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How can radar measurements of Venus help determine the distance from Earth to the Sun?

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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 of Venus do not rely on the parallax method, which involves measuring angular shifts against background stars. Instead, radar provides a direct and highly accurate way to determine the astronomical unit—the average distance between Earth and the Sun—by measuring the time it takes for a radio signal to travel to a planet and back.12

By the 17th century, astronomers had developed a proportional model of the solar system, meaning they understood the relative distances between planets even without knowing the absolute scale in kilometers. To establish this scale, they only needed an accurate measurement of the distance between any two objects at a specific time.2

Modern radar systems bounce signals off Venus and measure the round-trip time of flight. Because the speed of light is known, this timing allows scientists to calculate the precise distance between Earth and Venus. Once this distance is known, the proportional model of the solar system can be used to derive the absolute distance from Earth to the Sun with great precision.132