The confusion happens because you are following “Sun-Sign astrology” which is not real astrology.
The “Sign” you find from the month-day of birth is only the Sign that Sun appeared to be moving through on that day. Just 2% of the total astrological influence on a person. Okay .. an AVERAGE of 2% .. sometimes the influence of the Sun is more than 2%, sometimes it is less.
The reason that Sun-sign astrologer persists is because it doesn’t take 2 hours of calculating 40-50 factors for the MOMENT that person was born, as the sky appeared from WHERE they were born. No … instead, just look up the month-day of birth on a list, and you get that 2% Sun-sign influence. It is not SUPPOSED to be astrology. Sun-signs are supposed to be “entertainment only” and you are not supposed to take them seriously … even BY those who write about Sun signs (assuming they have any basic understanding about astrology).
As for cusps. They are a way to cover up getting the Sun Sign wrong. Because the dates for Sun Signs will vary, depending on the year of birth, the place and exact time THAT person was born. So it is common to get the Sun sign wrong.
But since Mercury and Venus are never far from the Sun, you MIGHT have Sun in one Sign, and Mercury and/or Venus in the next-door Sign .. so you SEEM like BOTH Signs. And you ARE, but not because of Sun’s location.
In truth, EVERYONE has ALL 12 zodiac signs active in their chart. It is just that whichever Signs had any of the 10 “planets” (including Sun) in them … those Signs will have more of an influence of your personality than the Signs which had no planets moving through them.
Let’s take your example of someone born on September 24th:
(1) SOME years Sun leaves Virgo and enters Libra SOMETIME during September 22nd, and OTHER years, it makes this change sometime during September 23rd. So anymore one born before Sept 22 has Sun in Virgo. And after Sept 23, their Sun is in Libra. IF born on either Sept 22 or Sept 23, the ONLY way they can find out WHICH Sign THEIR Sun was in , is to do up their unique birthchart, using the exact hour-minute of birth, the full birthdate (including year), and the location of their birth. Cusps are bogus.
(2) Let’s say someone was born Sept 24, 2019. They have:
Sun in Libra
Moon in either Pisces or Aries, depending on their actual birth time and
Mercury in Libra
Venus in Scorpio
Mars in Aquarius
Jupiter in Scorpio
Saturn in Capricorn
Uranus in Taurus
Neptune in Pisces
Pluto in Capricorn
They could have ANY of the 12 zodiac Signs rising on the eastern horizon,
again depending on the birth time and birth location.
The “aspects” (distances) between the planets is FAR MORE important than
Signs, and everyone born on that date will have their planets in different Houses … but I don’t want to burden you with too much detail.
NOTE that this person ONLY has 2 out of 10 planets in Libra, so they will not be much LIKE a Libran.
(3) Compare this with someone born Sept 24 in 2000:
Sun in Libra
Moon in Leo
Mercury in Libra
Venus in Libra or Scorpio, depending on the time and place of birth
Mars in Virgo
Jupiter in Gemini
Saturn in Gemini
Uranus in Aquarius
Neptune in Aquarius
Pluto in Sagittarius
They have 2, maybe 3 planets in Libra … and a LOT of planets in Air Signs, so they will be much more like a Libra than the person born in 2018.
When it comes to astrology, I am not an idiot. My mother was an astrologer. I started studying astrology in 1972, and later wrote and passed the examination for Professional Certification from the American Federation of Astrologers.
Forget about Sun-sign astrology. It is a scam only.
Forget about cusps. They are a tool of the scammers.
Go to alabe.com or caféastrology.com and put in YOUR complete birthdata (make sure the birthtime is precise and not rounded-off to the nearest hour or half-hour), and starting using REAL astrology.
And understand that you cannot do astrology about anyone UNLESS you have their full birthdata (date/time/place), and that the Signs are not NEARLY as important as the “aspects” between planets. So don’t even be THINKING that someone is ONE Sign.