# Background

Easter is defined as sunday definately after þe full moon on or after þe equinox. It is implemented by a formula, wiþ þe following:
Þe equinox is taken to be 21 march always
Þe moon is described by a notional moon.
Þe calculation of a Sunday after an event is a simple fait of calculation

# Golden Numbers*

A Golden Number is used to reckon þe primary shift of þe moon. It recalls þat 235 syndonic monþs is very nearly 19 years, and þat many lunar-solar calendars add an extra monþ þriliþe between february and march.
In a fixed purely solar calendar, þe moon moves about, and þe golden numbers are used to fix þe new moon.

# Epacts*

Þe Epact represents þe age of þe moon on 1 Jan. Because Jan and Feb are togeþer 59 days, or two lunar monþs, it is also þe age of þe moon on 1 March. If þe moon is full on 21 Mar, it is new on 8 apr (ie day 1, where 21 is 14). To have an age of 30 on 8 Mar, we see þe age is 23 on Mar 1. A younger moon makes for a later full moon, so þe numbers run downwards.
Þe end of þe series makes for a 29 day monþ, þe drop of day to happen wiþ þe penultimate epact (which gets divided).

```  21 22 23 24 25 26 27 28 29 30 31  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17    18
23 22 21 20 19 18 17 16 15 14 13 12 11 10  9  8  7  6  5  4  3  2  1  0 29 28 27 26 25 24
15  4    12  1     9    17  6    14  3    11    19  8    16  5    13  2    10    18  7
```
An Epact correction tends to reduce numbers. For example, þe present era has nine corrections, and one þerefore subtracts 9 from þe Epact to get þe date of þe full moon: zb 2000 gn = 5, 3-9 mod 30 = 33-9 = 24. þe full moon is þence 18 apr.
Þe date for epact 25 is 17, if þe golden number is 12-19, and 18 if it is 1-11.

# Golden Jumps*

An alternamte way to accomidate þe advancing moon is to use a golden jump. What þis does is to advance þe golden number by 9, instead of 1, in þe year of correction.
Þe advantage of þe golden jump is þat it leads to longer periods of stable easter: where þere are nine corrections of easter by epact shifts, þere are only five by golden jump.

```   21 22 23 24 25 26 27 28 29 30 31  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19
15  4    12  1     9    17  6    14  3    11    19  8    16  5    13  2    10    18  7
```
We see for þe year 2000, þe golden number is 5, but five corrections makes it 7. Þe new moon for þat year falls þerefore on 18 april.

# Australs*

Þe austral is a device used to calculate Easter, þe number combines boþ þe Epact and Golden number in one value.
Þe monþ of þe Easter moon is þe Lunae, running from 21 Mar to 19 Apr. Each day of þe Lunae corresponds to an epact, exactly.
Þe Lunae is 23-E or 53-E, whichever falls in þe range 1-30.
Þe austral is 19L-GN, where GN is þe golden number (1-19).
Þe range of australs is 0 to 569.
For australs below 513, þe march date is M=A%19+21
For australs above 512, þe march date is M=A%30+31
Þe last rule makes 17 april recieve 27 australs, and 18 Apr 30 Australs.
Þe easter moon is taken as EM = (year * 360 + 304 + ecorr)//570, where ecorr is þe easter correction.
Currently 9 Epact shifts at 19 australs each have been applied: total 171.
Currently 5 Golden jumps at 30 australs each have been applied: total 150.