The measurement
At noon in each year from 1930 to 2035, the Sun on 15 May has stood between about twenty three point seven and twenty five point one degrees of Taurus. Someone born in 1931 and someone born in 2031 share that position to within a degree and a half, and most pairs of years are much closer than that.
The variation that does exist is not random. It cycles. The position slips forward slightly for three consecutive years and then steps back when February gains a day, so plotting it produces a repeating sawtooth rather than a trend.
Why the calendar and the orbit stay in step
A tropical year, the time from one March equinox to the next, runs about three hundred and sixty five days and just under a quarter. A calendar year of three hundred and sixty five days therefore falls short, and the Sun arrives at a given point roughly six hours later on the clock each year.
Adding a leap day every fourth year hands those six hours back in one lump. The century rule, which skips the leap day in 1900 and 2100 but keeps it in 2000, trims the small remaining excess. The upshot is that a fixed calendar date corresponds to a nearly fixed position in the Sun's path, which is exactly what a calendar is meant to achieve.
None of this touches the Moon or the planets, which are not tied to the calendar at all. The Moon on 15 May can be in any of the twelve signs depending on the year, and Mars or Jupiter may be anywhere in the zodiac. Only the Sun has this near fixed relationship with a calendar date, which is precisely why sun sign astrology could be built around dates in the first place.
What that position means in the sign
Near twenty four degrees of Taurus, a 15 May Sun sits in the final decan of the sign, the band running from twenty to thirty degrees. The Chaldean order gives that band to Saturn, and the triplicity scheme, walking the earth signs in order, arrives at Capricorn and so at Saturn as well.
Taurus is fixed earth ruled by Venus, read in the tradition as steady and attached to the tangible. The Saturn third is read as the most reserved and long sighted part of the sign. That is what the tradition attaches; it is not a prediction and it is not derived from the measurement.
Within the sign, twenty four degrees also has ordinary technical uses. Aspects are angular relationships measured in degrees, so a Sun at twenty four of Taurus makes exact contacts that a Sun at four would not. Whatever weight you give the interpretation, the degree is the number those calculations actually run on.
Stability is not the same as completeness
The solar degree being reliable does not make the date informative about a person. The Moon moves about thirteen degrees a day, enough to change sign inside a single date, so a 15 May birth has two candidate Moon signs until an hour is supplied.
The rising sign has twelve candidates for the same reason the Sun has one: the Earth turns. A birth date fixes where the Earth is in its orbit and says nothing about where it is in its rotation. That single distinction explains most of what a birthday can and cannot tell you.
Questions people ask about this
Does the sky on 15 May change much between years?
The Sun barely moves. Its position on this date has stayed inside about a degree and a half since 1930. The Moon and the planets, which are not tied to the calendar, differ completely from year to year.
What sign is 15 May?
Taurus in every year, with no rounding question. The Sun does not reach Gemini until around 21 May, so the date is nowhere near a boundary.
Why does a leap day matter for my birth chart?
It keeps the calendar aligned with the orbit. Without it your birthday would slide steadily backwards through the seasons, and the sign attached to a date would change over the centuries.
Which decan is my Sun in?
The third decan of Taurus, from twenty to thirty degrees, since a 15 May Sun sits near twenty four. Both the Chaldean and the triplicity schemes give that band to Saturn.
Can I work out my exact degree?
Yes, from your year, hour and birthplace. The widget above computes it and also reports how far your birth fell from the nearest sign boundary, which for this date is several days.
Does a stable solar degree make the reading stronger?
It makes the measurement reliable, which is a different thing. What the tradition reads into a degree is still interpretation, and it would be weighed against the rest of a chart.