Degrees and approximate dates
Ten to twenty degrees of Aquarius, the second of the sign's three ten degree divisions. The Sun passes through in roughly ten days, generally from around 30 January to around 9 February.
Those dates are guides rather than fixtures. The tropical year is a little longer than three hundred and sixty five days, so the whole sequence of ingresses drifts and each boundary can sit up to a day and a half from its position the previous year. A birthday on 30 January or 9 February sits close enough to an edge that only the degree will settle which decan holds it.
Both counts arriving at Mercury
The Chaldean order is a plain repeating cycle, Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, begun at zero degrees of Aries and carried through all thirty six decans without restarting. Counted through, the middle decan of Aquarius comes out on Mercury.
The triplicity scheme reaches the same planet another way. It hands each decan to a sign of the same element in order, and the air signs run Gemini, Libra, Aquarius. An Aquarius sequence therefore goes Aquarius, Gemini, Libra, so this decan belongs to Gemini, whose ruler is Mercury.
Agreement of this kind is uncommon. Across most of the zodiac the schemes name different planets, and popular writing quotes one without saying which, so readers meet contradictory rulers for the same degrees and assume someone has made an error. Here that problem does not arise, which makes the decan a useful illustration of what the schemes actually are.
Gemini inside Aquarius, and what Mercury adds
Aquarius shaded by Gemini is read as the sign's detachment becoming conversational. Where the opening decan is described in terms of holding a position regardless of company, this one is described in terms of circulating, comparing sources and finding the argument interesting whether or not it is settled. The tradition attaches to these degrees an appetite for information about people rather than involvement with them, which read unfavourably becomes commentary offered where participation was wanted.
Mercury named a second time by the Chaldean count is taken to state that character rather than balance it. Older material on this face speaks of writing, messages, instruction and dealings conducted at a distance. Because nothing in either scheme introduces a contrasting planet, the usual reading of the decan is verbal and analytical throughout, with the qualification that Mercury describes how something is handled and says nothing about what is being handled or why.
Any placement, and a sense of proportion
The decan applies to whatever occupies the degrees. An Ascendant at fourteen degrees of Aquarius takes this one, and so does a Moon at nineteen, and the Ascendant's decan was read closely in older practice, well before the Sun became the popular reference point.
On the standing of the technique: decans came into astrology from Egyptian star lists, groups of stars used to mark the hours of the night, and were later matched to the ten degree divisions and given planetary rulers. The history is genuine, and it is longer than that of some material treated as more central. Even so, decans refine rather than found a reading. Sign, house and aspect set the picture, and the decan adjusts the shading. Presented as the key to a chart, ten degrees are being asked to carry more than the tradition ever put on them.
The check above computes your solar degree from your birth date and names the decan it falls in, which no date range can do for a birth close to a boundary.
Questions people ask about this
When is the second decan of Aquarius?
Approximately 30 January to 9 February, moving by up to a day and a half between years. If your birthday falls at either end, the degree rather than the date decides which decan applies.
Do both schemes really agree here?
They do. The Chaldean order counts to Mercury, and the triplicity scheme assigns the decan to Gemini, which Mercury rules. That kind of agreement is unusual across the zodiac.
Why is Gemini connected to an Aquarius decan?
Triplicity distributes each sign's decans among the signs of its element in order. Starting from Aquarius, the air sequence continues to Gemini, so Gemini takes the middle decan.
Does the doubled Mercury make the effect stronger?
The traditional reading is that nothing contrasting is introduced, so the Mercurial character is stated rather than tempered. That is an interpretation of the arrangement, not a measurement of strength.
What about Uranus, the modern ruler of Aquarius?
Uranus rules the sign in modern practice and is often mentioned for the sign as a whole. It does not enter this decan under either scheme, since triplicity here points to Gemini and the Chaldean count to Mercury.
Do decans apply outside the Sun?
Yes. Every placement sits in a ten degree division, and the technique is applied to the Ascendant and the Moon as readily as to the Sun. This page is written around the Sun because that is what most readers know.