A degree short of the next band
At noon from 1930 to 2035, the Sun on 9 May runs from seventeen point nine to nineteen point three degrees of Taurus. The decan boundary is at twenty. Even the most advanced year in the range leaves the better part of a degree in hand.
An evening birth in one of those late years closes the gap a little further, since the Sun covers about a degree a day, but it does not close it entirely. The following date is where the question genuinely opens up.
Stated plainly: every 9 May birth in the sampled century falls in the second decan of Taurus, and that is a firm answer rather than a likely one.
What sits on either side of that line
Below twenty degrees is the second decan of Taurus, given to the Moon by the Chaldean order of faces and to Virgo by the triplicity scheme. Above it is the third, which the Chaldean order gives to Saturn and the triplicity scheme gives to Capricorn, whose ruler is also Saturn. The two schemes happen to converge on that one, which they rarely do.
So this birthday sits in the band where the schemes still disagree, one degree short of the band where they agree. The tradition reads the middle face as steadying or as sharpening the sign depending on which ruler you accept, and the third as bringing Saturn's caution to it. These are interpretive layers over a measured position, and they should be held as such.
The convergence on Saturn for the third band is worth noting because it is unusual. In most decans the two schemes name unrelated planets, and a source quoting one without the other can be quietly misleading. Checking which scheme a writer used is usually more useful than deciding which one is right.
Degrees matter for more than decans
A solar degree is not only a decan marker. Aspects are measured in degrees, so two people with the same sun sign but positions a fortnight apart form quite different contacts with the rest of a chart. House cusps are placed in degrees too, which means the Sun's exact longitude decides which house it falls in once the birth time is known.
That is the practical reason for wanting a figure rather than a label. Taurus alone will not tell you whether your Sun opposes a planet or sits on a house cusp. Nineteen degrees of Taurus might, once the rest of the chart is on the table.
This is also why an approximate birth time is worth chasing down. The Sun changes house long before it changes sign, and the house it occupies is usually read as the area of life the placement applies to.
What the date leaves undecided
The Moon crosses a sign in about two and a half days, so a 9 May birth has the Moon in one of two signs and only the birth hour separates the two of them. The rising sign is entirely open, since the whole zodiac passes the horizon once a day and the rate varies with latitude. A birth in Edinburgh and one in Singapore at the same clock time give different answers.
The Sun is what a birthday fixes, and it is the placement shared most widely. The widget above computes the exact solar degree from your own year and birthplace instead of assuming noon, and reports how far the birth stood from the nearest sign boundary, which settles both the sign and the decan without any rounding.
Questions people ask about this
Does 9 May reach the third decan of Taurus?
No. The Sun stops between about eighteen and nineteen degrees at noon in every year from 1930 to 2035, and the third decan begins at twenty.
Is 9 May certainly Taurus?
Yes. The Sun is nineteen days into the sign and about eleven degrees from the Gemini boundary, so no year or hour changes the answer.
Who rules the decan this birthday falls in?
The Moon in the Chaldean order and Virgo, and so Mercury, in the triplicity scheme. The schemes disagree on this band, unlike the one that follows it.
Why do the two schemes agree on the third Taurus decan?
Coincidence of construction. The Chaldean sequence reaches Saturn there, and the triplicity scheme reaches Capricorn, which Saturn also rules. Most bands do not line up like that.
Why does my exact degree matter?
Aspects and house cusps are measured in degrees. Two people sharing a sun sign can have Suns that behave completely differently in a chart because of where in the sign they fall.
What do I still need my birth time for?
The Moon sign, which has two candidates on this date, the rising sign, which has twelve, and every house position, which follows from the rising sign.