Two blue moons, or not two
blue moons, that is the question!
January 2018
- This page was originally posted in 1999 (notice I mention the 2018 "double
month, double moon"!). I'm leaving the page intact beyond this top part - as
it is still correct. The newer terms are "seasonal blue moon" (what
the purists, such as me, refer to as the blue moon), and the "monthly blue
moon" which is the second moon in a month. Double-month double-moons are
more rare. 1999 was one year, this year is the next time (Jan/March). Can't
have three months in a year with double moons (between 12 and 13 moons a
year, if you have 3 months of double, that's 6 .. even skipping February {a
month that doesn't always get a moon .. such as in 2018} - there are 8
months that must .. that's 14 moons - no year has that.
This is one of the hills I will die on (figuratively/literally is
another) - to me, a true Blue Moon is the seasonal blue moon (third of four
in a "season" {between spring and summer, or between summer and fall,
etc.}). This happens about every 2.7 years (so, not really a rare event, but
it is the original definition!!) - Recent years when this (seasonal blue
moon) has/will happen(ed) : Feb 2000, Nov 2002, Aug 2005, Feb
2008, Nov 2010, Aug 2013, Jun 2016, Feb 2019, Aug 2021, Aug 2024, Feb 2027,
Nov 2029. The double-moon months occur about 2.5 years (slightly more
frequently). The double-moon-double-months are more rare - in 100 years
between 1930 and 2030, happened only 3 times in : 1961 (Jan/Apr), 1999
(Jan/Mar), and 2018 (Jan/Mar) (haven't done the calculations for after
2030).
Some stats for years 1930-2030:
|
1249 |
=
Number of full moons |
| 41 |
= number of double moon
months |
| 37 |
= Number of seasonal blue
moons |
| 3 |
= Number of double-moon
double months |
***** Notice - the Seasonal Blue Moon refers to the
lunar cycles and the seasons [that is, celestial
demarcations] ... the double-moon or double-month ones rely on
artificial (human) constructions of months and years. *****
So, what's the big deal about Jan 31st, 2018?
Well, this double-moon happens to show up near the perigee for the moon -
close-approach distance to the earth (a little over a day away from the
closest). *Some* call this a "super moon" (whereas I prefer "perigee-syzygy"
moon which refers to the full moon lineup with the perigee). Apparently last
time this happened was 150 years ago - haven't checked on this [need to
refine my way of doing some of these calculations].
Here are two current links that might be of interest:
https://www.timeanddate.com/astronomy/moon/blue-moon.html
http://earthsky.org/astronomy-essentials/when-is-the-next-blue-moon
So, the Jan 31st 2018 semantic hill upon which I may figuratively die
... this isn't a "supermoon" .. it is a double-moon month, but this won't be
a blue moon, and Detroit will only see a partial eclipse and the moon will
set before the end of the eclipse. Other than that - enjoy!!
What
is the big deal about "blue moons"?
In March of 1999, the public was caught up
in a mild "blue moon fever" - there were several articles in
newspapers and astronomy magazines talking about the definition of "blue
moon" - but the story is a complicated one. The publicly used
definition is that if there are two months in a year each with 2 moons in
it, then the last of those moons is called the "blue moon" (this
happened in Jan and March of 1999 - thus the last moon in March was
labeled a "blue moon". But, after some careful research, it was
revealed that the current "definition" was created from a
misunderstanding of the original definition! Originally, it was defined as
being the third moon in a four moon lunar season (most years have
12 moons, each with a "name", and most lunar seasons have 3
moons and so the extra one didn't have a proper name .. thus it was called
a blue moon. There is a weird and
fascinating story surrounding the naming of the moons, and this page will
try to fill you in on the details.
First,
the "two moons months in a year" issue! The
two moons in a month happening twice in a year is still a rare occurance -
last time was 1961, next time is 2018 .. and 1999 was the last time it
will happen in the 20th century (regardless of how you decide the start of
the new millenium!). See, the period of the moon's orbit is about 29.5
days .. and the year is 365.25 days .. that puts 12.4 moons per year. So,
some years have 12 and some have 13 moons (so far, so good). Now, since
the period of the moon is greater than the days in February, you could
occasionally have two moons in January or March, and none in February.
Even more rarely, you could have two in January, none in February, and
then two in March (that's what happened in 1999).
What
are the other blue moon definitions?
There are currently about 7 usages of the phrase "blue moon" :
- absurd (oldest usage) - "He would argue that the Moon was blue."
- to mean "never" - "I'll marry you, m'Lady, when the
Moon is blue."
