Halos are optical phenomena that appear in the sky near or around the Sun or Moon, for example "sun-dogs" or the parahelia, or a paraselene or "mock-moon." There are many other types of halos, but they are all caused by ice crystals high in the atmosphere. The particular shape and orientation of the crystals is responsible for the type of halo observed. Optical phenomena such as halos were used as an empirical means of weather forecasting before meteorology was developed.
Halos can also have unusual shapes, for example a cross. Emperor Constantine I of the Roman Empire is said to have seen such a halo in 313 near Trier. This sign is supposed to have prompted him to become a Christian.