A century of births, a degree and a half of spread
Take every 6 January from 1930 to 2035, measure the Sun at noon, and the answers fall between roughly 15.0 and 16.4 degrees of Capricorn. A great-grandparent and a great-grandchild born on this date have Suns within a degree and a half of each other.
People often assume the opposite, because they have heard that sign dates move. They do move, by a day or so, at the very edges of a sign. In the middle of a sign that same movement is invisible, since a day of solar motion is only about one degree.
That is worth stating plainly, because the usual framing invites the opposite conclusion. People hear that the zodiac has shifted relative to the constellations and assume their birthday has moved through signs. The tropical zodiac used here is measured from the March equinox rather than from the stars, so it does not drift in that way at all.
The mechanism behind the steadiness
A tropical year is close to 365.2422 days. The calendar counts 365, so the Sun arrives at any longitude about six hours later each year against the clock. Three such steps accumulate to eighteen hours, and then February gains a day and the calendar jumps forward, cancelling almost all of it.
What is left is a small residual, corrected further by the century rules that skip a leap day in 1900 and 2100 but keep it in 2000. The net effect over a normal human lifetime is a band of roughly one and a half degrees, oscillating rather than drifting.
The band and its rulers
Fifteen to sixteen degrees falls in the second decan of Capricorn, which covers ten to twenty degrees. The Chaldean sequence of decan rulers assigns it Mars. The triplicity scheme assigns it Taurus, and therefore Venus. Both are in use, and neither is a mistake.
Capricorn itself is cardinal earth under Saturn, and the tradition reads it as deliberate, long-horizoned and comfortable with delay. The decan is read as a modifier: Mars adds force and impatience to that framework, Taurus adds solidity and an attachment to the tangible. These are inherited readings and this page presents them as such rather than as claims about anyone's character.
The decan is also the layer that distinguishes one Capricorn birthday from another. Everyone born across roughly thirty days shares the sign. Only a third of them share this particular band, and the band carries its own ruler and its own inherited description.
Where the certainty stops
The degree is calculable, the decan follows from it, and the interpretation is tradition. Beyond that the date gives nothing. The Moon covers about thirteen degrees in a day and may sit in either of two signs on 6 January depending on the hour. The rising sign runs through all twelve in twenty four hours and is unknown without a time and a place.
The widget above computes your own solar degree from your year and birthplace rather than using the noon figures here, and reports how far your Sun lies from the nearest sign boundary.
Two facts close most of the gap: the time on the birth record and the town. With both, the Moon becomes exact rather than probable, the Ascendant becomes calculable, and every planet in the chart acquires a house.
Questions people ask about this
What sign is 6 January?
Capricorn, every year. The Sun is roughly halfway through the sign on this date, so no year, hour or time zone brings a boundary into range.
Why do sign dates shift if the degree does not?
Because a shift of a few hours only matters where a boundary is. At the edge of a sign it can change the answer. In the middle of a sign the same shift moves the degree by a fraction and changes nothing.
What degree is the Sun on 6 January?
About 15.0 to 16.4 degrees of Capricorn at noon across 1930 to 2035. Births earlier or later in the day sit slightly below or above that range.
Which decan and which ruler?
The second decan of Capricorn, ruled by Mars in the Chaldean order and by Venus through Taurus in the triplicity scheme.
Does the leap year affect my sun sign?
Only if you were born within a day of a sign boundary, which on 6 January you were not. The leap day shifts solar positions by around eighteen hours, enough to matter at an edge and not in the middle.
Is my rising sign also Capricorn?
Only by coincidence. The rising sign depends on the time and place of birth, not the date, and it has a one in twelve chance of matching the Sun.