A decan is a ten degree slice of a sign. There are three to each sign and thirty six in all, and they are one of the oldest surviving pieces of the tradition, older in origin than the twelve signs laid over them.
Where they come from
The Egyptians divided the night sky by which stars rose at which point in the year, producing thirty six groups that marked out the hours and the ten day weeks of their calendar. When Hellenistic astrology absorbed Egyptian material, those thirty six divisions were mapped onto the zodiac as ten degree segments, and they have travelled with it ever since.
The word face is the older term in western texts and decan the more common modern one. They mean the same division, though some traditional sources assign the rulers differently, which is part of why there is more than one answer to who rules what.
Two schemes, and why that matters
The Chaldean order runs a fixed planetary sequence, Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, beginning at zero degrees of Aries and repeating through all thirty six decans. Because thirty six is not a multiple of seven, the sequence does not line up with the signs and every decan gets whatever comes next.
The triplicity scheme works from the elements instead. The first decan of a sign goes to that sign itself, the second to the next sign of the same element, the third to the last. So the first decan of Aries is Aries, the second is Leo and the third is Sagittarius.
The schemes agree on only a handful of decans. Most articles on the subject quote one of them without naming which, which is how the same decan ends up with two different rulers depending on where you read about it. The calculator gives both and says which is which.
Why the degree matters more than the date
Decan boundaries fall at fixed degrees, not fixed dates. The Sun covers about a degree a day, so ten degrees takes roughly ten days, but where those days land shifts by up to a day and a half from year to year for the same reason the sign dates shift.
If you were born within a day of a boundary, a date range cannot answer the question and a calculated solar degree can. That is what this page does: it computes where the Sun actually was and reports the band it falls in, along with how far into it you are.
Beyond the Sun
Every placement sits in a decan, not just the Sun. The Ascendant and the Moon are the two most often read after it, and the Moon is included here for that reason. With no birth time the Moon is calculated for noon, which is fine unless it sits near a boundary, and the page says so when that is the case.
Decan work is a refinement rather than a foundation. The sign, the house and the aspects of a placement are read first, and the decan adds a shading to that. Anyone presenting a decan as the key to a chart has the order of things backwards.
Common questions
What is a decan?
A ten degree division of a sign, three to each sign and thirty six in total. The system comes originally from Egyptian star lists and predates a good deal of what was later built around it.
Why are there two sets of decan rulers?
The Chaldean order runs a fixed planetary sequence from the start of Aries, while the triplicity scheme assigns each decan to a sign of the same element. They agree on only a few decans and most articles quote one without naming it.
Do I need my birth time?
Only if you were born within about a day of a decan boundary. The Sun moves roughly a degree a day, so ten degrees takes about ten days and the time of day matters only at the edges.
Can other planets be in decans?
Yes, every placement sits in one. The Ascendant and the Moon are the ones most often read after the Sun, and the Moon is included here for that reason.
Are decans the same as the thirty six faces?
Yes. Face is the older term and decan the more common modern one. Some traditional texts use them with slightly different rulership assignments, which is part of why the schemes differ.
How much weight should I give a decan?
It is a refinement rather than a foundation. The sign, house and aspects of a placement are read first, and the decan adds a shading to that rather than overturning it.