The Descendant is exactly opposite the Ascendant on the western horizon. It represents partnerships, marriage, and how you relate to others. It often describes the qualities you look for in a partner or the traits you "project" onto others.
The astronomy in these guides is computed, not copied: sign boundaries, aspect angles and planetary positions are calculated on StarBind's own servers from the same JPL planetary data NASA uses, accurate to fractions of an arcsecond.