Ogam And Martyrological Wheels

This is a slightly edited copy of a note, that I sent to CALNDR-L on 1 November 2001. I've updated it on 5 November 2003 and on 9 December 2005.

Go to Martyrological Wheel

Ogam Wheel

The Ogam Wheel is a rotary calculating device that calculates an approximate moon phase for a date in a 13-month Celtic Tree Calendar.

It consists 3 concentric wheels that can rotate relative to each other. The outer wheel is divided into 30 sectors and is marked with years of the 19-year Metonic cycle and some dates from the year of Celtic Tree calendar. The next smaller wheel also has 30 sectors. These sectors correspond to phases of the moon and are marked with runes from a 16-rune alphabet forwards for the dark moon to full moon inclusive and backwards for the full moon to dark moon inclusive. The inner wheel acts as a pointer to a sector in the other two wheels.

The outer two wheels are set for a given year by placing the FULL MOON sector of the moon phase wheel by the sector of the year on the outer wheel.

Normally the inner wheel is turned one sector clockwise for each day passed.

Because the mean lunar month is about 29.53 days rather than 30 days a moon phase rune occasionally needs to be skipped. This is known as the sacrifice. The sacrificed rune is always one of the two uruz runes adjacent to the dark moon rune. One sacrifice occurs in each of a particular six tree calendar months. I'll refer to the days of these 6 tree months as the sacrifice periods. An additional uruz rune is sacrificed once every 19 years, to form the 19-year Metonic cycle.

Usually but not always, there is a choice of two uruz runes, one of which must be sacrificed. Charles chooses one so that the dark moon rune occurs (always?) on the dark moon rune day.

I show you the years and dates on the Outer Wheel of the Ogam Wheel. The years are for the current 19-year cycle 1988 to 2006 (Golden #1 to #19) and the dates that are the equivalents of the first and last day of each tree months. More precisely these dates mark the start and end of the tree months, inclusive of any sacrifice. The extra day (Dec 23) is also shown. The number at the start of each row is the position number referred to in other notes.

 1: 2000         Jan 20  Dec 22
 2: 1989  Jan 21         Dec 23
 3:       Dec 24
 4: 1997  
 6: 2005
 7: 1994
 9: 2002
10: 1991
12: 1999 
13: 1988         Nov 24
14:       Nov 25
15: 1996         Oct 27
16:       Oct 28
17: 2004         Sep 29
18: 1993  Sep 30 Sept 1
19:       Sept 2
20: 2001         Aug  4
21: 1990  Aug  5 July 7
22:       July 8 
23: 1998         June 9
24:       Jun 10 May 12
25: 2006  May 13
26: 1995         Apr 14
27:       Apr 15 Mar 17
28: 2003  Mar 18
29: 1992        Feb 17
30:       Feb 18

In a leap year of the tree calendar, the wheel must be turned exactly as many times as in a common year. This can be done by cancelling one of the six sacrifices. The tree calendar leap day then occurs at or close to this cancelled sacrifice. Alternatively, it can be done by simply not turning the Ogam Wheel on a leap day.

Charles Moyer's original notes about the Ogam wheel and pictures are at ogindex.html to which I have added a caveat about leap days and the 19-year cycle.

Also there is

The Martyrological Wheel

The outer wheel of the Martyrological wheel has its 30 sectors corresponding to epacts numbered in anti-clockwise order. Each sector is also given the corresponding martyrological letter. The moon phase wheel has sectors numbered 1 to 30 clockwise. 1 represents the new moon, 14 the full moon and 29 (presumably) the dark moon. These numbers define the age of the moon.

The outer two wheels are set for a given year by placing the NEW MOON sector (value 1)of the moon phase wheel by the sector of the epact/Martyrological letter of the year on the outer wheel. For January 1, the pointer on the inner wheel is pointed to position #0 on the outer wheel.

As with the Ogam wheel there are 6 sacrifices in each year plus an extra sacrifice once every Metonic cycle (at the end/start).

The sacrifice occurs by skipping moon age 30, whenever it would otherwise occur within each of the 6 sacrifice periods. These sacrifice periods are

Feb 5 - Mar 5
Apr 5 - May 3
Jun 3 - Jul 1
Aug 1 - Aug 29
Sep 29 - Oct 27
Nov 27 - Dec 25

when the golden number of the year is no greater than 11, else the sacrifice periods are one day earlier than shown above. This difference only matters in years with epact 25. Note that since each sacrifice period has exactly 29 non-leap days, there is always exactly one occurrence of 30 that is sacrificed.

The position of each date on the outer wheel the same as its epact, except for a date within a sacrifice period before the sacrifice. Such days are one place anti-clockwise/up from the epact position.

Whereas, the Ogam wheel has its sacrifice periods beginning at different positions on the outer wheel, the martyrological wheel has its sacrifice periods beginning at the same position on the outer wheel within each year. This is position 25 for years with golden number no more than 11 and position 26 for other years. Below, I show some years and dates on the outer wheel.

00: P  2006  Jan 1
29: N  1995  Feb 1  Apr 1
28: M        
27: H  2003  Jun 1  
26: G        start of sacrifice period (Golden > 11)
25: F  2011  start of sacrifice period (Golden < 12)
24: E  2000  Jul 3
23: D        Sep 1
22: C  2008
21: B        Nov 1
20: A
19: u  2005  Dec 31
18: t
17: s  2013
16: r  2002
15: q
14: p  2010
13: n  1999
12: m
11: l  2007
10: k  1996  Mar 21
09: i
08: h  2004
07: g
06: f  2012
05: e  2001
04: d
03: c  2009 
02: b  1998
01: a        Sep 23       
NB: 2011 has Golden number 17 so, its sacrifice periods begin early at position #26.

Now for some examples:

One thing I notice about the wheel is that for any year, the sum of the epact in the outer wheel and the moon age in the moon phase wheel in any given sector is constant. This constant is equal mod 30 the moon age on January 1, which is 1 greater mod 30 than the annual epact. For the 1 November 2001 example, this sum is 6 mod 30, giving annual epact of 5.

Was such a wheel ever used?