- actually appears blue - When Kracatoa in 1883 erupted, the moon
appeared blue due to the dust in the sky - certain fires have created
similar circumstances
- to mean "infrequent" - since the Moon occasionally, but
rarely, actually appears to be blue .. "Once in a blue moon ..."
- symbol of sadness/loneliness - such as the songs, including
everyone's favorite : "blue Moon" (don't get me started!)
- a drink - made with curacao, gin and a lemon twist
- current folklore to refer to the second moon in a month
How
did that last definition come into being?
Well, sit right back and you'll hear a tale, a tale of a fateful slip!
Much of this information comes from the
March
99 and
May
99 issues of Sky and Telescope
(it is fitting to come from this magazine as you will soon see!). [ I've
done a few calculations using some astronomy software that I have, and
wrote a program to do the 28
year calendar thing {wait, you haven't gotten that far ..
nevermind}.] Here is the timeline for that last (and strongly held)
interpretation :
- Maine Farmer's Almanac - 1937 From 1932-1957, the Maine
Farmer's Almanac listed various calendar events, including dates for
blue Moons. Now, there were different names for the various moons that
occured during the year (normally 12 of them), and they were related to
the ecclesiastical view of the seasons (as set down in the Gregorian
calendar reform of 1582). Their view of the seasons were that the first
day of Spring was March 21st, regardless of where the Earth was relative
to the Sun. (Currently we have an "astronomical" definition
which is related to when the tilted plane of the Earth's equator lines
up with the plane of the Solar system .. which wanders around the 21st
of March {within a day}.) Lent begins on Ash Wednesday which is 46 days
before Easter, and Lent MUST contain the Lenten Moon (last one of
Winter). The first full moon of the Spring (called the Egg or Easter or
Pashal moon) MUST fall within the week before Easter. They named the
moons by the events that are near to them .. in order, the moons are
named as follows : Spring (Egg, Milk, Flower), Summer (Hay, Grain,
Fruit), Fall (Harvest, Hunter's, Moon before Yule), and Winter (Moon
after Yule, Wolf, Lenten). That gives 12 moon names. But occasionally
there are 13 moons in a given year .. why? Well, days in a year = 365.25
{the 0.25 is for the leap day every 4 years} and the period of the Full
Moon is about 29.5306 days [this is the synodic month - related to it's
opposition to the sun]. Thus 365.25/29.5306 = 12.368 moons per year. In
about 19 years, we have 7 moons more than just 12 per year - a
thirteenth moon! (take the .368 and multiply it by 19 to get 7 extra
moons in that 19 year period) .. since the period for the full moons is
less than most of the month [except for poor February], then there has
to be two moons in a month occasionally (but never in February - it gets
cheated ... or does it?). To have two months like that in a given year
is relatively rare, as stated before. Ok, so where are we .. the Maine
Farmer's Almanac had this system to name the moons according to the
times of year that they come .. along comes this extra moon - that would
throw off the names (and since it doesn't come at regular intervals, it
can't have a permanent location in the sequence). The "unlucky
thirteen" myth might have roots here - the thirteenth moon
in a year tried to upset the naming scheme. Ok, the Almanac called this
extra moon a "blue Moon", and for example, they listed the
August 21st, 1937 moon as being a blue Moon. Also note that the blue
Moon was the THIRD one in a season (for example, from Dec 21st to March
21 - that was the Winter season .. the moon before March 21st would HAVE
TO BE the Lenten Moon {by their definitions}, so if that period had 4
moons, the first would be the Moon after Yule, the second was the Wolf
moon, leaving the last to be the Lenton .. it's the third that is
unnamed - they called it the blue Moon (no information exists as to why
the word blue). By the way, the editor for this almanac was Henry
Trefethan (this will be interesting later!)
- 1943 Sky and Telescope mentions the blue moon in a Q&A
column written by L. Lafleur (referring to the 1937 almanac). Lafleur
obviously had a copy of the Almanac in front of him because he cited
passages from it. He notes that 13 moons ocassionally occur in a year,
but doesn't mention if this is a "seasonal year" (as outlined
above) or not, and says nothing about 2 moons in a month (nor the
dreaded "blue" word).
- March 1946 - Sky and Telescope has an article by James Pruett
that mentions both the 1937 Almanac and the 1943 article. Pruett makes
the unfortunate statement : " .. this gives 11 months with 1 full
moon each, and one with two. This second in a month, so I interpret, was
called the blue Moon." Remember, he is refering to the 1937
Almanac, but he probably did not have one in front of him because it
listed the blue moon as being on Aug 21st! Why is that significant?
