retrogradus

Retrogradus is a term used in several disciplines to denote backward or reverse direction. Derived from the Latin retrogradus, meaning "going backward" (retro- meaning "back," gradus meaning "step" or "degree"), the word lends itself to descriptions of motion, sequence, or orientation that proceeds in reverse relative to a standard forward progression.

In astronomy and celestial mechanics, retrogradus appears as a Latin-style descriptor for retrograde motion, the apparent reversal in a planet's path as observed from Earth. Historically, retrogradus was used in manuscripts to label scenes of planetary motion that appear to move against the usual order; the modern understanding attributes this to relative motion between Earth and other planets in a heliocentric model.

In linguistics and textual studies, retrogradus is sometimes invoked metaphorically to describe processes that reverse sequence or chronology, though this usage is uncommon and usually described as retrograde rather than retrogradus. In music theory, Latin-era or scholarly writings may refer to "retrogradus" as a stylistic concept indicating a melodic line performed in retrograde, a technique related to the more common term "retrograde" in fugues and canons.

Today, retrogradus appears most often in historical discussions of Latin terminology in astronomy and in discussions of retrograde techniques in music theory. It is rarely used as a current technical term outside scholarly or historical contexts.