1. BLUE MOON
March 17, 2013 Tofique Fatehi
tofiquef@yahoo.com
http://tofique.fatehi.us
BLUE MOON
The time period between one full moon and the next (lunation) is approximately 29.53 days. A
Gregorian year has approximately 365.25 days. So, in a year there will be 365.25/ 29.53 =
12.369 lunations. Therefore, there will be at least 12 full moons in a year, usually, one full moon
per month.
The month of February has 28 or 29 days, which is a fraction less than a lunation. So, a situation
can arise that the full month of February fits entirely within a lunation, and that there is no full
moon in February. To compensate for this, any one of the other months in the year may have
two full moons.
The first of these full moons will occur in the beginning of that month, on the 1st or 2nd of the
month and the second one will occur a lunation later, that is in 29.53 days, but before the month
gets over, that is on the 30th or 31st of the month.
A second full moon in the same month is called a “blue moon”.
With 12.369 lunations in a year, at the end of each year, a fraction of a lunation, 0.369, is left
over, which will be carried forward and added to the next year. When all these fractions add up
to one or more, there will be 13 full moons in that year!
This would imply that there is at least one month in that year which has two full moons in it, and
that all other months have one full moon each. In other words, this will be a blue moon year.
This is a fairly rare phenomenon, occurring, on the average, once in two or three years. This is
why any rare occurrence is spoken of as “once in a blue moon”. It has nothing to do with the
color of the moon.
An even rarer phenomenon can occur with two blue moons in a single year! At first sight, this
may seem to imply that there will be 14 full moons in such a year. However, this is not so. Since
the number of a lunations in a year is 12.369 which is a fraction more than 12 and a fraction less
than 13, there can only be either 12 or 13 full moons in a year. Hence, if there are to be 2 blue
moons in a year, there has to be one month in that year without a full moon. Again, only
February can be that month.
Blue Moon
© 2013 Tofique Fatehi
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2. SUMMARY
 There can be either 12 or 13 full moons in a year.
 If there are two full moons in the same month, the second full moon is termed as a “blue
moon”.
 If there are 12 full moons in a year, then either
o each of the 12 months has one full moon, or
o February has no full moon and one of the other 11 months has a blue moon. All
other months have a full moon each.
 If there are 13 full moons in a year, then either
o Any one month (except February) in the year has a blue moon, and the other 11
months (including February) have one full moon each, or
o February has no full moon and two of the other 11 months have a blue moon. (It
may be mentioned that the first blue moon is always in January, and the second
blue moon is in March or April.)
See the table of blue moons below.
Blue Moon
© 2013 Tofique Fatehi
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3. TABLE OF BLUE MOONS
Here is a table listing all the blue moons in this century, from 2001 to 2099. There are only 3
years with two blue moons in them. They are marked in blue.
Date Time Date Time
Friday, November 30, 2001 20:51:10 Wednesday, July 30, 2053 17:08:32
Saturday, July 31, 2004 18:06:05 Friday, March 31, 2056 10:26:26
Saturday, June 30, 2007 13:50:25 Thursday, October 31, 2058 12:54:58
Thursday, December 31, 2009 19:14:20 Tuesday, August 30, 2061 22:20:53
Friday, August 31, 2012 13:57:45 Friday, May 30, 2064 10:37:03
Friday, July 31, 2015 10:45:46 Friday, December 31, 2066 14:42:26
Wednesday, January 31, 2018 13:27:46 Wednesday, March 30, 2067 20:11:05
Saturday, March 31, 2018 12:37:46 Monday, September 30, 2069 18:12:11
Saturday, October 31, 2020 14:51:29 Tuesday, May 31, 2072 22:19:43
Thursday, August 31, 2023 1:37:05 Tuesday, April 30, 2075 18:40:43
Sunday, May 31, 2026 8:46:46 Sunday, October 31, 2077 10:39:00
Sunday, December 31, 2028 16:49:45 Wednesday, July 31, 2080 19:16:42
Tuesday, September 30, 2031 18:58:02 Monday, May 31, 2083 9:43:39
Monday, July 31, 2034 5:56:35 Monday, December 31, 2085 0:00:58
Saturday, January 31, 2037 14:05:42 Thursday, September 30, 2088 15:28:47
Tuesday, March 31, 2037 9:54:43 Monday, July 30, 2091 12:03:20
Monday, October 31, 2039 22:36:54 Sunday, January 31, 2094 12:40:11
Sunday, August 31, 2042 2:05:14 Friday, April 30, 2094 13:58:47
Tuesday, May 30, 2045 17:53:13 Wednesday, October 31, 2096 11:20:22
Friday, January 31, 2048 0:16:32 Sunday, August 30, 2099 17:59:15
Friday, September 30, 2050 17:32:47
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Blue Moon
© 2013 Tofique Fatehi
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