= Transit radius
{title2=$R_{\mathrm{tr}}$}
The <transit radius> is the effective radius inferred from the fraction of stellar <radiative flux> blocked during an <exoplanet transit>. For a uniformly bright <star>, a completely overlapping opaque planet gives transit depth $\delta=(R_{\mathrm{tr}}/R_\star)^2$. Thus a one-percent depth corresponds to a radius ratio of about one tenth.
Real inference accounts for <limb darkening>, transit geometry and contaminating <light>. A planetary atmosphere can make the effective occulting area depend on <wavelength>: a grazing ray traverses a long slant path, so absorption need not correspond to one sharply defined material surface. A quoted transit radius therefore belongs to a measurement band and a stated model.<Analytic Light Curves for Planetary Transit Searches by Kaisey Mandel and Eric Agol>[Sections 2, 4 and 5, geometry, limb darkening and small-planet approximation]{ref}
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