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Blue Moon in July

Blue Moon in July

Carter Observatory
The National Observatory of New Zealand

July is a Blue Moon month as it has two Full Moons, the first on July 1 at 01 49 am and the second on July 30 at 12 48 pm . This phenomenon occurs in New Zealand in July but because of the various time zones, it happened in May and June in other countries.

Blue Moons happen every 2½ to 3 years, hence the modern meaning of the phrase “Once in a Blue Moon”. However we understand that the expression “Once in a Blue Moon” meaning “not very often” was in use long before the two Full Moons in a month explanation.

As far as we can tell, the Blue Moon definition comes from the Maine Farmers’ Almanac, which started publication towards the beginning of last century. This almanac’s calendar ran from one Winter (Northern hemisphere) Solstice to the next. Each year was divided into four fixed seasons of equal length. Most years contain 12 full Moons - three each in Winter, Spring, Summer, and Autumn - and each was named for an activity appropriate to the time of year (such as the Harvest Moon and specific Moons associated with Easter and Christmas). But occasionally the year contained 13 Full Moons, so one of the seasons had an extra Full Moon. The third Full Moon in the season was called a Blue Moon. The third one received this honour, because only then did the names of the other Full Moons fall at the proper times relative to the Solstices and Equinoxes.

Now comes the mistake. An article, by James Hugh Pruett, entitled “Once in a Blue Moon” appeared in the March 1946 edition of Sky & Telescope. Pruett was an amateur astronomer and frequently wrote articles on a variety of topics, especially fireballs and meteors for Sky & Telescope. The article mentions, as a basis, the Maine Farmers’ Almanac, but unfortunately, in the article he says:
“Seven times in 19 years there were - and still are – 13 full moons in a year. 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”.

Pruett must not have had the almanac handy, or he would have noticed that the almanac’s Blue Moon fell on the 21st of the month (obviously not the second full Moon that month). This new erroneous definition of Blue Moon crept into folklore, presumably because Sky & Telescope was more widely read and authoritatively accepted than the Maine Farmers’ Almanac.

Another accepted meaning of the term Blue Moon is when certain extreme atmospheric conditions cause the Moon to take on a bluish colour. Dust in the atmosphere usually turns the Moon a reddish colour.

Webster’s Dictionary does not mention the two Full Moon explanation. The entry says, “the rare blue appearance of the moon that is due to dust particles in the high atmosphere”. Blue Moon is not even mentioned in the Oxford English Dictionary or the Encyclopædia Britannica.

Wikipedia agrees with the explanations above.

Just to clarify this, 12 48pm is 48 minutes past noon.

For more information about this subject, or any other astronomical issue contact
Brian Carter, Senior Astronomer
Phone: Direct Dial (04) 494 8321, Carter Observatory (04) 472 8167
or Fax (04) 472 8320

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