Ten degrees of the year
Each sign carries three decans of ten degrees, thirty six around the ecliptic. This is the second in Cancer, opening once the Sun has passed ten degrees and closing at twenty.
The Sun is generally in it from around 2 July to around 12 July. That window is approximate: the tropical year does not divide evenly into days, so every boundary in the zodiac slides by up to a day and a half depending on the year, and on where in the day you were born. Birthdays in the first days of July or around the twelfth are the ones most likely to be misfiled by a printed table.
Mercury against Scorpio
Under the Chaldean order, rulers cycle through Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter from zero degrees of Aries onward. Counting from there, this face falls to Mercury.
The triplicity scheme does something else entirely. It distributes the decans of a sign among the signs sharing its element, in order, so Cancer's water decans belong to Cancer, then Scorpio, then Pisces. That makes this the Scorpio face, and modern writing attaches Pluto to it as Scorpio's contemporary ruler, with Mars standing behind it in older texts.
Mercury and Scorpio are not variations on the same theme, and this is a good example of why naming the scheme matters. A great deal of published decan writing quotes the triplicity ruler as if it were the only one, so a reader who has been told this decan is Plutonian has usually been given half the tradition.
Knowing which count is in use also makes it possible to compare sources sensibly instead of assuming one of them is wrong.
How each is interpreted
The Scorpio colouring is read as Cancer with the surface removed. The sign's protectiveness is still assumed, but where the first face is described as sheltering, this one is described as watchful: attention paid to what is not being said, a long memory for how people behaved under pressure, and reluctance to explain its own reasoning. Traditional writing associates the water triplicity with feeling that does not announce itself, and this face is where that reticence is taken to be strongest.
The Chaldean Mercury is read as adding articulation to material that would otherwise stay unspoken. In older texts Mercury here is linked to record keeping, to correspondence and to the handling of family or household matters in writing. Placed against a Scorpio backdrop, that is usually interpreted as a capacity to put difficult things into precise words when there is a reason to.
All of this applies to any placement, not only the Sun. Decans are read on the Ascendant and on the Moon routinely, and a water decan is arguably more informative on a Moon than anywhere else. The technique is old, descending from Egyptian star lists that predate the zodiac in its familiar form, but it refines a chart rather than founding one.
Why a date range is not enough
Decan boundaries are degrees of longitude, not calendar days. If you were born on 2 or 12 July, whether your Sun sits in this decan depends on the year and on the hour, because the Sun covers roughly one degree in a day.
The check above computes the Sun's actual position for your date and states the degree along with the ruler each scheme gives. For anyone near an edge that is the only reliable answer.
Questions people ask about this
What dates correspond to the second decan of Cancer?
Roughly 2 to 12 July in most years. The edges move by up to a day and a half, so a birthday at either end needs the solar degree worked out rather than looked up.
Is this decan ruled by Pluto or by Mercury?
The triplicity scheme points to Scorpio and therefore to Pluto in modern practice, or Mars in older practice. The Chaldean order gives Mercury. Both readings are traditional and this page shows each rather than picking one silently.
Does the Scorpio face make a Cancer more intense?
That is the usual claim, on the grounds that the second water sign is blended in. It describes emphasis, not character in full, and a Sun decan is easily outweighed by the Moon, the Ascendant and the aspects.
Why do decan rulers differ between sources?
Because there is more than one scheme in circulation and most writers do not say which they used. Once you know whether a source counts in Chaldean order or by triplicity, the disagreements stop being mysterious.
Can decans be applied to the Moon?
Yes, and many practitioners prefer them there. Any point in a chart falls in one of the thirty six decans, and the Ascendant is read this way just as often as the luminaries.
How seriously should decans be taken?
As a genuine but secondary layer. They come from a documented Egyptian and Hellenistic lineage, so they are not a modern invention, but they add detail to a reading rather than carrying it.