Where the Sun is, and when
A sign contains three faces of ten degrees, and there are thirty six around the ecliptic. This is the first in Aquarius, running from the ingress up to ten degrees.
The Sun usually occupies it from about 20 January to about 30 January. The lower edge is the sign change, which explains why the Aquarius start date is printed as 19 or 20 January depending on the source. Every boundary in the zodiac wanders by up to a day and a half between years, because the tropical year is not a whole number of days, so late January birthdays near either end deserve an actual calculation rather than a lookup.
Its own sign, and Venus
Under the triplicity scheme, a sign's faces go to the signs of its element in order. Aquarius, Gemini and Libra share the air triplicity, so the first Aquarius face stays with Aquarius, ruled by Uranus in contemporary practice and by Saturn in the older tradition.
The Chaldean order takes no account of that. It runs Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter in an unbroken loop from zero degrees of Aries, and the thirty first face falls to Venus.
Venus and Uranus are an awkward pair, and a Venus ruler is the last thing most descriptions of this decan mention. That is because the majority of published decan writing uses triplicity without naming it, so the Chaldean answer simply never appears. Naming the scheme is what makes the disagreement legible instead of confusing.
Once the method is stated, the accounts stop looking like contradictions and start looking like separate layers.
What is read into each
With no second sign blended in, the triplicity reading treats this as Aquarius at full strength. The traits usually given to the sign are taken without moderation: a preference for reasoning a position out rather than inheriting it, comfort with being the dissenting voice, and interest in how a group works rather than in any one member of it. Older texts using Saturn add a structural note, describing rules kept for their own sake as readily as rules broken.
The Chaldean Venus is read as an unexpected warmth in that. Venus is associated with attachment and with taste, so here it is interpreted as the point at which detachment meets a genuine liking for people, often expressed through friendship and shared interests rather than through intimacy. Traditional writing on this face links it to being sought out for company by those who do not otherwise fit in.
Any placement in these degrees carries this, not the Sun alone. The Ascendant and the Moon are read by decan as a matter of routine. The system itself comes down from Egyptian star lists that predate the twelve signs in their familiar form, which is why it has survived, but it stays a refinement over a chart rather than a foundation under one.
Near the ingress
Because this decan opens at the sign change, a birthday on 19 or 20 January raises the prior question of whether the Sun had reached Aquarius at all. The Sun moves about a degree a day, so the hour of birth can decide it.
The check at the top of the page computes your solar longitude for the date you enter and reports the exact degree along with the ruler each scheme gives. That answers both questions together, which a table of dates cannot.
The neighbouring face is named too, so a narrow result is easy to place.
Questions people ask about this
What dates make up the first decan of Aquarius?
About 20 to 30 January in most years. The opening edge is the Capricorn to Aquarius ingress, which moves between the 19th and 20th, so birthdays there need calculating.
Is this decan ruled by Uranus, Saturn or Venus?
Uranus in modern triplicity practice, Saturn in the older version of the same scheme, and Venus under the Chaldean order. All are traditional answers to slightly different questions, which is why this page gives them rather than picking one.
Does a first decan make someone more typical of the sign?
That is the usual argument, since no second sign is mixed in. It describes emphasis and remains an interpretation, and a Moon or Ascendant elsewhere can easily outweigh it.
Why does Venus rarely appear in descriptions of this decan?
Because most popular writing uses the triplicity scheme without naming it, and Venus only shows up in the Chaldean count. It is not that one source is wrong, just that the method is left unstated.
Can decans be read on the Moon and Ascendant?
Yes. Every placement sits in one of the thirty six decans, and many practitioners read the Ascendant this way before the Sun. The Sun is used here because it needs only a date.
How much does a decan really add to a reading?
A layer of detail, no more. The lineage is genuine, running from Egyptian star lists into Hellenistic astrology, but the sign, the house and the aspects carry far more weight in practice.