Where the decan sits
Every sign divides into three parts of ten degrees each, called decans or faces. The first decan of Aries occupies the first ten degrees of the zodiac itself, beginning at the March equinox.
The Sun crosses it in roughly ten days, from around 20 March to around 30 March. The exact dates shift by up to a day and a half from year to year, for the same reason the sign dates shift: the tropical year is not a whole number of days.
Why both schemes say Mars
The Chaldean order assigns rulers to the thirty six decans in a fixed sequence, Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, repeating from the start of Aries. Since Aries is where the count begins, its first decan takes Mars.
The triplicity scheme works differently: each decan goes to a sign of the same element, taken in order. The first decan of a fire sign goes to the first fire sign, which is Aries, ruled by Mars.
The two schemes disagree about most decans. Here they land on the same answer, which is why this decan is often described as Aries at its least diluted.
What the doubling is taken to mean
The traditional reading is that a planet here gets the sign's qualities without any of the modification the later decans bring. Where the second decan of Aries lends it something of Leo and the third something of Sagittarius, the first adds nothing and subtracts nothing.
In practice that is read as directness, speed and a short interval between deciding and acting. It is also read as the least patient stretch of the zodiac, which is the same trait described from the other side.
How much weight to give any of this is a judgement. Decan work is a real part of the tradition, older than the signs in some respects, but it is a second-order refinement rather than a foundation.
Checking your own degree
A decan boundary is a specific degree, so the answer depends on your solar degree rather than on your birthday alone. Someone born on 30 March may be in the first decan or the second depending on the year and the time of day.
The check above computes the actual position of the Sun for your date and reports the degree along with both rulers. If you were born close to a boundary it will tell you which side you fall on, which a date range cannot do.
Questions people ask about this
What dates are the first decan of Aries?
Roughly 20 to 30 March, though the boundaries move by up to a day and a half between years. The check above gives your exact solar degree rather than a range.
Who rules the first decan of Aries?
Mars, in both of the schemes in common use. The Chaldean order starts its sequence with Mars at the beginning of Aries, and the triplicity scheme gives the first fire decan to Aries itself.
What is a decan?
A ten degree division of a sign. Every sign has three, giving thirty six in total. The system is older than much of what surrounds it and comes originally from Egyptian star lists.
Do decans apply to planets other than the Sun?
Yes. Any placement sits in a decan, and the technique is applied to the Ascendant and the Moon as often as to the Sun. This page is written around the Sun because that is what most people are looking for.
Are decans the same as the modern decan meanings online?
Not always. Many popular articles use the triplicity scheme without saying so, which is why a decan ruler quoted elsewhere may differ from the Chaldean one given here. Both are shown so the difference is visible.
Does the first decan make someone more Aries?
That is the traditional claim, on the grounds that nothing else is mixed in. It is an interpretation rather than a measurement, and this page marks it as one.