The effect of an aspect is felt even if the two planets in aspect are not exactly in the mutual distance required. Therefore a so-called orb of aspect - and this varies in each aspect- is allowed for each aspect type (Opposition aspect = 180°, Trine aspect = 120°, Square aspect = 90°, sextile aspect = 60°).
For example, if Moon is 15° longitude and Mercury is 134°, 134 - 15 = 119°, so if the Trine orb if more than 1°, the Moon and Mercury are said to be in Trine.