A safe sign and a close face
The sun sign for this date needs no discussion. Across every year from 1930 to 2035 the noon Sun has been between about eight and two fifths and nine and four fifths degrees of Sagittarius, having crossed the boundary from Scorpio more than a week earlier.
The decan boundary is a different matter. The first face of Sagittarius ends at ten degrees, and the upper end of that band comes within a fifth of a degree of it. At noon it has never crossed, but noon is a convention, and a birth late in the evening in a year at the high end of the range gets very close indeed.
This is the sort of distinction a date range cannot make. Only a calculation from a real time and place will say which side of ten degrees a particular birth falls on.
Why the band sits where it does
The Sun advances a little under a degree a day, so consecutive dates in early December step forward by almost exactly one degree each. The width of the band for any single date, a little over a degree, comes from the leap year cycle rather than from anything about the date itself.
Each ordinary year the Sun reaches a given longitude about six hours later against the calendar, which shows up as a slightly lower degree on a fixed date. Every fourth year the leap day pulls it back by nearly a full day, restoring the position. The sawtooth repeats, and the calendar and the orbit stay in step.
A slow residue survives, worth about a day per century, which the Gregorian rule on century years keeps from accumulating. It is the reason the lowest figures in the record belong to the most recent years.
What the first face carries
Decans are the ten degree divisions inside each sign, thirty six around the zodiac, inherited from Egyptian star lists that marked the hours of the night. The first face of Sagittarius is where this date sits, and the two rulership schemes in common use disagree about it.
The Chaldean order, cycling the seven traditional planets from the start of Aries, delivers Mercury. The triplicity scheme starts a sign's faces from the sign itself, so this one stays with Sagittarius and with Jupiter. The tradition reads the Jupiter version as the sign at full strength and the Mercury version as that outlook made to argue in particulars.
Anyone born on this date who finds the face described in flatly different terms in two places has probably found two schemes rather than an error. Both are legitimate and neither is usually named.
The rest of the chart
None of this reaches the placements that differ between two people born on the same day. The Moon covers roughly thirteen degrees a day and can be in either of two signs on this date. The Ascendant turns through the whole zodiac in twenty four hours, changing by about a degree every four minutes, so a birthday tells you nothing about it.
The Sun is the least personal placement in a chart for the simple reason that everyone born within about a month shares it. It is worth knowing precisely, and it is not worth mistaking for a description of a person.
The widget above computes your solar degree from your own year and birthplace and reports how far you were born from the nearest sign boundary. On this date the sign boundary is remote, but the degree it returns is what decides the decan, and here that is a question worth asking.
Questions people ask about this
What sign is 1 December?
Sagittarius, in every year on record. The Sun sits around nine degrees into the sign, so no hour of birth or time zone can put it in Scorpio.
Which decan is a 1 December birthday?
The first decan of Sagittarius at noon in all years checked, though the position is close to the ten degree boundary. A late evening birth in some years is very near the edge.
How close to the decan boundary is it?
The upper end of the noon band comes within a fraction of a degree of ten degrees. Since the Sun covers roughly a degree a day, half a day either side of noon shifts the answer by about half a degree.
Who rules the first decan of Sagittarius?
Mercury under the Chaldean order and Jupiter under the triplicity scheme. Both are in use, and a description that names neither could be built on either.
Does the degree affect anything practical?
It decides which aspects the Sun forms with other planets and which house it occupies, both of which are measured in degrees. It has no bearing on the sign.
What else should I look up?
The Moon and the rising sign, since those are what separate two charts sharing this birthday. Both need a birth time as well as a date and a place.