- These maps are intended to show the
levels of pollution in the atmosphere rather than the stellar
visibility, but the colour code can give an estimate:
Red: approximately one hundredth of stars, or few more, is visible over 30 degrees of elevation
Orange: the milky way is invisible or quite difficult to see by an average observer in normal clear nights
Yellow: artificial sky brightness equal to the natural so that the total sky brightness is doubled
Blue: artificial sky brightness over 10% than the natural brightness which is the definition of "light polluted sky"
- The maps show the artificial sky brightness at the zenith in clear nights in V band.
One of the difficulties in implementing MARPOL arises from the very international nature of maritime shipping. The country that the ship visits can conduct its own examination to verify a ship's compliance with international standards and can detain the ship if it finds significant noncompliance . When incidents occur outside such country's jurisdiction or jurisdiction cannot be determined, the country refers cases to flag states, in accordance with MARPOL. A 2000 US GAO report documented that even when referrals have been made, the response rate from flag states has been poor.