When I started studying astrology one of the (many) concepts that confused me was aspects. If two planets are in the same sign but twenty degrees apart are they still conjunct? Fifteen years into my astrology career I've come up with some answers that work for me. I'll share them in case anyone else is as confused as I was.
There is a general orb of allowance for all aspects although there is no standard. The further apart two planets are from the exact degree that forms the aspect, the weaker the aspect becomes. For example, when two planets are zero degrees apart they are exactly conjunct and the aspect is strongest. If they are one degree apart the conjunction becomes slightly weaker, five degrees, even weaker, and so on. So how far apart can they be before the aspect loses all strength? This depends on the strength of the aspect and planets. Certain aspects (like conjunctions and oppositions) and certain planets (like the Sun and Moon) have more oomph. It's like tossing a stone into a pond: a bigger stone will create ripples that go further.
From my personal experience and studies here is a list of what I feel are the maximum allowable orbs for major aspects: Conjunction: 7 degree orb, 9 degrees with Sun or Moon Sextile: 5 degree orb Square: 7 degree orb, 9 degrees with Sun or Moon Trine: 7 degree orb, 9 degrees with Sun or Moon Opposition: 7 degree orb, 9 degrees with Sun or Moon
Please note this is not a definitive list. Some astrologers use very small orbs (two to three degrees max). Others use orbs up to twelve degrees. One thing most astrologers agree on is when the Sun and Moon are involved the orb can be increased.
So does this mean if Mars is at two degrees Capricorn and Mercury is at eleven degrees Libra they have no effect on each other? They are still square by sign and only slightly out of aspect (they are nine degrees apart). Early astrologers used sign aspects: as long as these two planets were in Capricorn and Libra they would be considered square to each other. Deborah Houlding's excellent site on classical astrology (www.skyscript.co.uk) has a detailed article on this.
My answer would be that the two planets would weakly affect each other. If Mercury square Mars makes someone argumentative, this weak square might make them inclined to dispute things but the urge would be overridden by the Libra desire for harmony. A tighter square might mean they still suppress the urge to disagree but boil over with resentment later on.
Astrology is a technique that defines human characteristics so it's difficult to slap any hard and fast rules on it. However, some rules are needed. It's my belief that somewehere between these two statements is an area where astrology works.