The degree, and why it lands where it does
Taken at noon in every year from 1930 to 2035, the Sun on 10 May has stood between about eighteen point nine and twenty point three degrees of Taurus. That is a spread of roughly a degree and a half across more than a century, which is a much tighter figure than most people expect.
The reason is the leap year cycle. A tropical year runs a little under three hundred and sixty five and a quarter days, so without correction the calendar would slide against the seasons. Adding a day every fourth year pulls it back. The position on any given date therefore drifts forward through a four year cycle and then jumps back, tracing the same small band over and over rather than wandering off.
One consequence is that the printed date ranges you see in books and on charts are rounded summaries of that band, not statements about a particular year. They work well enough in the middle of a sign and badly at the edges, which is where nearly every argument about birthdays starts. Here the sign is safe from that problem and the decan is not.
Second decan or third
Each sign divides into three bands of ten degrees, called decans. The second decan of Taurus runs from ten to twenty degrees and the third from twenty to thirty. A 10 May Sun sits within about a degree of that dividing line, so the answer genuinely differs from year to year and, within a year, from morning to evening.
In the older Chaldean order the second decan of Taurus answers to the Moon and the third to Saturn. Those are not close readings of one another. The Moon band is described in the tradition as domestic, accumulative and attached to comfort, and the Saturn band as reserved, durable and slow to part with anything. If you want the correct one you need the year and the hour, which the widget above works out from your own details.
What the sign itself fixes
Taurus is fixed earth, ruled by Venus. Fixed describes the middle stretch of a season, the part that holds a condition rather than starting or ending it. Earth is the element the tradition associates with the tangible and the maintained. Venus as ruler is read as an interest in comfort, in value and in what is pleasant to the senses.
All of that is interpretation, and it is the same interpretation for every Taurus birthday from late April to late May. It is precisely because the sign label is shared so widely that the degree and the decan are worth having.
Venus rulership also has a practical use rather than only a descriptive one. Because Venus rules the sign the Sun occupies, the tradition treats the condition of Venus in the chart as bearing on the Sun's affairs, which means the sign and house Venus falls in matter as much as the label attached to Taurus. That is a further placement the date cannot supply on its own.
What a date cannot tell you
The Sun is one body in a chart. The Moon travels about thirteen degrees a day, so over the twenty four hours of 10 May it crosses roughly half a sign, and depending on where it began it can be in either of two signs by the end of the day. A date alone cannot choose between them.
The rising sign is looser still. The horizon turns through all twelve signs in a day, changing roughly every two hours, so a birth date tells you nothing at all about it. Two people born on the same 10 May, hours apart, can share a Sun to the degree and share almost nothing else. That is the ordinary case, not the exception.
Questions people ask about this
Is 10 May always Taurus?
Yes. The Sun enters Taurus around 20 April and leaves around 21 May, so this date sits well inside the sign with no boundary anywhere near it. The year, the hour and the birthplace make no difference to the sign.
Which decan is a 10 May birthday in?
That is the part the date does not settle. The Sun sits within about a degree of the twenty degree line, so some years fall in the second decan and some in the third. The widget above resolves it from your year, hour and birthplace.
What is the difference between the two decans here?
In the Chaldean order the second decan of Taurus is given to the Moon and the third to Saturn. The tradition reads the first as attached and accumulative and the second as reserved and durable. Both are readings, not measurements.
Why does my solar degree hardly change between years?
Because the leap year cycle keeps the calendar in step with the orbit. The degree drifts forward for three years and then a leap day pulls it back, so the position stays inside a band of about a degree and a half.
Do I need my birth time for this date?
Not for the sign, which is fixed. For the decan it helps, because a birth in the morning and one at night sit at slightly different degrees and this date is close to a band boundary.
Does the decan change my sun sign reading?
It refines it rather than replacing it. The sign stays Taurus throughout, and the decan is the layer the older material used to tell one part of a sign from another.