The position the argument sits on
At noon in every year from 1930 to 2035 the Sun on 23 April stood between roughly two and a half and four degrees of Taurus. Three degrees is the working figure, and the spread across more than a century is under a degree and a half because the leap year cycle keeps the date and the orbit in step.
That places the birth in the first of the three faces of Taurus, the band from zero to ten degrees. Every year in the sample is inside it, so the date settles the face as firmly as it settles the sign.
Worth stating plainly that the sign is not affected by any of what follows. Three degrees is about three days past the Aries boundary, which is more margin than the year to year drift in the crossing, so no birth hour or time zone moves the answer. The disagreement below is entirely about the layer inside the sign.
How the Chaldean count reaches Mercury
The Chaldean order arranges the seven visible planets by apparent speed: Saturn, Jupiter, Mars, Sun, Venus, Mercury, Moon. Decan rulership walks that loop starting at zero degrees of Aries with Mars, and continues without a break through all thirty six faces of the zodiac.
Counting forward, Aries takes Mars, Sun and Venus, and the loop then carries on into Taurus with Mercury. So the first face of Taurus belongs to Mercury under this scheme, and the assignment is arithmetic rather than symbolic. Its logic is entirely in the sequence.
How the triplicity count reaches Venus
The other scheme ignores the planetary loop and works from the elements. Each sign's three decans go to the three signs sharing its element, taken in their zodiacal order. The earth signs are Taurus, Virgo and Capricorn, so the first earth decan belongs to Taurus, the second to Virgo and the third to Capricorn.
The first face of Taurus is therefore ruled by Taurus itself, and its planet is Venus, which already rules the whole sign. Under this count the band doubles the sign's own ruler rather than introducing a new one, which is the opposite of what the Chaldean answer does.
What follows from each, and what does not
Marked as tradition, the Mercury reading puts a mobile and articulate layer inside a slow sign, and is often used to explain why the earliest Taurus birthdays read less heavily than the later ones. The Venus reading goes the other way and treats the band as Taurus at its most characteristic, with nothing else mixed in.
Neither answer changes the sun sign, which is Taurus at any degree, and neither predicts anything. Nor does the date settle the Moon, which can be in either of two signs depending on the hour, or the ascendant, which moves through all twelve in a day. The widget above uses your year, hour and birthplace to give the exact degree and the margin to the nearest boundary.
Questions people ask about this
What sign is 23 April?
Taurus, in every year checked at noon from 1930 to 2035. The Sun is around three degrees past the boundary, which is roughly three days of margin.
Is the decan ruler Mercury or Venus?
Both, depending on the scheme. The Chaldean face order gives Mercury and the triplicity scheme gives Venus. Sources usually quote one without naming which system it came from.
Which scheme should I use?
Whichever the tradition you are reading uses, consistently. Mixing them produces contradictions, and there is no neutral authority that settles which count is correct.
What degree of Taurus is a 23 April Sun?
Near three degrees, within a range of about two and a half to four across the sampled century. Your exact figure depends on your year, hour and birthplace.
Does the decan override my sun sign?
No. A decan is a subdivision inside the sign, so the Sun is fully Taurus either way. The face ruler adds a second layer, it does not replace the first.
Does Venus rule this birthday twice?
Under the triplicity scheme, in effect yes, since Venus rules both the sign and the first earth decan. Under the Chaldean order the sign is Venus and the face is Mercury.