What Is A Blue Moon?

Full moon of March 19, 2011

We all understand that the phrase,
“Once in a blue moon,” refers to a rare event. But what exactly is a Blue Moon?

This Friday July 31 will be a Blue Moon. What does that mean?

Blue Moon was once defined in such a complicated astronomical manner that it was practically impossible to decipher. But the modern meaning for Blue Moon is that it is the second full moon of a calendar month.

Typically calendar months have a single full moon. But July 2015 had a full moon on July 2 and will have a second full moon, called a Blue Moon, in two days on July 31.

Sometimes Blue Moon refers to an “extra moon,” such as the 13th full moon of a calendar year. Also, the third full moon of a season which has four can be called a Blue Moon; the first, second, and fourth (last) are regarded as the regular full moons of the season, and the third one is considered the extra or blue moon.

Today a Blue Moon has nothing to do with the color Blue. Blue moons are usually grayish white in color, just like normal full moons.

However, on rare occasions the moon does look blue. This is likely the origin of the centuries-old expression “once in a blue moon” to mean a rare event. The rare blue color is caused by unusual atmospheric interference.

Map showing Krakatoa in the Sunda Strait
between Java and Sumatra

Krakatoa Turns Moon Blue

The Indonesian volcano Krakatoa blew in 1883 before earthquake magnitudes were measured, but it’s estimated to be of magnitude 8.8 – 9.2, one of the largest ever. Surges were recorded on tide gauges 6,000 miles away in Alaska and 11,000 miles away in the English Channel. The explosion is believed to be the loudest sound ever heard in modern history. People reported hearing it 3,000 miles away.

Following the Krakatoa explosion, the moon looked blue for many days. Clouds of volcanic ash ascended through earth’s atmosphere and acted as a blue filter, turning the moon “blue.”

Many volcanic ash particles are 1 micron (=0.001 mm) across. This is about the wavelength of red light, so red light was blocked and scattered by the Krakatoa ash. The ash clouds acted as a filter by only allowing blue light of much shorter wavelength to get through, thereby making the moon look blue.

Huge forest fires can have the same effect on the appearance of the moon.

Sometimes when the moon is low on the horizon it looks red. Tiny droplets in the atmosphere, much smaller than a micron across, scatter the blue light but let the red through. Red-color moons are much more common than blue-color moons.

Friday’s Blue Moon will hit its peak fullness around 6:42 am EDT Friday morning. So you can get a good view of it either Thursday night or Friday night. Look for it above the eastern horizon just after sunset. During the night, it will cross the southern sky to set on the western horizon at sunrise.

The last Blue Moon was August 31, 2012. The next few Blue Moons will be January 31, 2018, March 31, 2018, and October 31, 2020.

Questions to Ponder

1. Have you ever seen a bluish-colored moon? Do you know what caused it?
2. Why does earth have a moon? Of what benefit is it?
3. How would life be different without the moon?

Share your thoughts on these questions in the comments below. It could encourage or help another reader.

Soli Deo Gloria.

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©William T. Pelletier, Ph.D.
“contending earnestly for the faith”
“destroying speculations against the knowledge of God”
(Jude 1:3; 2 Cor 10:4)
Wednesday July 29, 2015 A.D.

It is the Lord of hosts who made the earth by His power, Who established the world by His wisdom, and by His understanding He stretched out the heavens. (Jeremiah 51:15)