Position in the sign and in the year
Decans are ten degree divisions, three to a sign and thirty six around the wheel. This one begins at the exact ingress into Gemini and ends at ten degrees.
In calendar terms the Sun is normally here from about 21 May to about 31 May. The lower boundary is the sign change itself, which is the reason the Gemini start date moves between 20 and 21 May in different years. All decan boundaries drift like this, by as much as a day and a half, so the dates are an approximation and nothing more.
The upper boundary at ten degrees is quieter but just as movable, so late May birthdays sit close to a line as well.
Where the schemes part company
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and then repeats, beginning at zero degrees of Aries. Counted through, the seventh face lands on Jupiter.
The triplicity scheme has nothing to do with that sequence. It gives the decans of a sign to the signs of the same element in order, so the air decans of Gemini go to Gemini, Libra and Aquarius. The first therefore stays with Gemini and with Mercury, its own ruler.
So one method doubles the sign's ruler and the other introduces a planet traditionally opposed to Mercury in temperament. Popular articles almost always quote whichever suits their framing without naming the method, which is how the same decan ends up described as Mercury-heavy on one site and Jupiter-flavoured on another.
Readings attached to each
Under the triplicity scheme this is Gemini undiluted: no second sign is mixed in, so the traits usually attributed to the sign are read at full strength. Traditionally that means attention that moves quickly between subjects, a preference for gathering information over settling it, and fluency in speech and in writing. Because nothing modifies it, the same texts describe it as the face where the sign's restlessness is least offset.
The Chaldean Jupiter is read as adding scale. Where Mercury collects details, Jupiter is taken to want the general principle behind them, so this face is often described as a mind that keeps trying to assemble what it has picked up into something larger. Traditional writing also links Jupiter here to teaching and to publishing, meaning information passed on rather than merely held.
These readings apply to any placement in the decan. The Ascendant and the Moon are common targets for the technique, and in practice a decan tells you rather more about a Moon than about a Sun. The system itself descends from Egyptian star lists that predate the twelve signs, which is why it survives, but it remains a refinement laid over the chart rather than a load-bearing part of it.
Checking the boundary
Because this decan opens at the sign change, anyone born around 20 or 21 May faces two questions at once: whether the Sun had reached Gemini at all, and if so by how much. Birth time matters here, since the Sun moves roughly one degree a day.
The check at the top of this page computes your actual solar longitude and reports the degree with the ruler under each scheme. That resolves both questions in a way a printed date range cannot.
A birth time is not essential for it, though a known time narrows the answer considerably when the degree is within a fraction of the boundary.
Questions people ask about this
When is the Sun in the first decan of Gemini?
Usually between about 21 and 31 May. The opening boundary is the Taurus to Gemini ingress, which moves between years, so a birthday on 20 or 21 May needs an actual calculation.
Which ruler should I use here, Jupiter or Mercury?
Whichever scheme you are working in, provided you say which. The Chaldean order gives Jupiter, the triplicity scheme gives Mercury. Mixing them without noticing is what produces contradictory descriptions of this decan online.
Does a doubled Mercury make someone more Gemini?
That is the traditional argument for the first decan of any sign, since nothing else is blended in. It is an interpretation rather than a measurement, and other factors in a chart can easily outweigh it.
What are decans, historically?
Ten degree divisions of the ecliptic, thirty six in all. They originate in Egyptian star lists used for timekeeping at night and were later absorbed into Hellenistic astrology and given planetary rulers.
Can I use this for my Moon or rising sign?
Yes. Every placement sits in a decan, and the Ascendant and Moon are read this way as often as the Sun. This page centres on the Sun only because that is the placement most people know.
Is Jupiter in Gemini considered a difficult combination?
Traditionally Jupiter is in detriment in Gemini, and some writers carry that judgement into the decan. It is better read as a description of friction between breadth and detail than as a verdict, and the rest of the chart changes it.