Harmonics are a generalization of aspects. Two planets/angles are in Nth harmonic relation if the angle seperating them is (within orb) 360/N. So, conjunction is a 1st harmonic [360/1 = 360 = 0 (mod 360),] opposition is a 2nd harmonic [360/2 = 180] and so forth.
The 20th Century English astrologer John Addey pioneered the study of harmonics. Here is a two-bit tour of the subject. Not surprisingly, some people try to assign numerological signifigance to harmonics. Your humble obedient blogger is ignorant of numerology, so cannot intelligently comment on the matter.
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