Why is it called a Blue Moon

Blue Moon –

Designation of the second full moon in a calendar month
The “Blue Moon” translated as “Blue Moon” in German is a rare lunar event that occurs on average every two and a half years. It marks the second full moon in a calendar month.

In fact, the word translated from English does not exist in German usage. The name comes from the English expression “Once in a Blue Moon”, which translates as: “once in each jubilee year”.

However, the name has nothing to do with the blue glow of the moon.

This post is about the origin of the name. Part of this comes from an old Wikipedia article of the same name.

The term was first used in the English-speaking world around 1528 and refers to a rare occurrence there.

The association with the Moon was probably invented in the early 19th century and it was there that it was first associated with the Moon.

Due to the period of revolution of the Moon around the Earth, there can be four full moons instead of three in one season. In the “Main Farmer’s Almanac“, the third full moon of such a season was previously referred to as the “Blue Moon“.

After the loss of the “blue moon” concept in the early 20th century, it was first mentioned again in March 1946 in an article in “Sky & Telescope” magazine.

The amateur astronomer James Hugh Pruett (1886–1955) wrote an essay there entitled “Once in a Blue Moon”. There he looked after the “Main Farmer’s Almanac”.

Its misinterpretation resulted in the connection of the second full moon in a calendar month.

Despite the name, Purnima is no different. Scientifically speaking, a Blue Moon is not a significant event, but it is rare.

Sometimes you can see the moon twinkling blue during a volcanic eruption or wildfire. This is due to changes in the atmosphere during such rare events. At the core of this phenomenon, only the blue color of the moon is allowed to pass.

A calendar month without a full moon?


In fact, it is possible that a calendar month does not have a full moon. This is possible only in February: February has 28 days, leap years have 29 days.

It takes 29.5 days for the Moon to orbit the Earth. This is the time it takes for the Moon to reflect the same phase of the Moon again. In astronomy, this period is called the “synodic month“.


If the full moon falls on the last or last day of January, the full moon will not return for 29.5 days. Due to the short month of February, the next full moon may not fall till March.

Black Moon – equivalent of Blue Moon


Ultimately, each phase of the moon (the first and last quarter, the new moon) can occur twice in a calendar month.

The term “Black Moon” was coined in German English “Schwarzer Mond” for the appearance of a second new moon within a calendar month.

It takes 29.5 days from a full moon to the second full moon. If it matches exactly, there will be two full moon positions in the same month.

About every two and a half years there are two full moons per month. The second full moon in the English speaking world is called the blue moon. The expression “Once in a Blue Moon” means something like “all the years of the jubilee”.

But this year there are two Blue Moons. There was already a second full moon in January. But February turned out to be completely empty. This happens every 19 years.

Image by Lee_seonghak from Pixabay

The date of the Chinese New Year depends on the course of the Moon (NASA)
A blue moon also appears silvery gray in the sky (NASA)

This definition of a blue moon apparently dates back to an article in the American magazine “Sky and Telescope” in 1946. The author read about the Blue Moon in the traditional farmer’s rent a few years ago.

However, he had misunderstood the explanation. According to the yearbook, a blue moon is the third of four full moons that occur during a season. According to this rule, the next Blue Moon will be on May 18 of the coming year.

It doesn’t matter which version you choose: Blue Moons are quite rare. And they have nothing to do with the color in which the moon shines.

In fact, moonlight can be blue. But it doesn’t depend on the timing, but on dust particles and ice crystals in the air.


Blue moon: definition


The two different definitions of a Blue Moon are:

Seasonal Blue Moon: The third full moon of an astronomical season have four full moons. every seasons have only three full moons.
Monthly blue moon: The second full moon of a month with two full moons. There is only one full moon most months.

Why are there different definitions?


The basic definition of a seasonal blue moon phenomenon is. It is based on the old practice of giving a name to the full moon each month.

The full moon of January was called Wolf Moon, Old Moon, or Ice Moon, depending on the culture; In February, the moon of snow, storm, or hunger stood in the night sky, in March the moon of fasting, worm, or raven — and so on.

However, because the months of the Gregorian calendar do not correspond to the length of a single lunar month, there are 13 full moon years at certain intervals.

In order not to confuse the traditional order of full moon names with the months and seasons of the calendar, the extra full moon was nicknamed the Blue Moon.

The definition of a monthly blue moon was born out of the mistake of amateur astronomer James Hugh Pruett (1886-1955).

Due to a misunderstanding when calculating the data for the Blue Moon season, he published the monthly definition in 1946, which quickly integrated into popular culture and even appeared in an edition of the board game Trivial Pursuit Gave .

In the meantime, this statement has become so irrevocably natural that it is generally considered to have an equivalent second definition.

Why does the moon look bigger when it rises and sets?

What is the rarity of a blue moon?


Compared to other astronomical events — for example, planetary transit during a solar eclipse — a blue moon is not particularly rare. If both definitions are taken into account, the phenomenon can be experienced every two to three years. There are 408 seasonal blue moons and 456 monthly blue moons in the 1100 years between 1500 and 2650.

Ashen Ball: What is Earth Shine?

The moon may be blue
A blue moon is not really blue. In very rare cases, however, the Moon may take on a bluish hue.

When the atmosphere is surrounded by particles of dust or ash of a certain size: the diameter should be slightly larger than 900 nanometers. These particles disperse the red part of the moonlight, thanks to which the blue part comes automatically and the moon appears blue. This process is called My Scattering.

For example, particles needed to disperse mines can be released into the atmosphere by a dust storm, forest fire, or volcanic eruption. After the eruptions of the Krakatau volcano (1883) in Indonesia, El Chichón (1983) in Mexico, Mount St. Helens (1980) in the United States and Pinatubo (1991) in the Philippines, eyewitnesses reported a blue moon.

Blood moon: The moon can also be red

(*) A jubilee year in the Roman Catholic Church is the term of a year during which the punishment for a sin can be completely forgiven. Part of the word Jubilee comes from the old Hebrew word ram, jobel. The Israelites declared a year of celibacy with the shofar, a wind instrument made of ram’s horns.

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