Remember the reason for having two in a month .. 29.5 days between
moons, and the months are usually (poor February) longer than that. But
that means that for two moons to fit in a month, they have to come near
the very beginning and very end (no more than 1.5 days either way) of
the month - August 21st could not possibly have been the second moon of
August!! By my calculations, the moons of that season were : Jun
23rd(Hay), July 23rd(Grain), Aug 21st(BLUE!), and Sept 20th (Fruit).
Thus, the August 21st moon is listed as BLUE, but cannot be the second
one of the Month! So, Pruett mistakenly called the "second moon in
the month a blue moon" - this is only the beginning of the
troubles!
- May 1950 - Sky and Telescope uses that mistaken definition in
an article. In a weird twist, the data for the article came from non
other than Henry Trefathen - the original editor of the 1937 Almanac! To
his credit, Trefethan never used the term blue Moon for the second one
in a month, but the article using HIS data repeated the interpretation
of the 1946 article that misinterpreted HIS 1937 Almanac!
- Popular usage : Whew! How did the term get further into
popular culture? A 1980 NPR program by Deborah Byrd used the information
from the 1946 article (you can see her "take" on this whole
mess here, but that seems to be
repeating the 'twice in a month' idea), and the information was also
picked up by Kids' World Almanac for 1985, and was even included in a
1986 edition of Trivial Pursuit!
Was
1999 a "blue moon year"? If we
go with the original (and so far only verifiable) use of the term blue
Moon, then what happened in 1999 was rare (never again in 20th century,
not until 2018), but was not a blue Moon event.
Now,
which definition do we choose? The
Maine algorithm seems to be the original usage, and it appears we can
trace the misinterpretation, but as the Sky and Telescope people point out
.. "With two decades of popular usage behind it, the
second-full-Moon- in-a-month (mis)interpretation is like a genie that
can't be forced back into its bottle." Maybe we should let the
controversy rage .. it gives me something to put on my website!
Will
there be a "true" blue moon this year, in 2000?
The good news is that this February .. the month that sometimes get
skipped will have a blue Moon!! February 19th will have a moon that is the
third in the current lunar season .. thus it doesn't have a "traditional"
name, and so it is a blue Moon!
Can
we look at the origin of this Feb 2000 blue moon in more detail?
Sure, we can do anything in Physics! First, what is the best resource for
some of this "lunar season" stuff .. the
Farmer's Almanac! (I played around
with writing some programs to do the same calculations, but they are not
ready for "prime-time" yet.) The starting dates for the
ecclesiastical seasons of 1999-2000 are defined as follows :
- Fall 1999 - Sept 23rd
- Winter 1999 - Dec 22nd
- Spring 2000 - Mar 20th 2:35 AM EST
- Summer 2000 - Jun 20th
- Fall 2000 - Sept 22nd
- Winter 2000 - Dec 21st
What do we do with that information? Remember, these are the "lunar
seasons" that define when we get a "true" blue moon. Since
the Winter season started on the 22nd of December, 1999 and ends on March
20th of 2000, that defines the winter season where a blue moon will occur
... there must be 4 moons, and the third one is the blue one! When are the
full moons? Dec 22nd 1999 (Moon after Yule), Jan 21st, 2000 (Wolf), Feb
19th (BLUE!), and March 20th (Lenten) (only
a few hours before the end of the season - whew!) [Some technical
information .. this is the end of the 954th Lunation .. just thought you'd
like to know! .. Hmm, I wonder what that means .. at about 28.5 days per
moon cycle .. that makes it about 75 years ago when "Lunation 0"
would be ... Why then? ...And that means that Lunation 1000 will happen in
about 3 years ... Uh oh .. a L1K issue! I'll have to look into this!]
Resources
for moon stuff? There have been some
links above that you can follow to look at this particular issue. Also, I
highly recommend the
Inconstant Moon
website for moon-specific information (and some great background
music!). When you see this page, you might have some music playing .. if I
can get the "clearance" to use it .. it will be Moonlight
Sonatta. [Every time I hear that, I flash back to the last episode of the
Northern Exposure first season!] This information will also be up on the
Astronomy Wall on the second floor of the Science building (after this
friday) if you want to take a peek.
When
will there be other BLUE moons, or "double-moon double-month years"? Funny
you should mention that .. you can see exhaustive detail on the Second
floor of the Science Building. Short answer : "double-moon
double-month" - not until 2018, but for the blue moon .. we need only
wait until November of 2002! (I think .. those ecclesiastical seasons are
a bear to calculate - I'll have to double check that!)
28 year cycle? The cycles mentioned
above result from the difference between the lunar cycles and the
ecclesiastical lunar seasons, and the tropical year. Other calendar cycle
information can be found at the year 28 page on
my website!
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