Radar Ranging & Stellar Parallax: Observation, Measurement & Evidence

Radar distance follows distance = light speed × round-trip time / 2. Stellar parallax uses Earth's orbital radius as a baseline; distance in parsecs is the reciprocal of parallax in arcseconds for the standard small-angle geometry.

Radar echoes from planets establish precise Solar-System scales, while ground and space astrometry measure parallaxes for nearby stars. Missions such as Hipparcos and Gaia have expanded geometric stellar distances to enormous samples.

A nearby star viewed from opposite sides of Earth's orbit appears to shift against much more distant background stars. The same triangulation principle used in surveying becomes extraordinarily demanding because stellar angles are tiny.

Radar echoes, spacecraft telemetry, Hipparcos, Hubble, and Gaia establish geometric scales. Repeated observations across seasons separate parallax from linear proper motion and binary wobble.