Orb

The margin of deviation / inexactness? allowed for an aspect to remain valid. For example, for two planets to be in opposition they do not have to be separated by exactly 180 degrees - the aspect would still be valid if the angle was 175 degrees or 185 degrees. Determining the size of the orb used is a matter of interpretation. Psychological astrology tends to use larger orbs, whereas horary astrology tends to work with tighter orbs. A plactic aspect is one which is strictly speaking just out of orb but still considered significant enough to include in a horoscope interpretation.

The faster moving a planet is the larger its orb - up to 10 degrees margin of deviation on each side. For example, this means that fairly large orbs are often used for aspects involving the Moon (which is the fastest moving planet). The type of aspect also plays a role in deciding the size of its orb: Larger orbs are used for the major aspects, whereas a smaller ones are used for the minor aspects (generally 1 to 3 degrees). The table on the left hand side gives a general idea of the size of orb for the major aspects.