# What formula would be used to calculate the aphelion distance of Halley's Comet from the sun? Halley’s comet has a perihelion distance of 0.6 AU & an orbital period of 76 years,

Kepler's third law relates the orbit period T in years to the semi-major axis distance a in AU using the equation ${T}^{2} = {a}^{3}$. If $T = 76$ then $a = 17.94$.
Given that the comet's orbit is an ellipse then the sum of the perihelion distance and the aphelion distance is twice the semi-major axis ${d}_{a} + {d}_{p} = 2 a$ or ${d}_{a} = 2 a - {d}_{p}$. We have ${d}_{p} = 0.6$ and $a = 17.94$ then ${d}_{a} = 2 \cdot 17.94 - 0.6 = 35.28 A U$.
A direct equation relating the three values would be: ${d}_{a} = 2 \cdot {T}^{\frac{2}{3}} - {d}_{p}$