Thirukkanitham (which means the results that can be observed using your eyes). If you calculate Venus and Saturn using Vakyam and look in the sky to try and measure the angle between them, it’s not the same result as the Vakya Siddhanta gives you. The moon’s motion is very erratic and needs lots of corrections to arrive at accurate readings. How can you define the moon’s motion in one simple sentence where modern astronomers make pages and pages of corrections. The Moon requires corrections in the algorithm every 72 years.
The Surya Sidddhantam has not been updated in 1500 years and the Vakyam Siddhanta has not been updated either. The eclipse calculations in the Vakya panchangam and others are copied from the Thirukkanitham calculations. The Thirukkanitham relies on modern calculations using spherical trigonometry or NASA’s JPL. The inaccuracies in the Vakyam and Surya Siddhatam calculations can have errors of up to two - four hours in thithi and nakshatra, yoga, and karana end times, along with planetary positions.