Example of the line core fit of the line Fe Iλ_lab 5379.5740 Å described in Sect. 3. Left panel: normalised flux of a HARPS-LFC spectrum (black thin line) of the Moon, labelled s429 in Table 1, centred on λ_lab = 5379.5740 Å together with a single Gaussian fit (red dashed line) in a region of 0.5 Å (vertical red dashed lines) centred on the laboratory wavelength (vertical green dashed-dotted line). Right panel: parabolic fit (blue solid line) to five points of the line core compared to the single Gaussian fit (red dashed line). Vertical lines show the wavelength position of the centroids of the parabolic fit (blue dashed-three dotted line) and the Gaussian fit (red dashed line), together with the laboratory wavelength (vertical green dashed-dotted line).