The column mass (ie, the spatial density multiplied by the length of the observer’s line of sight through the material) is highest at the limbs when looking at

The column mass (ie, the spatial density multiplied by the length of the observer’s line of sight through the material) is highest at the limbs when looking at a gaseous shell. So shells like this look like rings from all angles. A related effect called limb darkening is the edges of the sun always look less bright, independent of viewing angle