Where the decan rulers come from
The Chaldean order lists the seven classical bodies by how fast they appear to move against the stars: Saturn, Jupiter, Mars, the Sun, Venus, Mercury and the Moon. Starting from the first degree of Aries and assigning one planet to each ten degree band, the sequence runs continuously around the zodiac and closes exactly, since thirty six divides by seven with the cycle repeating cleanly over the full circuit.
The same order underlies the planetary hours and the naming of the days of the week, which is why the scheme has stayed in use for so long. It is a piece of arithmetic rather than a doctrine, and it can be checked by counting.
Counting it out is the quickest way to see that it holds. Aries takes Mars, the Sun and Venus across its three bands, Taurus follows with Mercury, the Moon and Saturn, and the cycle carries on unbroken to the end of Pisces.
The position on 6 August
Taken at noon across every year from 1930 to 2035, the Sun on this date has stood between roughly thirteen and about fourteen and a half degrees of Leo. Every value in that window falls in the stretch from ten to twenty degrees, so the second band applies to the whole date without qualification.
Counting the Chaldean sequence around from Aries, that band lands on Jupiter. Leo remains ruled by the Sun, so the position carries a solar sign with a Jupiterian band inside it rather than one ruler replacing the other.
The lowest values in that window belong to leap years and the highest to the third year after one, which is the same four year rhythm that governs every date in the calendar.
What the texts attach to it
Older writing reads Jupiter inside Leo as warmth with room around it. The words that recur are generous, expansive, drawn to teaching and to hosting, more comfortable with a large gesture than a careful one, and inclined to take responsibility for a group rather than only for itself.
All of that is inherited convention. It comes from the tradition's view of Jupiter combined with its view of Leo, not from anything observed about people born in early August, and it should be held at that distance. What is not convention is the band assignment, which follows from the degree and the count.
It is also worth noting what the scheme does not claim. The decan ruler is a second layer over the sign ruler, not a replacement, and no text treats it as overriding the sign.
The parts a birthday cannot reach
The Sun is the only body a date fixes. The Moon crosses about half a sign in a day, so it can be in either of two signs on this date depending on the hour, and across different years it has been in all twelve.
The Ascendant depends on the minute of birth and on how far the birthplace lies from the equator, since the signs rise at uneven rates away from it. The widget above resolves the solar degree for a given year and place, and reports the distance from the birth to the nearest sign boundary, which is the figure people generally wanted when they looked the date up.
What remains after that is a chart rather than a date, and it needs a birth record instead of a calendar.
Questions people ask about this
Which planet rules the decan for 6 August?
Jupiter. The Chaldean order assigns the second ten degrees of Leo to it, while the sign as a whole stays under the Sun, so the two operate as separate layers.
What is the Chaldean order?
A ranking of the classical planets by apparent speed, from Saturn as the slowest to the Moon as the fastest. It is used for decan rulers, for planetary hours and for the sequence of the weekdays.
What degree is the Sun on this date?
Roughly thirteen to about fourteen and a half degrees of Leo at noon, depending on the year. The widget above returns a single figure once a year and birthplace are given.
Is the decan reading a prediction?
No. It reports what the tradition attaches to a Jupiter band inside a solar sign. It is a convention carried down through the texts rather than a claim about how anyone's life will go.
Could a birth time move this into another band?
No. The Sun advances a little under a degree a day and the whole window sits several degrees clear of both band edges, so any hour of 6 August stays in the second decan.
Does the sign ruler or the decan ruler matter more?
The tradition treats the sign ruler as the primary one and the decan ruler as a qualification on it. The band narrows the reading rather than overturning it.