Where this decan sits in the year
Each sign splits into three parts of ten degrees, called decans or faces. This is the first of the three in Taurus, running from zero degrees to ten degrees of the sign.
The Sun takes roughly ten days to cross it, usually from about 20 April to about 30 April. Those dates are approximate on purpose. The tropical year is not a whole number of days, so the boundaries drift by up to a day and a half from one year to the next, and a birthday at either end can fall on the other side of the line depending on the year and the hour.
Mercury or Venus, and why the answer differs
The Chaldean order runs Mars, Sun, Venus, Mercury, Moon, Saturn, Jupiter and then repeats, starting from the beginning of Aries and continuing without a break through all thirty six decans. Counting forward from that starting point, the first decan of Taurus arrives on Mercury.
The triplicity scheme is built differently. It gives each decan of a sign to a sign of the same element, taken in order, so the earth decans go to Taurus, then Virgo, then Capricorn. The first earth decan therefore belongs to Taurus itself, and its ruler is Venus.
So the schemes disagree, and plainly. Most popular writing on decans quotes only one of them and does not name which, which is how the same ten degrees end up described as Mercurial in one article and Venusian in the next. Neither is wrong. They are answers to slightly different questions.
What the tradition reads into it
Under the triplicity reading, this decan is Taurus doubled: Taurus coloured by Taurus, with nothing borrowed from Virgo or Capricorn. That is taken to mean the sign's characteristics show with the least modification, so the emphasis falls on steadiness, physical comfort, a slow rate of change and a strong dislike of being hurried. Read from the other side, the same description becomes reluctance to revise a settled position.
The Chaldean assignment of Mercury adds something quite different. Mercury is read here as bringing speech, handling, counting and exchange into a stretch of the zodiac otherwise associated with holding still. Traditional texts on the faces of Taurus mention work with the hands, goods and their valuation, and the practical negotiating that goes with them. It is a narrower claim than the general sign description and worth treating as separate from it.
Beyond the Sun, and how much weight to give it
Decans apply to any placement, not only the Sun. A Moon or an Ascendant in the first decan of Taurus is read the same way, and in older practice the Ascendant degree mattered more than the solar one. This page is framed around the Sun because that is what most readers arrive with.
Decan work is genuinely old. It descends from Egyptian star lists used to mark the hours of the night, long before it was folded into the Greek and later Arabic material that produced the rulership schemes above. That history is worth knowing, but it does not make the technique foundational. Decans refine a reading that already rests on sign, house and aspect. Treat them as a second-order detail rather than the thing everything else hangs from.
The check at the top of this page computes your actual solar degree from your birth date, which is what settles the question for anyone born near 20 or 30 April. A date range cannot do that.
Questions people ask about this
What dates does the first decan of Taurus cover?
Approximately 20 to 30 April, with the boundaries moving by up to a day and a half between years. If your birthday sits at either end, use the check above rather than the range.
Is the ruler of this decan Mercury or Venus?
Both answers are in circulation. The Chaldean order gives it to Mercury, while the triplicity scheme gives it to Venus by way of Taurus itself. This page shows both so the disagreement is visible instead of hidden.
What is a decan?
A ten degree division of a sign. Every sign has three of them, giving thirty six across the zodiac. The idea comes originally from Egyptian star lists that marked the passing hours of the night.
Does the decan apply to my Moon or Ascendant too?
Yes. Any point in a chart falls in some decan, and the technique was applied to the Ascendant and the Moon at least as often as to the Sun. The Sun is simply the placement most people know.
Why does another site give a different ruler for these degrees?
Almost certainly because it is using the other scheme and has not said so. Check whether the article names the Chaldean order or the triplicity method before assuming one of you has made a mistake.
Does being in the first decan make someone more Taurean?
That is the traditional claim under the triplicity scheme, on the grounds that nothing from another earth sign is mixed in. It is an interpretation rather than a measured effect, and the Chaldean reading would describe the same degrees differently.