December Star Sign: What Star Sign Is December 10, 12, 15, and 18?

If you were born in December, your star sign is either Sagittarius or Capricorn. December 1 through December 21 falls in Sagittarius — mutable fire, ruled by Jupiter. December 22 through December 31 falls in Capricorn — cardinal earth, ruled by Saturn. The boundary sits at the winter solstice, where philosophical range yields to structural patience.

This post covers the December star signs in detail, with specific lookups for the dates people search most often: December 10, 12, 15, and 18.

December Star Sign Dates

• Sagittarius ♐︎ November 22 – December 21 · mutable fire · ruled by Jupiter
• Capricorn ♑︎ December 22 – January 19 · cardinal earth · ruled by Saturn

Find your December birthday in those ranges. That is your star sign. What follows covers each sign’s character and provides detailed lookups for specific December dates.

What Is a Star Sign?

A star sign — also called a sun sign or zodiac sign — is whichever of the twelve zodiac signs the Sun occupied on the day you were born. The Western tropical zodiac divides the ecliptic (the Sun’s apparent annual path) into twelve 30-degree arcs, each named after a constellation. The terms “star sign,” “sun sign,” and “zodiac sign” all describe the same placement. “Star sign” is the more common term in British and Australian English; “zodiac sign” and “sun sign” are more common in American usage.

The Sun’s position on your birthday is determined by where the Earth was in its orbit — the same date falls in the same sign every year, with minor variation at the boundary dates where the Sun transitions from one sign to the next.

10 December Star Sign: Sagittarius

December 10 falls in Sagittarius — the mutable fire sign ruled by Jupiter. Sagittarius runs from November 22 through December 21, placing December 10 eighteen days into the sign and eleven days before the Capricorn boundary. There is no cusp ambiguity for this date.

At eighteen days in, the Sun is well past the Scorpio-Sagittarius transition and still a comfortable distance from the Capricorn ingress. People born on December 10 carry the full mutable fire signature: philosophical range, intellectual curiosity, directness in speech, and an instinct to test a principle across multiple contexts before accepting it as settled.

The fire element gives Sagittarius its outward orientation. Fire signs (Aries, Leo, Sagittarius) share a quality of direct engagement, but each expresses it differently depending on modality. Sagittarius fire is mutable: it travels. Where Aries fire (cardinal) initiates a single powerful action and Leo fire (fixed) sustains a single creative vision, Sagittarius fire moves between contexts — carrying its heat from idea to idea, testing what it finds against what it has already learned. December 10 sits deep enough into the sign’s season that this comparative quality is fully operational.

Jupiter as ruler gives December 10 Sagittarius its characteristic relationship with scope. This is not optimism in the motivational sense — it is a structural orientation toward the question behind the question. Jupiter expands the field of view. People born on December 10 carry this expansion at a point in the season where the sign has had time to develop its range: the reading is broad and the material gathered is substantial.

The day number for December 10 is 1 (1 + 0 = 1). One is the number of the Initiator in Pythagorean numerology — independence, self-direction, and comfort with acting ahead of consensus. Paired with Sagittarius’s mutable fire, the 1 adds decisive force to the sign’s philosophical character. People born on this date tend to reach conclusions and act on them with a directness that does not wait for the room to catch up. The philosopher who picks a heading and moves.

For the full birthday profile including BaZi Day Master, see Born on December 10.

12 December Star Sign: Sagittarius

December 12 falls in Sagittarius — twenty days into the sign, nine days before the Capricorn boundary. This is late-mid Sagittarius, well past the season’s opening and approaching its final third. The placement is unambiguous: December 12 is Sagittarius.

The mutable fire quality at this point in the season is fully articulated. Sagittarius has been running for nearly three weeks. The comparative intelligence that defines the sign — learning by moving between frameworks, testing what it finds in one context against what it found in the last — has had time to accumulate material. People born on December 12 carry the sign’s quality at its most informed: the philosophical range is not just active but furnished with what it has gathered over the course of the season.

