Sketch of the Earth-Comet-Sun plane, with the positions of each of the three comets at the epoch of the respective observations. In addition (at a different scale given by the 10 000 km bar), three regions in the comae are plotted: in the shaded area, the region opaque to Lyman α (i.e. where opacity at 121.6 nm towards the Sun is larger than 1.0); in dashed line, the spherical region whose radius corresponds to the Haser scale length of HCN (assuming a constant mean value for the expansion velocity); within the dotted lines, the region where less than 1−1/e of the HCN molecules are photodissociated (≈ equivalent to its scale length) when the screening is taken into account.