Where the decan sits
Each sign holds three faces of ten degrees. This is the middle one of Capricorn, from ten degrees to twenty, reached about a week and a half after the December solstice.
In calendar terms the Sun crosses it from roughly 1 January to roughly 10 January. Do not treat that as fixed. The solstice ingress itself moves between years, and everything downstream of it moves too, by up to a day and a half. New Year birthdays are close enough to the first boundary that a table cannot reliably answer the question.
Venus through Taurus, or Mars directly
The triplicity method takes the earth signs in the order Taurus, Virgo, Capricorn, and starts each sign's decans from itself. Capricorn is the last of them, so its decans run Capricorn, Taurus, Virgo, and the middle one falls to Taurus, whose ruler is Venus.
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, repeating from the start of Aries and taking no notice of which sign a decan happens to be in. Counted through, the twenty ninth decan is Mars.
Venus and Mars give quite different readings of the same ten degrees, and the split is worth flagging because most short accounts of this decan name a ruler without naming a scheme. If a description you have read sounds nothing like another, the schemes are usually the reason.
What each layer is taken to add
Capricorn shaded by Taurus is read as ambition that wants something tangible at the end of it. Capricorn on its own is described in terms of structure, patience and position; the Taurean overlay is taken to attach that to material results, property, holdings, a settled standard of living, rather than to status in the abstract. The tradition also reads it as the slowest moving part of an already slow sign, meaning the least easily diverted once a direction has been set.
Mars, from the Chaldean side, is read as putting force behind all that. The associations are effort, contest and a willingness to cut something off, and here they are taken to mean the patience is not passive: the goal is worked at rather than waited for. Older texts describe this pairing as productive, in the plain sense that it gets things finished.
The proportion still matters. Decans are a real inheritance, traceable to Egyptian star lists used to divide the night, but they are a refinement on top of a chart rather than the thing the chart is built from.
The modes are worth a line as well. Capricorn is cardinal and Taurus is fixed, so the triplicity colouring adds staying power to a sign already inclined to plan long. Traditional readings take that as the reason this decan is described as difficult to redirect, more so than the opening decan of the sign, where Capricorn's own initiating quality is unmixed.
Beyond the Sun, and the New Year problem
Decans apply to any placement. The Ascendant is read this way as standard practice, since it is derived from birth time and place and is the most precise figure in the chart, and the Moon is treated the same. A Saturn or Moon between ten and twenty degrees of Capricorn sits in this decan whatever the Sun is doing.
The check above works out your actual solar degree from your birth date. For a birthday on 31 December or in the first days of January, that is a meaningfully different exercise from reading a range, because the range shifts from one year to the next and you may be on either side of it.
Questions people ask about this
What dates are the second decan of Capricorn?
Approximately 1 to 10 January, though the boundaries move by up to a day and a half depending on the year. A calculated solar degree is the reliable way to place a birthday near either edge.
Who rules the second decan of Capricorn?
Venus if you follow the triplicity scheme, which colours this decan with Taurus. Mars if you follow the Chaldean rotation. Both are in ordinary use and they rest on different logic.
What does Taurus add to Capricorn?
The tradition reads it as a pull towards the tangible, ambition measured in holdings and comfort rather than rank. It is also read as making the sign harder still to divert. That is interpretation, offered as such.
Does Mars here mean conflict?
Not as the tradition frames it. Mars is read as effort and force applied to the decan's aims, which in a Capricorn setting is usually described as sustained work. What would alter the reading is Mars's own condition in the chart.
Can decans be used for the Moon?
Yes. Any placement falls in a decan, and the Moon and Ascendant are both read this way routinely. The Sun simply gets the most attention because it is what most people know.
Why do decan date ranges differ between websites?
Because they are approximations of a moving boundary. The Sun's entry into each sign shifts between years, so any fixed set of dates is right for some years and a day out for others.