Transit zone angle geometry. The yellow and blue circles represent the Sun and planet, respectively (sizes not drawn to scale). R_⊙ is the solar radius and R_p is the planet’s radius. The average distance between the planet and the Sun is r (1 au for Earth), and ϕ_TZ is the full transit zone angle. The transit zone is a projection of ϕ_TZ onto the sky. Any theoretical sensor in this strip of sky could detect the planet’s existence via the transit method. Using similarity reasoning, we calculate ϕ_TZ =.