What a date can and cannot fix
The Sun covers about one degree a day, so a calendar date pins it down tightly. On 11 December it sits near nineteen degrees of Sagittarius, and across the past century the noon figure has stayed between roughly eighteen and a half and just under twenty.
Everything else in a chart moves faster or depends on where you were standing. The Moon travels twelve to fifteen degrees a day. The Ascendant travels the entire zodiac in a day and is different in Lisbon from what it is in Manila at the same instant.
That is the whole reason birth times are asked for. Not ceremony, and not precision for its own sake, but the fact that half the chart is unreadable without one.
It also explains why two people born on the same day can be given charts that look nothing alike. They share a Sun and a face, and they may share very little else.
The Moon has two answers here
The Moon changes sign roughly every two and a half days. On any given date there is a fair chance it crosses a boundary at some hour, which means a person born in the morning and a person born late in the evening can have different lunar signs while sharing a birthday.
Since the Moon is read as the seat of habit, mood and what somebody needs rather than what they pursue, the difference is not a minor one. Popular birthday pages skip it entirely, because a date alone cannot supply it.
If your birth time is unknown, a chart calculation will still tell you whether the Moon was safely inside one sign all day or whether it crossed. That distinction is worth having even when the exact answer is not available.
The horizon is unconstrained
The rising sign has no relationship to the date at all. Any of the twelve can rise on 11 December, and the one that does changes every couple of hours, shifting degree by degree in between.
Because houses are measured from the Ascendant, an unknown birth time leaves not one gap but a whole layer. The planets are in the same signs either way, and the areas of life they are read into are entirely undetermined.
Where the time is genuinely lost, some of it can be recovered from known life events by rectification, though that is a skilled and slow procedure with results that remain provisional. The plainer route is to work only with what is certain and mark the rest as unknown.
What is settled, stated plainly
The Sun on this date is in Sagittarius, near nineteen degrees, in the second face of the sign. Under the Chaldean order that face belongs to the Moon while Jupiter rules the sign itself, and the traditional reading of the pair is a Sagittarian reach kept largely to itself.
The Sun is also, on the arithmetic above, the least individual thing a chart contains. It is shared with a month of other people, while the Ascendant is shared with those born within a couple of hours in the same region.
The widget above returns the exact solar degree for your year, hour and birthplace, and reports the distance to the nearest sign boundary so you can see how settled the answer really is.
Questions people ask about this
Is 11 December definitely Sagittarius?
Yes. The Sun does not leave Sagittarius until around 21 December, so this date carries no boundary risk in any year and needs no birth time to confirm.
What is my Moon sign if I was born on 11 December?
The date cannot answer it. The Moon may hold one sign all day or cross into the next, so a birth time and a chart calculation are needed.
Can my rising sign be guessed from the date?
No. Every sign rises at some point during the day, so the Ascendant depends wholly on the hour and the birthplace rather than the date.
What degree is the Sun on 11 December?
Close to nineteen degrees of Sagittarius, with about a degree of variation across the last century of births at noon.
Which decan does the date fall in?
The second decan of Sagittarius, ten to twenty degrees, ruled by the Moon in the Chaldean order and by Aries in the triplicity scheme.
Is the sun sign the most important placement?
The tradition does not treat it that way. It is the one most widely published, largely because it is the only one a date alone can supply.