What is a Cusp?

'In astrological terms, a "Cusp" is the imaginary "boundary line" between two Signs, or between two Houses. It is described by the degree of a Sign where it is located, such as "0 degrees of Aquarius" or just "0 Aquarius". Sometimes you may see this written as "00 AQU 00" or where the AQU is replaced by the glyph or symbol for the Sign of Aquarius. If the Sun is at 0 Aquarius (00 AQU 00), or between 0 Aquarius and 0 degrees plus 30 minutes of Aquarius (zero plus 30/60ths of a degree, or 00 AQU 30) - then it is "on the Cusp". If it is at 1 degree Aquarius, it isn't. If it is between 29 degrees plus 30 minutes of Capricorn (29 CAP 30) and 0 Aquarius (00 AQU 00) it is also "on the Cusp", but actually in the previous Sign.'

'The Sun can only be in one Sign or another, not in two at once. The center point is considered to be such a small point, that it would cross from one Sign to another in less than a minute; and thus will go from 29 degrees and 59 minutes of one Sign, to 0 degrees and 0 minutes of the next Sign, in a very short time (Signs have 30 degrees, and a minute is 1/60th of a degree). The position of the Sun or any planet is never described as being in two Signs at once - it is in one Sign or the other, even if it is barely into the next Sign, or almost leaving the previous Sign. The only time it would be "on the fence", so to speak, is if its center point were exactly at the Cusp line, which is a rather rare occurance, since the center point and the Cusp line are infinitely small.'

'You might say it is sort of "on the fence" - but its center is going to be on one side of that fence or the other, since a Cusp is considered to be a very fine line of zero width. Think of a tiny peppercorn sitting on a fine pencil line - even though it is wider than the line and extends to each side, its center point is on one side of the line or the other. Only in rare cases is the exact center point of the Sun so close to the Cusp line that, within the limits of accuracy available to us, would it have to be said to be exactly "on the Cusp". And that's the only time you would really have some characteristics associated with each of the two Signs.'

-continued...
'Since Mercury is the Planet of "the Intellect" which indicates how you think and how you "communicate" (by talking or writing or gesturing), and it can often be in the next or previous Sign from your Sun, this can explain why you may think you have some traits of that Sign as well as the traits of your Sun Sign. You are observing the traits of your Mercury, not your Sun. Venus too, is always close to the Sun, and cannot be more than two Signs away from the Sun in anyone's horoscope. In either of these two common cases, those observed traits of the next or previous Sign are not the result of your Sun being "on the Cusp" at all - but because you have Mercury and/or Venus and/or other Planets (or the Ascendant) in the adjacent Sign.'

'Some astrologers differ in their opinion about this, so you will have to decide for yourself whether this opinion (which is shared by some of the most highly respected world-famous astrologers like Liz Green in England) makes more sense to you than saying the Sun is "on the cusp" if it is crossing the cusp line at any time on your day of birth - or saying it is "on the cusp" if it was located within one or two or even five degrees of the cusp line at the time of your birth.'

I always interpreted a cusp as meaning you ARE in fact two signs. Like my ex boyfriend's birthdate is June 20th and technically he is a Gemini BUT the next day June 21st is the start of Cancer so I am pretty sure that moron has water in his chart otherwise we wouldn't have made it as long as we did.

all that fine line stuff is crap....im 1 degree into cancer and there are definitely gemini traits in me. there is an overlap. like the sign of cancer chages as u go through.....i know plenty of cancers that are closer to leo and they are definitley a different type of cancer than me ....way more stubborn pride.

With this definition of a cusp, I couldn't prove to myself one way or the other.
I personally do believe in cusps, but I don't think they're extra large. IE:1-2 days MAYBE outside, if at all, planets aren't that big. Unless there essence lingers behind? Like some sort of snail trail?
I have a sneaking suspicion that with astrology, you can't just draw lines and put things in neat little boxes.

Does anybody have a Sun sign cusp-ing between SignA and signB, a rising, Mercury, and Venus in B? or vice versa? Do you still exhibit any A?

I bet we could take a survey or something, somehow

i knew of a Scorpio born only 1 degree away from being a libra, and it's strange without any other planets inside Libra..*he still exibited some of that Libra flair, aura, influence, you could just sense it. 'Another good survey would be to find cuspers born 'without' any other same-sign planets exacting their cusp, especially of inner planets, or ascendent.' That would be very interesting and telling. i do tend to agree though, that the orb/degree of influence is probably much tighter than most people might think...