- Aspects show an influence in the period leading up to exactness and a diminishing effect as they separate from perfection. The time-span in which their effect may be expected to manifest and linger is defined by the period that they are said to be 'in orb'.
Many modern astrologers apply an orb of 8° before and after the most powerful aspects (conjunction, square, trine and opposition) to indicate the periods of application and separation. A smaller orb of 4° is similarly applied to the sextile, with 2° applied to the remaining, minor aspects. (For further details see An Introduction to Aspects and Chart Shaping by Nick Campion).
Traditionally however, orbs of influence were applied to planets, not the aspects themselves, with the Sun and Moon recognised as having a greater 'virtue' and orb of influence than the planets. For example, one popular list defines the Sun's orb as 15°; the Moon's orb as 12°, and the orb of Mercury, Venus and Mars as 7°. To know if two planets are 'in orb of application' their orbs are added together and halved - if the planets are separated by less than that distance they are said to be 'in application'.
Sun and Moon: Orbs are 15° + 12°; so 27° divided by 2 = 13°30'. Thus the Sun and Moon are 'in orb' of any aspect when they are less than 13°30' from the point of exactness.
Sun and Mars: Orbs are 15° + 7°; so 22° divided by 2 = 11°. Thus the Sun and Mars are 'in orb' of any aspect when they are less than 11° from the point of exactness.
A reference table for the most popular orbs applied in traditional literature, with pre-calculated limits for application, is available in the Table to Show Orbs for Aspectual Contact.
The section 'Application & the Development of Orbs'
in the article The Classical Origin and Traditional Use of Aspects explains the logic and method of determination more fully.
© Deborah Houlding