An Etymological Dictionary of Astronomy and Astrophysics

A suffix denoting spatial or temporal direction, as specified by the initial element. Also -wards (Dictionary.com).

Etymology (EN): M.E., from O.E. -weard “toward,” literally “turned toward;” PIE *wert- “to turn, wind,” from *wer- “to turn, bend;” cf. Pers. gard-, gardidan “to turn, change,” → revolve, → evolve.

Etymology (PE): -su, → direction.