The septile is an aspect of about fifty-one and three-sevenths degrees, formed by dividing the circle into seven equal parts. Because seven does not divide three hundred and sixty into a whole number, the septile falls at an irrational angle, and this sets it apart from the aspects built on simpler fractions. It is a minor aspect, given a very narrow orb, and it was not among the classical Ptolemaic set; interest in it grew with the harmonic astrology of the twentieth century. The tradition that uses it reads the septile as a relation touched by fate, compulsion, or inspiration, an irrational, otherworldly quality fitting its irrational angle, often linked to creativity and the sense of destiny. These are the interpretive conventions of the astrological tradition for this angular relation, one of the subtler aspects derived from dividing the circle by seven.
Because seven has no factor in common with the other small harmonics that generate the recognised aspects, one, two, three, four and six, the septile family stands apart from the semi-sextile, semi-square, sesqui-quadrate and biquintile, all of which are built from divisors already present in Ptolemy's five. The septile belongs to the wider revival of harmonic aspect theory that follows Kepler's Harmonices Mundi of 1619, though Kepler's own attention fell mainly on the fifth harmonic; twentieth century harmonic astrologers, prominently John Addey, did the systematic work of extending the approach to the seventh and other primes. Because 360 divided by seven never resolves to a whole number of degrees, software and older ephemeris-based practitioners can differ by a small fraction of a degree in where they place it, an ambiguity the aspects built on whole-number divisions do not share.