Schematic of the configuration between the Sun (yellow), the comet (black), and a water molecule to determine the column density at a given (r, χ) in the coma. s is the abscissa along the line between the molecule and the Sun. s₀ is the lower bound of integration (s₀ < 0 and χ > π∕2 in the figure as s increases sunward). r_s is the cometocentric distance for a given s, which is .