Schematic of the Milky Way divided by a set of rings centered at the Galactic center. The two thick arrows originating from the Sun (shown as a circle) correspond to two lines-of-sight, one looking toward the inner Galaxy and the other toward the outer Galaxy. The dashed lines represent the distance to the Galactic center (R_⊙) and the distance to the tangent point (R_tan) for the line of sight that goes towards the inner Galaxy. The thin lines point to the different path lengths crossed by the lines-of-sight that correspond to the different cases considered in the text.