What two key observational inputs are required to apply Newton's refined Kepler's Third Law?

Orbital period (P) and semi-major axis (a)

To transform the equation derived from Newton's version of Kepler's Third Law ($P^2 = rac{4 ext{pi}^2}{G(M_1+M_2)} a^3$) into a solvable tool for finding the sum of the masses ($M_1 + M_2$), astronomers must first secure two fundamental observational parameters describing the orbit. The first is the orbital period ($P$), which involves tracking the stars' positions over complete cycles, a measurement that can range from days to decades. The second essential input is the semi-major axis ($a$), which represents the true average separation between the two orbiting bodies. While determining the period is relatively straightforward, measuring the true separation ($a$) is complicated because observations only show a two-dimensional projection; deriving the true spatial separation requires knowing the system's distance, often found via parallax.