Where the ten degree bands came from
Long before the planetary rulers were attached, the thirty six divisions were a clock. Egyptian astronomers watched groups of stars rise in succession through the night and used them to mark the hours, and the scheme that resulted mapped onto the zodiac as thirty six bands of ten degrees.
Hellenistic writers, and Arabic ones after them, gave each band a planetary ruler. The layer of meaning that birthday pages occasionally mention is therefore later than the divisions themselves, which began as practical timekeeping and had nothing to say about character at all.
Two routes, one planet
The Chaldean scheme orders the seven classical planets by apparent speed, from Saturn down to the Moon, and runs that sequence continuously from the first degree of Aries. Counted through to the closing band of Leo, it arrives at Mars.
The triplicity scheme works differently. It assigns a sign's three bands to the rulers of the signs that share its element, taken in zodiacal order. Fire runs Aries, Leo, Sagittarius, so Leo's third band goes to Aries and therefore also to Mars. Two conventions with no common derivation returning the same planet is not proof of anything, but it does mean a reader on this date is not being asked to pick a side.
What is read into these degrees, marked as reading
The tradition describes Leo as fixed fire under the Sun: constant, visible, organised around a centre. The closing band adds a Mars edge, and the texts read it as the more contentious and less accommodating part of the sign, quicker to push back.
Those are descriptions carried down in writing. They are not derived from observation, they do not follow from the geometry, and they say nothing about what any particular person born on the sixteenth of August is like. A date confers nothing. What it does is locate a position, and the position is what the tradition then comments on.
The measurable part and the missing part
The measurable part is the longitude, which at noon across the last century has run between about twenty two point seven and twenty four point one degrees. The window stays narrow because leap years correct the calendar against the orbit before the drift accumulates, so the same date holds nearly the same position for generations.
The missing part is most of the chart. The Moon travels up to fifteen degrees a day and may occupy either of two signs on this date; the Ascendant runs through all twelve in twenty four hours and shifts with the latitude of the birthplace. The widget above computes the solar degree exactly from the year, hour and place, states the distance to the nearest sign boundary, and fills in the placements a calendar cannot.
There is one further thing a date cannot supply, which is the house the Sun falls in. Houses are set by the horizon and the meridian at the moment of birth, so they depend on the same time and place data as the Ascendant. The same solar degree can land in any of the twelve houses, and in ordinary practice the house is read as at least as informative as the sign.
Questions people ask about this
Is 16 August Leo or Virgo?
Leo, in every year on record. The Sun does not reach Virgo until around the twenty second or twenty third of August, so this date has never been near the boundary.
Which decan of Leo is 16 August?
The third, from twenty to thirty degrees. The date's noon positions across the last century all sit inside it, so the band does not change with the year.
Who rules the third decan of Leo?
Mars, in the Chaldean order and in the triplicity scheme alike. Most decans have two competing answers, and this is one of the few where the schemes coincide.
Where did the decans come from?
From Egyptian timekeeping, where thirty six star groups rose in turn and marked the hours of the night. The planetary rulers were added later by Hellenistic and Arabic writers.
Does the decan predict anything?
No. It is a division of the zodiac with a traditional interpretation attached. Descriptions built on it are inherited readings, not forecasts, and nothing about a birth date determines a life.
How exact is the degree for my birth?
Exact to a fraction of a degree once the year, the hour and the birthplace are supplied. The range quoted here is a noon figure across the last century, not a limit on the precision available.