Retrograde — negative degrees per day

Apparent retrograde motion is the body moving backward in ecliptic longitude as seen from Earth. The Swiss Ephemeris returns a signed speed. The code tests speed < 0. Mercury retrograde and Venus retrograde are the same test as Saturn retrograde. There is no special case by planet and no special case by sign.

A sign cannot be retrograde. Aries does not “go retrograde.” Mercury can be retrograde while in Aries, or while in any other sign, or direct in Aries. The sign pages do not bake a retrograde story into the sector. The sky table shows the flag for the observation time, from Paris.

Stations, the moments speed crosses zero, are a finer question than the boolean flag. The flag on a single timestamp says which side of zero the body is on at that instant. It does not by itself give the station date.

A speed of minus 0.4° per day is retrograde. A speed of 0.0 is not, and a speed of 1.2° per day is not. Mercury can post minus 0.4 while sitting at 10° Aries, and it can post plus 1.2 at that same degree a few weeks later. The Aries page does not change its ruler or its 0°–30° window when Mercury’s speed flips. Only the sky row’s flag flips. One chart gives the sign of the speed, not the date the speed changed.