decans and the later zodiacal divisions connect, so this page does not present one as simply turning into the other.

The decans used on this site belong to the second strand: equal ten-degree thirds of each Tropical sign. They make no claim about which stars or constellations stand behind those degrees today.

The two ruler conventions

Most of the interpretive interest in decans comes from giving each decan a ruling planet. Two conventions are in wide use, and they often give different answers for the same decan.

Chaldean faces

The older convention is usually called the faces, or the Chaldean decans. It uses only the seven bodies visible to the naked eye, listed in what astrologers call the Chaldean order, from the slowest-moving to the fastest: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. The sequence starts with Mars at 0° Aries and then steps through that order, one planet per decan, repeating the seven as many times as needed.

So the three faces of Aries are ruled by Mars, the Sun and Venus. The faces of Taurus continue the run with Mercury, the Moon and Saturn, and the sequence carries on around the zodiac. Because 36 is one more than five times seven, the run ends with Mars in the last decan of Pisces, and Mars rules the first decan of Aries as well.

This is a traditional seven-body scheme. Uranus, Neptune and Pluto have no faces, because the scheme was fixed long before they were discovered. In traditional astrology the faces count as a minor essential dignity, a small extra condition beyond the four main ones.

Triplicity decans

The second convention, common in modern Western writing, gives each decan to a sign of the same element, taken in zodiac order. The first decan belongs to the sign itself, the second to the next sign of that element, and the third to the one after. Aries, a fire sign, has decans associated with Aries, Leo and Sagittarius. Cancer, a water sign, has decans associated with Cancer, Scorpio and Pisces. The decan is then said to be ruled by the ruler of that sign.

For Scorpio, Aquarius and Pisces, Astrologist.com gives the modern ruler first and keeps the traditional ruler in view, as explained on modern and traditional rulers. So the Cancer decans are ruled by the Moon, then Pluto (traditionally Mars), then Neptune (traditionally Jupiter). The elements themselves are covered on zodiac elements explained.

The name can cause confusion. Traditional astrology also has a separate essential dignity called triplicity, in which each element has its own set of day and night rulers. The triplicity decans described here are a different idea: a way of assigning decans by element. The two should not be mixed up.

Neither is the site's default

Astrologist.com does not treat either convention as the standard. They are two different traditions, and they agree on some decans and disagree on others. The third decan of Aries, for example, is ruled by Venus in the Chaldean faces and by Jupiter in the triplicity decans. If you use decan rulers, pick one convention, say which one you are using, and do not switch between them within a reading. Both are laid out side by side, with all 36 decans, on decan rulers for all 12 signs.

How astrologers use decans

In practice, a decan is a secondary shading within a sign. The site's working order for a placement stays the same: the planet says what, the sign says how, the house says where, and aspects describe how planets interact. A decan only refines the sign layer.

An astrologer who uses decans might note that a planet in the second decan of Cancer has a face ruled by Mercury, or sits in the decan associated with Scorpio, and take that as a small extra colour on the Cancer reading. The planet is still in Cancer, it still has Cancer's ruler as its dispositor, and its house and aspects still matter far more. Many astrologers read charts well without decans at all.

Decans also give a way to talk about parts of a sign that the sign alone does not separate. That is their main use, and it is a modest one. Like the rest of astrology on this site, they describe tendencies and patterns, not fixed outcomes.

What a decan is not

• Not a new sign. A planet in the second decan of Aries is in Aries. Readings that say "second-decan Aries is really a Leo" go further than the technique supports, and this site does not use them.
• Not a house. Decans divide signs by degree. Houses are a separate division based on the time and place of birth, and a sign degree is not a house cusp.
• Not a strength score. Astrologist.com does not rank decans or call any decan better, stronger or weaker than another. The site's dignity lookup on planetary dignities explained uses only domicile, detriment, exaltation and fall, and faces are not scored.
• Not a personality type. There are no 36 decan personalities here, and no decan compatibility or forecasts.
• Not a cusp. Being near the start or end of a sign is a different question, covered on born on the cusp.

Other traditional subdivisions of a sign, such as terms (also called bounds), use unequal spans of degrees and are not covered on these pages. Readings based on a placement being in an early or late degree are also a separate topic; ascendant degree vs rising sign touches on that for the Ascendant.

Decans and the rest of the sign

Decans sit inside the structure every sign already has. Each sign keeps its element and its modality across all three decans; zodiac modalities explained covers the cardinal, fixed and mutable groups. In the triplicity convention, the decans borrow the other signs of the same element, so a decan never introduces an element the sign does not already have. In the Chaldean faces, the decan ruler can be any of the seven traditional bodies, which is one reason the two conventions read so differently.

Next steps

To read the decans of your own placements, open the free birth chart, note the degree of each placement, and follow what decan am I? step by step. The site's calculation choices, including the Tropical zodiac and the modern-first rulers, are set out on the methodology page. If you want to talk a decan through, you can ask the Astrologist and tell it the sign and degree you are asking about.

Related articles

• What Decan Am I? How to Find Your Decans
• Decan Rulers for All 12 Signs
• Zodiac Elements Explained
• Zodiac Modalities Explained
• Planetary Dignities Explained
• Modern and Traditional Rulers
• Born on the Cusp?
• Ascendant Degree vs Rising Sign
• Zodiac Signs