Quick answer: you have exactly one Sun sign. At the instant you were born the Sun stood at one specific degree of the ecliptic, and that degree falls inside one sign. What varies is the DATE the Sun changes sign, which shifts by up to a day from year to year, and that is where the cusp idea comes from.
"Born on the cusp" is one of the most common things people believe about their chart, and it is a misunderstanding of a real fact. The real fact is that the date ranges printed in newspapers are approximations. The misunderstanding is that this leaves you between two signs. It does not, and the reason is worth understanding because it explains something about how the calendar works.
Why the dates move
A tropical year is about 365.2422 days, not 365. The calendar absorbs that quarter day with leap years, which means the calendar drifts forward by roughly six hours a year and then jumps back by a day every fourth February.
The Sun does not care. It reaches 0 degrees of Libra when it reaches 0 degrees of Libra, and that moment lands about six hours later each year until a leap day resets it. So the ingress into a sign can fall on the 22nd one year and the 23rd the next.
That is the entire origin of the cusp idea. A magazine cannot print "Libra begins on 22 September at 18:44 UT this year and 23 September at 00:33 UT next year", so it prints "September 23 to October 22" and adds a hedge for people born at the edges. The hedge is editorial, not astronomical.
What actually happens at the boundary
The Sun moves roughly one degree a day, or about 2.5 arcminutes an hour. When it crosses from 29 degrees 59 minutes of Virgo to 0 degrees 00 minutes of Libra, that crossing takes an instant, and you were born either before it or after it.
There is no overlap and no blend. A chart calculated with your birth time gives the Sun a single longitude, and that number falls in one sign. The sun sign calculator will tell you which in a few seconds, and it uses your date rather than a printed range.
If you were born within a day of an ingress, your birth TIME becomes the deciding factor, which is the one situation where a Sun sign genuinely depends on the hour. For everyone else the date settles it.
The one thing that is true
A planet in the last degree of a sign is in an unusual position, and classical astrology has always noticed this. It is not between two signs; it is at the end of one, about to lose the entire context it has been operating in. Some traditions treat the final degrees as weak for that reason, under the heading of critical or anaretic degrees.
So the intuition behind the cusp question is not stupid. Something does change sharply there. What is wrong is only the idea that you get to be both.
Cusp means something else in a chart
The word has a precise technical meaning that has nothing to do with birthdays. A cusp is the boundary line of a HOUSE: the degree at which one of the twelve divisions begins.
House cusps are what a house system computes, and they genuinely do move. Two systems can disagree about which house a planet occupies while agreeing exactly on where the planet is, because the planet's position is astronomy and the cusp is a convention.
So when an astrologer says a planet is on a cusp, they almost always mean it sits near a house boundary, not near a sign boundary. The two usages get confused constantly and they are not the same thing.
How to settle it for yourself
Cast the chart. Your Sun's longitude will come back as a single number with a sign attached, and that is the answer regardless of what any date range says. If you were born within about 24 hours of an ingress, add your birth time, and if you do not know it to the hour then this is one of the few cases where a rough time is genuinely worth chasing.
What you will not find is a chart that gives you two Sun signs, because no such chart exists. The Sun sign article covers what the placement actually describes once you know which one you have.
Frequently Asked Questions
Can I be born on the cusp of two signs?
Not in the technical sense. At your birth moment the Sun occupied one degree of the ecliptic and that degree falls inside exactly one sign. What moves is the calendar date of the boundary, not your placement.
Why do sign dates change from year to year?
Because the tropical year is about 365.2422 days rather than 365, so the moment the Sun enters a sign drifts about six hours later each year until a leap day pulls it back. That is why a boundary can fall on the 22nd one year and the 23rd the next.
What if I was born on the day the sign changed?
Then your birth time decides it, and this is the one case where a Sun sign genuinely depends on the hour. Calculating the chart with your time gives a single answer.