The element for December 12 is fire — the most outward of the four classical elements. The mutable expression of fire is the most mobile: it does not hold position (fixed fire, Leo) or initiate from a single point (cardinal fire, Aries). It distributes itself. Sagittarius fire carries its quality across a wide field, lighting what it finds worth examining and moving on when the principle has been tested.

The day number for December 12 is 3 (1 + 2 = 3). Three is the number of the Expresser in Pythagorean numerology — creative articulation, verbal facility, and the capacity to give form to what has been perceived. Paired with a Sagittarius Sun, the 3 adds a communicative dimension to the sign’s philosophical range. People born on this date tend not just to perceive the larger pattern but to articulate it — the teacher, the writer, the person who translates between worlds because they have spent time in more than one.

December 12 is also the twelfth day of the twelfth month. In numerological terms, both components reduce to 3 (1 + 2 = 3), producing a doubled expressive signature. The month and day point in the same direction, amplifying the creative and communicative quality that defines this birthday’s numerological profile.

For the full birthday profile including BaZi Day Master, see Born on December 12.

15 December Star Sign: Sagittarius

December 15 falls in Sagittarius — twenty-three days into the sign, six days before the Capricorn boundary. This is late Sagittarius, past the midpoint and approaching the sign’s final quarter. Despite the proximity to the transition, December 15 is not a cusp date. The Sun is in one sign or the other at any given moment — it does not blend with the next sign as it approaches a boundary.

Late-season Sagittarius has a particular quality. The mutable fire has been running for more than three weeks. The sign’s characteristic mode — testing principles across contexts, reading situations for their implications beyond the immediate case — has covered substantial ground. People born on December 15 carry Sagittarius at its most experienced point: the philosophical breadth is not tentative or exploratory but confident, operating from a base of accumulated reading.

Jupiter as ruler gives this placement its relationship with meaning and scope. December 15 Jupiter-ruled fire does not just expand the field of view — it has been expanding it for twenty-three days. The range is wide. The reading is deep. People born on this date carry a quality of philosophical conviction that comes not from dogma but from the sheer amount of ground the sign has covered over the course of its season.

The day number for December 15 is 6 (1 + 5 = 6). Six is the number of the Caretaker in Pythagorean numerology — responsibility, service, and an awareness of community and domestic life that grounds broader ambitions in the people they affect. Paired with a Sagittarius Sun, the 6 introduces a quality the sign does not always display: rootedness. Where Sagittarius at large ranges wide and may leave the local in favor of the universal, the 6 pulls the philosophical instinct back toward the personal. People born on December 15 tend to be the Sagittarians who ask not just what is true in general but what it means for the people standing in front of them. The broad reading applied locally. The philosopher who comes home.

For the full birthday profile including BaZi Day Master, see Born on December 15.

18 December Star Sign: Sagittarius

December 18 falls in Sagittarius — twenty-six days into the sign, three days before the Capricorn boundary. This is the final stretch of Sagittarius. The Sun will cross into Capricorn at the winter solstice around December 21–22. Despite sitting this close to the end of the sign, December 18 is unambiguously Sagittarius. The Sun has not yet crossed into Capricorn.

At twenty-six days in, Sagittarius is at its most concentrated expression. The mutable fire has been running for nearly four weeks. Every theme the sign carries — philosophical range, comparative intelligence, directness, the capacity to read a situation for what it implies across a hundred cases — is operating at full articulation. Late-degree Sagittarius has the quality of a season approaching its conclusion: the reading is complete, the range is wide, and the material gathered over the course of the sign’s run is at its most substantial.

The proximity to Capricorn does not dilute the Sagittarius placement. The Sun occupies one sign at a time. A person born on December 18 has a Sagittarius Sun — not a blend, not a diminished version, not a transitional placement. If anything, some astrological traditions read the final degrees of a sign as carrying heightened intensity: the archetype at its most concentrated before yielding to the next.

