Movable longitude is the zodiac longitude of a heavenly body calculated from the vernal point
To get the Fixed longitude, you must subtract Ayanamsa to movable longitude.
This is a necessary correction to get zodiac sign the same as the constellation of the same name.
For example, if you don't subtract Ayanamsa a planet in Virgo his in fact in Leo constellation.
Notice that Hindus always use Fixed longitude.
See also: