Why is the Astronomical Unit (AU) inadequate for measuring the distance to Proxima Centauri?
The AU becomes too small to be practical outside the Sun's immediate gravitational influence
The Astronomical Unit (AU) is perfectly calibrated as a ruler for the relatively small distances within the solar system, as exemplified by Jupiter orbiting at $5.2 ext{ AU}$ and Pluto at $40 ext{ AU}$. However, when astronomers attempt to measure distances to other stars, such as the nearest star system, Alpha Centauri (or Proxima Centauri, its closest component), the sheer scale involved renders the AU impractical. Expressing interstellar distances in AU would result in unwieldy, massive numbers filled with excess zeros, hindering easy comparison and comprehension. This limitation necessitates the shift to larger, light-speed based units like the light-year or the parsec to manage interstellar gulfs.
