Yermette Lunar Calendar from Solar Calendar

Sometime ago (before I'd heard of the Ogam wheel), I thought of a way of make a solar calendar lunisolar.

It works with any solar calendar that has its leap day after the same date each leap year. I'll call this same date the pre-leap day. In the Gregorian calendar the pre-leap day is February 28.

Lunisolar months are defined as follows:

  1. Any month that contains the pre-leap day
  2. All other months of the year follow a 9-month cycle called a yermette.
  3. There is a yermette that begins 1,3,4,6,7,9,10,12 or 13 non-leap days after a pre-leap day.

Rule 3 ensures that rules 1 and 2 can be applied consistently. This would not be possible if an odd-numbered month of a yermette were to begin 29 days before the pre-leap day. Rule 3 prevents this from happening.

The months as defined will follow the 19-year cycle of 235 months.

This is best seen by dividing the solar year into 26 fortnights of 14 days followed by the pre-leap day and the leap day when it exists. In the Gregorian calendar, the first fortnight would begin on March 1. Each yermette will begin on the same day of the fortnight.

If the first yermette begins on the non-leap day after the pre-leap day (A01 e.g. March 1), the months begin on the following days of the fortnight calendar (letter is fortnight, number is day and ; marks end of yermette). Each counted month of the yermette begins on the same unique day of the fortnight (01,03,04, 06,07,09, 10,12,13). 

year 01:  A01 C03 E04 G06 I07 K09  M10 O12 Q13;T01 V03 X04 Z06
year 02:  B06 D07 F09 H10 J12 L13; O01 Q03 S04 U06 W07 Y09
year 03:  A09 C10 E12 G13;J01 L03  N04 P06 R07 T09 V10 X12 Z13
year 04:  B13;E01 G03 I04 K06 M07  O09 Q10 S12 U13;X01 Z03
year 05:  B03 D04 F06 H07 J09 L10  N12 P13;S01 U03 W04 Y06
year 06:  A06 C07 E09 G10 I12 K13; N01 P03 R04 T06 V07 X09 Z10
year 07:  B10 D12 F13;I01 K03 M04  O06 Q07 S09 U10 W12 Y13
year 08:  A13;D01 F03 H04 J06 L07  N09 P10 R12 T13;W01 Y03
year 09:  A03 C04 E06 G07 I09 K10  M12 O13;R01 T03 V04 X06 Z07
year 10:  B07 D09 F10 H12 J13;M01  O03 Q04 S06 U07 W09 Y10
year 11:  A10 C12 E13;H01 J03 L04  N06 P07 R09 T10 V12 X13 pld;
year 12:  C01 E03 G04 I06 K07 M09  O10 Q12 S13;V01 X03 Z04
year 13:  B04 D06 F07 H09 J10 L12  N13;Q01 S03 U04 W06 Y07
year 14:  A07 C09 E10 G12 I13;L01  N03 P04 R06 T07 V09 X10 Z12
year 15:  B12 D13;G01 I03 K04 M06  O07 Q09 S10 U12 W13;Z01
year 16:  B01 D03 F04 H06 J07 L09  N10 P12 R13;U01 W03 Y04
year 17:  A04 C06 E07 G09 I10 K12  M13;P01 R03 T04 V06 X07 Z09
year 18:  B09 D10 F12 H13;K01 M03  O04 Q06 S07 U09 W10 Y12
year 19:  A12 C13;F01 H03 J04 L06  N07 P09 R10 T12 V13 Y01;

Use with Gregorian Calendar

For the Gregorian calendar one can define a Yermette to begin on 1 March 1900 (A1), so the above 19-year cycle begins on 1 March 1900 or 1 March 1995. The fortnights would be defined as follows: 

A: Mar 01 - Mar 14    B: Mar 15 - Mar 28
C: Mar 29 - Apr 11    D: Apr 12 - Apr 25
E: Apr 26 - May 09    F: May 10 - May 23
G: May 24 - Jun 06    H: Jun 07 - Jun 20
I: Jun 21 - Jul 04    J: Jul 05 - Jul 18
K: Jul 19 - Aug 01    L: Aug 02 - Aug 15
M: Aug 16 - Aug 29    N: Aug 30 - Sep 12
O: Sep 13 - Sep 26    P: Sep 27 - Oct 10
Q: Oct 11 - Oct 24    R: Oct 25 - Nov 07
S: Nov 08 - Nov 21    T: Nov 22 - Dec 05
U: Dec 06 - Dec 19    V: Dec 20 - Jan 02
W: Jan 03 - Jan 16    X: Jan 17 - Jan 30
Y: Jan 31 - Feb 13    Z: Feb 14 - Feb 27
pld: Feb 28           leap-day: Feb 29.

For example, 30 September 2003 is P4 year 09. It is in the 8th lunar month, which starts on O13, so is the 6th day of the 8th lunar month of the year. This lunar month is the last month of a yermette.

Comparison with Ogam Wheel

Also, a direct comparison with the Ogam wheel is possible, if the Tree Calendar leap day is a second day between Elder and Birch. Then the fortnights are half tree-months and the year numbers shown are then Moyer golden numbers. The tree calendar used here would sometimes differ a day from the one used for the Ogam wheel, because of difference tree-calendar leap days.

Karl Palmen 5 September 2003