The position first
A 7 January birth puts the Sun between about 16.0 and 17.4 degrees of Capricorn, measured at noon across the years 1930 to 2035. The spread is under a degree and a half because the leap year cycle keeps the calendar and the orbit in step, resetting the six hour annual slip every fourth February.
Sixteen degrees sits in the second decan of Capricorn, the band from ten to twenty. That much is arithmetic and is not in dispute. The dispute begins with who rules the band.
Sixteen degrees is also a comfortable place to sit. The nearest decan edge is four degrees away, roughly four days of solar travel, so no birth hour or year in the modern calendar moves this date out of the band it belongs to.
The Chaldean order
The older of the two schemes runs a fixed sequence of the seven visible planets through all thirty six decans: Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter, then round again. The count begins at zero degrees of Aries.
Capricorn is the tenth sign, so its first decan is the twenty eighth in the sequence and takes Jupiter, its second takes Mars and its third takes the Sun. A 7 January birthday therefore falls in a Mars ruled decan under this scheme. The order is the same one behind the planetary hours and the naming of the days of the week, which is part of why it has survived.
The triplicity scheme
The other scheme ignores the planetary sequence and works by element. Each sign's three decans go to the three signs of its own element, starting with the sign itself. Capricorn is earth, so its decans go to Capricorn, Taurus and Virgo in that order.
The second decan is the Taurus one, ruled by Venus. This is the scheme most modern articles use, usually without naming it, which is why a reader told their birthday is Mars ruled somewhere and Venus ruled elsewhere concludes that one of the two must be an error.
Both are legitimate. The Chaldean order describes a rhythm running through the whole zodiac. The triplicity scheme describes an internal structure within each sign. They are answering different questions.
What that leaves you with
Under the Chaldean reading this band is Saturn's patience with a Martial edge, effort pressed against structure. Under the triplicity reading it is Saturn's patience with a Venusian steadiness, work directed at something solid and enjoyable. Both are inherited readings rather than findings, and there is no test that decides between them.
What is not interpretive is the position. The widget above computes your solar degree from your own year and birthplace, names the decan, and gives the distance from your Sun to the nearest sign boundary.
Beyond the Sun the date settles nothing else. The Moon covers about thirteen degrees in a day and may be in either of two signs depending on the hour, and the rising sign turns through all twelve in twenty four hours, so it needs a time and a birthplace before it can be worked out at all.
Questions people ask about this
Who rules the 7 January decan?
Mars under the Chaldean order and Venus under the triplicity scheme. Both attributions are traditional, and sources rarely state which one they are following.
Which decan scheme is correct?
Neither supersedes the other. The Chaldean order runs a planetary sequence through the whole zodiac, while the triplicity scheme divides each sign by its own element. They are different tools.
What sign is 7 January?
Capricorn, without exception. The date is well clear of both the Sagittarius boundary in December and the Aquarius boundary later in January.
What degree of Capricorn is 7 January?
Between roughly 16.0 and 17.4 degrees at noon over the years 1930 to 2035. Your own figure depends on your birth hour and time zone as well as your year.
Could I be in a different decan than this page says?
Not on this date. Sixteen degrees is several degrees clear of both the ten and the twenty degree boundaries, so no hour of any year moves the Sun out of the middle band.
Does the decan ruler change my sun sign ruler?
No. Saturn rules Capricorn regardless of which decan a placement falls in. The decan ruler is an additional layer rather than a replacement.