Why do astronomers use observations from widely separated places to measure solar parallax?

Astronomers use observations from widely separated places on Earth to measure solar parallax because they rely on the principle of trigonometric parallax to determine distances within the solar system.1

By observing a celestial object from two different locations, astronomers create a triangle where the two observing points form a baseline and the object serves as the apex. To achieve the greatest precision in these measurements, it is essential to use the longest possible baseline. When measuring the parallax of the Sun or other bodies within the solar system, the distance between two widely separated points on Earth’s surface provides this necessary baseline.21

The resulting angular displacement, or parallax, allows astronomers to calculate the distance to the object. Because the Sun is very distant, these measurements are often performed by observing other planets—such as Mars or Venus—as they approach Earth, as the smaller the distance to the object, the larger and more measurable the parallactic displacement becomes.21