PSBL geometry, where blue is the binary axis and orange is the source moving by with relative proper motion μ_rel. We are transforming from L to ﹩{L}^{{\prime} }﹩, where ﹩{L}^{{\prime} }﹩ is farther along in the direction of ﹩\hat{s}﹩ by d. The source’s point of closest approach to L is S at a distance u₀ at time t₀. The source’s point of closest approach to ﹩{L}^{{\prime} }﹩ is ﹩{S}^{{\prime} }﹩ at a distance ﹩{u}_{0}^{{\prime} }﹩ at time ﹩{t}_{0}^{{\prime} }﹩.