Blue Moons

Have you ever heard someone say "once in a blue moon"? A blue moon is not actually blue. It is the name for an extra full moon. Usually, there are three full moons in each season. When there are four, the extra one is called a blue moon. They do not happen very often.

Two Kinds of Blue Moons

There are two ways to define a blue moon. The older meaning is the third full moon in a season that has four full moons. The newer meaning is the second full moon in a single calendar month. Both types are uncommon. That is why people say "once in a blue moon" to mean something rare.

Can the Moon Really Look Blue

The Moon can actually look blue in very rare cases. This happens when there are tiny particles in the air from a big volcanic eruption or forest fire. These particles scatter red light and let blue light through. But this has nothing to do with what we call a blue moon. It is just a fun coincidence.

Fun Facts

• Blue moons happen about once every two to three years.
• The saying "once in a blue moon" has been used since the 1500s.
• In very rare years, there can be two blue moons in the same year.

Did You Know?

After the eruption of the volcano Krakatoa in 1883, the Moon actually looked blue for about two years because of all the dust in the air.