• The forest fires in British Columbia burned on Sept. 23, 1950. A day later, the smoke that had reached Eastern Canada was thick enough to completely block out the sun. The following day, the sun was blue. By Sept. 26, the smoke had crossed the Atlantic.

• Volcanic eruptions and forest fires are known to cause a blue-looking sun or moon. The particles in the dust or smoke scatter the red end of the spectrum, leaving an excess of blue.

• After the eruption of Krakatoa, an island volcano in Indonesia, in 1883, blue moons and suns were widely observed.

• Astronomers have two definitions for the term "blue moon." One definition refers to when two full moons occur in one month. An older definition of blue moon is the third full moon of four occurring in one season. The 500-year-old expression "once in a blue moon" is thought to predate the calendar meanings of the term.