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Why are arcseconds used to describe stellar parallax angles?

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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.

Arcseconds are used to describe stellar parallax angles because the angular shifts of stars caused by Earth’s orbit are extremely small. An arcsecond is defined as 1/3,600 of a degree, a unit of measurement necessary to quantify the minute apparent displacements of even the closest stars.12

For example, the closest star, Proxima Centauri, has a parallax of only 0.77 arcseconds. Because these angles are so tiny, astronomers utilize this precise unit to calculate stellar distances. This relationship is fundamental to the definition of the parsec, a unit of distance equal to 3.26 light-years, which is defined as the distance of an object whose parallax equals exactly one arcsecond. Using arcseconds allows for a standardized, manageable way to express the vast distances between Earth and other stars in our galaxy.13