Which observational concept relies on observing from two different vantage points to determine distance?

• A. Luminosity distance
• B. Standard candle
• C. Parallax
• D. Redshift

Answer: (C)

Parallax is the apparent shift of an object's position against distant background when your point of view changes. In astronomy, we observe a nearby star from two vantage points separated by a baseline—typically six months apart as Earth travels around the Sun. The small angle the star seems to move is the parallax angle, and distance is directly tied to that angle: larger baseline or larger observed shift means closer object, with distance in parsecs equal to one divided by the parallax angle in arcseconds. This is a geometric, direct way to measure distance without assumptions about the star’s brightness.

Other concepts involve how bright something appears or how its light is stretched by the expanding universe, not triangulation from two viewpoints. Luminosity distance uses intrinsic brightness and observed brightness; a standard candle is a object with known intrinsic brightness used for that method; redshift tracks cosmic expansion. None of these rely on observing from two different positions to deduce distance, which is why parallax is the one that fits.