Ten to twenty degrees, and the dates behind them
A sign splits into three faces of ten degrees. This is the middle one, so the Sun is past the raw opening of the zodiac but not yet at the handover into Taurus.
In calendar terms the Sun spends about ten days here, roughly from 30 March to 9 April. Treat those dates as approximate. The tropical year does not divide neatly into whole days, so a boundary can arrive up to a day and a half earlier or later depending on the year and on where you were born.
Ten degrees is also roughly the span the Sun covers in the time it takes the Moon to move through a third of a sign, which is one reason the divisions were convenient long before anyone attached planets to them.
Chaldean order and triplicity, side by side
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and then repeats, starting from zero degrees of Aries. The first face takes Mars, so the second takes the Sun.
The triplicity scheme ignores that sequence entirely. It gives each decan to a sign of the same element in order, so the fire decans go to Aries, then Leo, then Sagittarius. The middle decan of Aries therefore belongs to Leo, and Leo's ruler is the Sun.
The answer matches. That is worth naming, because most popular articles quote one scheme without saying which, and a reader who compares sources on other decans will find flat contradictions. Here there is nothing to reconcile.
What Leo and the Sun are read as adding
The Leo colouring is not read as extra warmth in a vague sense. It is read as a change in what the Aries impulse is aimed at. Aries on its own is described as acting before the situation is settled; with Leo mixed in, the tradition describes the same speed attached to something worth being seen doing, so the action wants an audience or at least a witness.
The solar face is taken to add a further thing: steadiness of direction. The Sun in this scheme is associated with holding a single line rather than scattering, so the reading softens the classic Aries complaint of starting more than gets finished. The corresponding cost, in the same tradition, is that a project once claimed becomes hard to abandon even when it should be.
None of this is measurement. It is what the sources say, and it is offered as interpretation rather than as fact about you.
One practical use of the face is comparative. Two people born a week apart in Aries can be given the same sign description and yet fall either side of a boundary, and the decan is the first thing in the tradition that distinguishes them without needing a birth time.
Beyond the Sun, and how to be sure
Decans apply to any point in a chart, not only the Sun. An Ascendant at fifteen degrees of Aries sits in this face, and so does a Moon at eleven degrees. Practitioners who use decans at all tend to read the Ascendant with them first, because the rising degree changes fastest and so distinguishes charts most sharply.
The wider caution is that decan work is genuine and old, descending from Egyptian lists of stars used to mark the hours of the night, but it is a refinement laid over the chart rather than something the chart is built on. Sign, house and aspect come first.
The check at the top of this page computes your actual solar degree from your birth date instead of matching a range. If you were born around 30 March or 9 April that is the only way to settle which face you are in.
Questions people ask about this
What dates does the second decan of Aries cover?
Approximately 30 March to 9 April. The exact crossing moves by up to a day and a half between years, so anyone born at either end should check a computed degree rather than trust the range.
Which planet rules this decan?
The Sun, under both schemes. The Chaldean sequence reaches the Sun on the second face of Aries, and the triplicity scheme assigns the middle fire decan to Leo, which the Sun rules.
Why do some sites give a different ruler?
Usually because they are quoting a scheme they have not named, or because they have miscounted the Chaldean sequence. Both schemes are stated here so the source of any disagreement is visible.
Does the decan override the sign?
No. It qualifies the sign. A Sun here is read as an Aries Sun first, with the Leo and solar material treated as a second-order adjustment to the description.
Can I use decans for my Moon or Ascendant?
Yes, and many practitioners prefer to. Any placement occupies a decan, and the Ascendant is the usual first choice because the rising degree moves quickly enough to separate charts born on the same day.
Where did decans come from?
From Egyptian star lists, where thirty six groups of stars marked divisions of the night. They were later mapped onto the zodiac and picked up planetary rulers, which is where the Chaldean order enters.