In astronomy, a kind of tremulous motion of the axis of the earth, by which in its annual revolution it is twice inclined to the ecliptic, and as O is the fifteenth letter, and the fourth vowel in the English Alphabet. The shape of this letter seems to have been taken from the circular configuration of the lips in uttering the sound. It corresponds in figure with the Coptic O, and nearly with the Syriac initial and final vau, and the Ethiopic ain. In words derived from the oriental languages, it often represents the vau of those languages, and sometimes the ain; the original sound of the latter being formed deep in the throat, and with a greater aperture of the mouth. O.S. stands for Old Style.

In astronomy, a kind of tremulous motion of the axis of the earth, by which in its annual revolution it is twice inclined to the ecliptic, and as often returns to its former position.