The day number for December 18 is 9 (1 + 8 = 9). Nine is the number of the Humanitarian in Pythagorean numerology — completion, broad vision, idealism, and an orientation toward the collective rather than the individual. Paired with a Sagittarius Sun, the 9 amplifies the sign’s already expansive character. Where Sagittarius reads across contexts, the 9 reads across people — the individual case expanding into the social field, the philosophical principle extending to its widest possible application. People born on December 18 tend to carry Sagittarius’s range at its most panoramic: the view is not just broad but oriented toward what serves the widest group. The philosopher who thinks in populations, not case studies.

For the full birthday profile including BaZi Day Master, see Born on December 18.

The Sagittarius-Capricorn Boundary: December 21–22

The transition from Sagittarius to Capricorn falls around December 21–22 each year. December 21 is the final day of Sagittarius in most years; December 22 is the first day of Capricorn. The exact moment the Sun crosses the 0-degree Capricorn point shifts by a few hours annually, which is why a small number of December 22 births fall in late Sagittarius and an even smaller number of December 21 births might fall in early Capricorn.

The term “cusp” is often applied to this boundary, implying that people born near it carry qualities of both signs. In the tropical zodiac, this is not how the system works. The Sun occupies one sign at a time. The transition is a boundary, not a gradient. When someone born on December 20 recognises Capricorn traits in themselves, the explanation is typically elsewhere in the chart: Mercury, Venus, or Mars may have already moved into Capricorn while the Sun was still in Sagittarius. Those inner planets often lead or trail the Sun by days or weeks.

If your birthday falls on December 21 or 22, a chart calculated from your exact birth time and year will show which side of the line your Sun occupies.

December Sagittarius: Traits at a Glance

People born December 1–21 carry the Sagittarius Sun — mutable fire, Jupiter-ruled. The core traits:

• Philosophical range. Sagittarius reads situations for their implications beyond the immediate context. The sign asks what a principle means across a hundred cases, not just the one in front of it.
• Directness. Mutable fire speaks plainly. Sagittarius states what it has found without diplomatic filtering or strategic withholding.
• Comparative intelligence. The sign learns by covering ground — moving between positions, contexts, and frameworks until the connecting pattern becomes legible.
• Mutable adaptability. Sagittarius adjusts method without abandoning direction. The sign is flexible about how it gets there, not about whether the destination is worth reaching.

December Sagittarius sits in the latter half of the sign’s season. The themes are fully established. The mutable fire is operating at its complete range.

December Capricorn: Traits at a Glance

People born December 22–31 carry the Capricorn Sun — cardinal earth, Saturn-ruled. The core traits:

• Structural patience. Capricorn builds with the understanding that what is worth making takes time. The sign measures progress in years, not weeks.
• Institutional thinking. Saturn-ruled, Capricorn reads systems as objects to be built, maintained, and improved. The sign thinks in terms of what endures after the initial effort is spent.
• Material intelligence. Cardinal earth assesses what can actually be constructed with available resources — not in theory but in practice, under real constraints.
• Cardinal initiative. Capricorn acts. The sign does not wait for conditions to improve. It begins the work in the conditions that exist — which, at the solstice, means building against the dark.

Early-degree Capricorn (December 22–31) carries the sign’s quality from the opening degree. The structural patience is present from day one of the season.

Beyond the Star Sign: What Else Your December Birthday Reveals

Your star sign describes one placement — where the Sun was when you were born. It speaks to core identity and temperament. But a full chart includes dozens of placements that the star sign alone does not capture.

The Moon sign (which changes every 2.5 days and varies from year to year on the same calendar date) describes emotional processing. The rising sign (which depends on the exact hour and location of birth) describes how you present to others and how the world first registers you. Mercury, Venus, and Mars each carry their own sign placements, often different from the Sun’s.

Beyond Western astrology, other traditions read the same birth data through independent frameworks. BaZi (Chinese astrology) maps your date and time to Five Elements across Four Pillars. Numerology reduces the date to structural numbers — Life Path, Personal Year, day number — each describing a different layer. Ziwei Doushu (Purple Star astrology) maps life domains to star-palace combinations.

Knowing your December star sign is the starting point. The full picture requires the full chart.

Get your chart across four traditions — free to start