Here's why:

* Right Ascension (RA): Think of RA as the celestial equivalent of longitude. It measures a celestial object's position eastwards along the celestial equator, starting from the vernal equinox.

* Retrograde Motion: This is an apparent backward motion of a planet as seen from Earth. It's an illusion caused by the relative motion of Earth and the planet as they orbit the Sun.

* Decreasing RA: During retrograde motion, the planet appears to move westward against the background stars. Since RA increases eastward, this westward motion means the planet's RA is actually decreasing.

Important Note: While RA decreases during retrograde motion, the planet's declination (celestial latitude) can either increase or decrease depending on the specific planet and its position in its orbit.