Parsec

Annual parallax uses the change of viewing direction produced by the observer’s solar orbit, with one au as the geometric scale. A smaller angular displacement corresponds to a greater distance.

The modern parsec is exactly 648,000/π au. The familiar one-au, one-arcsecond description uses the small-angle relation; it does not replace that exact definition with a tangent-based length.

References: [1]

In the usual small-angle model, d in parsecs is approximately 1/p when parallax p is in arcseconds. Thus 0.1 arcsecond corresponds to about 10 pc and 0.01 to about 100 pc. The parallax angle is not the full displacement between opposite orbital positions.

An exact unit does not make observations exact. Taking the reciprocal of a noisy parallax can give an unreliable distance, requiring attention to measurement uncertainty and the inference model.

References: [1] [2]

One pc is about 3.26 light-years; a kiloparsec is 1,000 pc and a megaparsec is one million pc. Unit conversion supplies another expression of the same length, not an independent measurement.

Absolute stellar magnitude uses a reference distance of 10 pc. Converting observed brightness to that standard also requires a specified passband and treatment of extinction. A distance unit alone does not establish intrinsic luminosity.

References: [1] [3]