A division the date sits on top of
Libra divides at ten and twenty degrees into three bands of ten. The spread of noon positions on 13 October straddles the second of those cuts, so in some years the Sun is still in the middle band and in others it has already moved into the closing one. Neither answer is correct for the date in general.
The hour and the birthplace matter as well. In a year where noon falls close to twenty degrees, a birth in the morning and a birth at night can end up on opposite sides of the line, because the Sun covers close to a degree in a day. This is one of the few things a birthday page can get genuinely wrong by stating a single decan flatly, and the widget above avoids it by calculating from your own details.
The two rulers in question
Under the Chaldean order the middle decan of Libra belongs to Saturn and the closing decan to Jupiter, both working beneath Venus as the ruler of the whole sign. Tradition reads the Saturn band as the stretch where Libra's weighing acquires a standard and holds to it, and the Jupiter band as the more expansive third, where the sign's attention to others widens rather than tightens.
Those are descriptions of a scheme, offered as tradition. What is checkable is the longitude, and on this date the longitude is what decides which description a practitioner would reach for. It is unusual for that decision to depend on the year of birth, and this is one of the dates where it does.
Why the position moves at all
A calendar year of whole days is about a quarter of a day shorter than the orbit, so each common year the Sun arrives at a given date slightly behind where it was, and the leap day repays that lag in one step. The result is a small repeating cycle of roughly one and a third degrees, which for most dates is far too small to change anything.
It only becomes consequential when a date happens to sit on a division, as this one sits on the twenty degree line. The same mechanism that makes most birthdays a reliable coordinate makes a handful of them genuinely year dependent at the decan level.
What remains open regardless
The Moon covers about thirteen degrees a day and crosses into a new sign every couple of days, so the date leaves it with two possibilities and sometimes three. The rising sign is not narrowed at all by the calendar, because the whole zodiac passes the eastern horizon in a rotation and only the hour and the latitude choose which sign was up.
The sun sign, meanwhile, is settled and shared with everyone born across the month. It is the least individual part of the chart, which is why the degree and the decan are the details worth having, and why on this date the decan is worth calculating rather than reading off a table. A table gives the same answer to everyone with this birthday, and on a date sitting across a division that answer is wrong for a substantial share of them.
Questions people ask about this
Which decan is a 13 October birthday?
It depends on the year. The noon position has fallen on both sides of twenty degrees since 1930, which is the line between the middle and closing decans of Libra. The widget above settles it from your year, hour and birthplace.
Is the sign in doubt as well?
No. Libra runs to two hundred and ten degrees of longitude, which the Sun does not reach until around the twenty third of the month. The sign is fixed on this date in every year of the range.
What is the difference between the two decans?
Under the Chaldean order the middle band answers to Saturn and the closing band to Jupiter, with Venus ruling the sign throughout. Tradition reads the first as the more exacting third of Libra and the second as the more expansive one.
Does the hour of birth affect the decan here?
In years where noon sits close to twenty degrees, yes. The Sun moves nearly a degree a day, so a morning birth and a late evening birth can land on opposite sides of the division.
Why do other pages state one decan for this date?
Because they work from a fixed table of date ranges rather than a calculated position. That approach is adequate away from a division and inaccurate on a date that sits across one.
Does the decan change how the chart is calculated?
No. The decan is an interpretive subdivision, so aspects, houses and everything else are worked out from the longitude itself. The decan only affects which traditional description is applied.