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'.
Traditionally, 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'. For example, 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.