The problem is with what a year is, which is arbitrary to begin with, but worse still, a year is not constant. The momentum of every planet is affected by the g

The problem is with what a year is, which is arbitrary to begin with, but worse still, a year is not constant. The momentum of every planet is affected by the gravity of every other planet, leading to fluctuations in solar orbital speed, which means a full orbit is likely never the same period as the previous or subsequent orbit. Maybe these fluctuations are small, but light travels so fast that over long distances it is sure to be significant. Thus the definition of a light year is defined as the distance light travels in a Julian year, 365.25 days, but this just pushes the error down to a different measure, because days are not constant either. Since the speed of light actually is constant, maybe we should define years and days by the speed of light instead of vice versa.