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How do radar measurements help determine distances within the solar system?

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Diagram illustrating how astronomers calculate the distance to a star using parallax: Earth is shown in two positions on opposite sides of its orbit around the Sun, with lines extending to a nearby star X and further to distant background stars. The apparent shift in star X's position, measured as angle A, is used to determine its distance, with the baseline labeled as 186,000,000 miles. — Parallax Calculating stellar distances.

Radar measurements determine distances within the solar system by calculating the precise time delay between the transmission of a radio signal from Earth and the detection of its reflection from a celestial body. Because the speed of light is known with high precision, this "time of flight" allows astronomers to calculate the exact distance to objects such as the Moon, Venus, Mars, and Mercury.123

While historical methods relied on trigonometric parallax—which involves measuring the apparent angular displacement of a planet against background stars from different locations—radar ranging provides a far more direct and accurate approach. By measuring the distance between Earth and another planet, astronomers can calibrate the scale of the entire solar system, effectively determining the astronomical unit (the mean distance between Earth and the Sun). This technique has largely superseded older parallax-based methods for establishing these fundamental distances.34