Degrees and approximate dates
A sign divides into three faces of ten degrees. This is the last of them in Aries, running from twenty degrees up to the boundary with Taurus at thirty.
The Sun takes about ten days to cross it, which in most years falls between roughly 10 April and 20 April. Those dates are not fixed. Because the tropical year is not a whole number of days, the boundary drifts by up to a day and a half depending on the year and on your time of birth, so a date range is only ever an approximation.
For that reason the ranges printed in most guides should be read as a guide to the middle of the decan rather than to its edges.
Venus by one count, Jupiter by the other
The Chaldean order walks through Mars, Sun, Venus, Mercury, Moon, Saturn and Jupiter in that fixed loop, starting at zero degrees of Aries. Counting forward, the third face of Aries lands on Venus.
The triplicity scheme ignores that loop entirely. It gives each decan of a sign to a sign of the same element, taken in order, so the fire decans of Aries go to Aries, then Leo, then Sagittarius. That makes Jupiter the ruler here, as the traditional lord of Sagittarius.
Venus and Jupiter are not close readings of the same thing, and this is exactly the sort of disagreement that popular writing tends to hide. Most articles name a decan ruler without saying which scheme produced it. Both are given on this page so you can see where the difference comes from.
What the tradition reads into it
The Sagittarius colouring is read as the point where Aries acquires a destination. The impulse to start is still there, but it is described as pointed at something outside the immediate moment: a plan, a journey, an argument being made rather than simply an action being taken. Traditional texts associate this face with movement over distance and with a willingness to state a position out loud.
The Chaldean Venus is taken to add something different again, and it sits oddly with the sign. Venus is read here as an interest in the response, in whether the thing done lands well with other people. In a decan otherwise described as forward and unhesitating, that is usually interpreted as a softening of the delivery rather than of the intent.
None of this applies only to the Sun. Any placement occupies a decan, and the technique is used on the Ascendant and the Moon at least as often. Decans are a genuine and very old part of the tradition, descended from Egyptian star lists that predate the zodiac signs in their current form, but they work as a refinement on top of a chart rather than as a foundation for one.
Why the degree matters more than the date
The boundary between this decan and the first decan of Taurus is a precise degree, not a calendar day. Somebody born on 19 or 20 April may be on either side of it depending on the year, and the same is true at the lower boundary around 10 April.
The check above works out the actual position of the Sun for your birth date and reports the degree together with the ruler each scheme assigns. If you were born near either edge, that is the only way to settle it. A published date range cannot.
It also reports the neighbouring decan, so you can see how close the call was.
Questions people ask about this
What dates does the third decan of Aries cover?
Approximately 10 April to 20 April. The boundaries shift by up to a day and a half between years, so anyone born close to either end should check their solar degree rather than trust the range.
Is this decan ruled by Venus or by Jupiter?
It depends on the scheme. The Chaldean order gives it Venus, while the triplicity scheme assigns it to Sagittarius and therefore to Jupiter. Neither is a mistake, and this page shows both so the source of the disagreement is visible.
Why do other sites give a different ruler?
Almost always because they are using the triplicity scheme without naming it. That is a legitimate approach, but quoting one ruler as though it were the only answer hides a real split in the tradition.
Does the Sagittarius influence make someone a Sagittarius?
No. The sign of the Sun is still Aries. The triplicity decan is read as a flavour laid over the sign, affecting emphasis rather than replacing the placement.
Do decans apply to the Moon and the Ascendant?
Yes, and many practitioners find them more informative there. Any point in a chart sits in one of the thirty six decans. This page is framed around the Sun because that is what most readers arrive looking for.
How much weight should a decan carry in a reading?
Less than the sign, the house and the major aspects. Decan work is old and well documented, but it is a second-order detail. Treated as the whole picture it tends to overstate itself.