Let 'P' be the planet. It is observed from two different positions A and B on the earth at the same time. A distant star S is in the direction of the planet.
Measure ∠SAP and ∠S'BP
Let ∠SAP = θ1, and ∠S'BP = θ2
∠APB=θ1+ θ2 =0 (say).
Let d be the distance between A and B,
i.e., AB = d
we have θ =Arc/Radius = d/D
or d/θ
D is the distance between then earth and the planet