Vakri (retrograde) grahas: what it means
Vakri (retrograde) describes a planet that appears to move backward against the stars as seen from Earth. It is an apparent motion, caused by the relative speeds of Earth and the planet.
What it is
Mars, Mercury, Jupiter, Venus and Saturn can retrograde. The Sun and Moon never do, and Rahu and Ketu always move backward by definition of the mean node.
How it is worked out or read
This site marks a planet as retrograde when its longitude one day later is smaller than today's. It does not use the classical rules for retrograde strength.
Where it appears on this site
The birth chart marks retrograde planets with R. The retrograde in the chart article explains how it is read.
Why planets appear to reverse
A planet appears to move backward when Earth overtakes it on the inside track, or when a planet on the outside track is passed by Earth. The effect is a result of relative motion, not of any change in the planet's own path. Retrograde motion is visible in the sky for weeks at a time, and it occurs regularly for Mars, Jupiter and Saturn, and for Mercury and Venus which lie inside Earth's orbit.
Retrograde and the classical view
The classical texts give retrograde planets several meanings. Some read a retrograde planet as stronger in certain respects, because it is said to be working inward, and others read it as weaker because its motion is unusual. The site does not apply a strength rule for retrograde planets. It marks them with R in the birth chart, so the reader can see which grahas were retrograde at birth and apply the rule the reader's tradition uses.
Checking a retrograde mark
The site marks a graha retrograde when its longitude one day later is smaller than its longitude today. A reader who doubts a mark can compute the position for two consecutive days and compare them, using the ephemeris or the birth chart tool. The Sun and Moon are never marked, and Rahu and Ketu are not marked because the mean node always moves backward by definition.
Why planets appear to reverse
Retrograde motion is an effect of viewing the planets from the moving Earth. An outer planet such as Mars, Jupiter or Saturn appears to move backwards when the Earth overtakes it on the inside track. An inner planet, Mercury or Venus, appears to move backwards when it overtakes the Earth while passing between the Earth and the Sun. Before and after each retrograde period the planet appears to stand still, at a station.
How the texts read it
The texts give retrograde planets high motional strength, cheshta bala, and many readers treat a retrograde planet as intensified or turned inward. The Sun and Moon are never retrograde, and Rahu and Ketu always move backwards. The planet table on this site shows whether each planet was retrograde at birth.
In short
Retrograde is apparent motion, not a planet truly reversing in space.