What Is a Blue Moon?

Get answers to:

  • How long does it take the moon to orbit around the Earth?
  • Where does the expression Blue Moon come from?
  • Why are there two different meanings for Blue Moon?

There are two meanings for Blue Moon. The older definition, which seems to have been forgotten, referred to the third full moon in a season when there were four full moons in that season. The newer definition refers to the second full moon in the same calendar month.

The time between one full moon and another is about 29-1/2 days1 and is called a Synodic Month.2

Seasons are measured between the solstice and equinox – approximately 91 days long.3

If the first full moon occurs near the start of the season, the second full moon occurs about 29.5 days into the month, the third full moon occurs about 59 days into the season, and a fourth full moon will occur about 87.5 days into the season. It was that third full moon that was, traditionally, called Blue Moon.

In March 1946, the American magazine Sky & Telescope printed an article by amateur astronomer James Hugh Pruett (1886-1955) titled Once in a Blue Moon in which he wrote:

This gives 11 months with one full moon each and one with two. This second in a month, so I interpret it, was called Blue Moon.

For some reason, his interpretation of Blue Moon stuck and most people today understand it to mean the second full moon in a calendar month.

The earliest reference to a Blue Moon occurs in an anti-clerical document written in 1528 titled Rede me and be nott wrothe, for I ſay no thynge but trothe4:

Yf they ſay the mone is blewe
We must beleve that it is true

Originally, it simply meant “nonsense”, but eventually shifted its meaning to being something rare, uncommon or unusual.

Sometimes the moon can appear to be bluish This is a rare event and occurs when very fine particles (about 0.7μm in size) are in the atmosphere. They scatter the light and can make the moon appear bluish in colour. The most common causes are forest fires and volcanic eruptions.

  1. The moon takes 27 days, 7 hours, 43 minutes and 11.6 seconds to complete one orbit around the Earth. However, because the Earth moves around the sun in an elliptical path, the angles between the Earth, Moon, and Sun keep changing. The actual time between phases of the moon varies between 29.18 to 29.93 days, but the average is 29.53 days.↩
  2. Synodic comes from the Greek word sunodos which means meeting.↩
  3. Because the Earth’s orbit is elliptical, the Fall (~89.8 days) and Winter (~89 days) seasons are a little shorter than the Spring (~92.8 days) and Summer (~93.6 days) seasons.↩
  4. Older English can be hard to read. The title of the book (in modern English) is Read me and don’t be angry, for I say nothing but the truth.The rhyme says: If they say the moon is blue / We must believe that it is true. The reference can be found here on line 16.↩