# Calculating moon phase for LaTeX calendar

How can I calculate the moon phase for LaTeX calendar?

My start point is D.E.Knuth's algorithm to calculate Easter is applied here https://texample.net/tikz/examples/birthday-calendar/

The number E is the epact, which specifies when a full moon occurs. However, I don't know how to access the variable \l_easter_E_int

If I could access E, then full moon would happen around 29 days after E.

The code

``````  Easter=-3, % Maundy Thursday
Easter=-2, % Good Friday
Easter,    % Easter Sunday
Easter=1,  % Easter Monday
Easter=39, % Feast of the Ascension
Easter=49, % Pentecost
Easter=50, % Whit Monday
``````

Seems to be an easy way to state days with respect to "Easter". I don't know how this command works.

How can I make the reference Epach=1, Epach=29, etc.?

• Welcome to TeX.SE. Knuth's algorithm is for the calculation of Easter Sunday. For moon phase calculations, other, much more precise algorithms exist. Run a google search for "moon phase calculator" to learn about some of them.
– Mico
Commented Jan 15, 2021 at 18:28
• @Mico Thanks. After 1 hour of search, I have not found any LaTeX calendar example with automatically calculated moon phases. Also, my calendar will in every casel contain Knuth's algorithm which already calculates a full moon occurence \l_easter_E_int. Commented Jan 15, 2021 at 18:38
• What about using a Lua script via LuaLaTeX?
– user226564
Commented Jan 15, 2021 at 18:57
• See tex.stackexchange.com/questions/34785/… for how to typeset the phase, once it is calculated. Commented Jun 15, 2021 at 11:33
• @StevenB.Segletes OK, I know this. But how can I typeset the Easter, once it is calculated? Commented Jun 17, 2021 at 9:16

I remember back in the 80s there being a plain TeX macro in a file `phoon.tex` which calculated the phase of the moon, but this appears to have disappeared along with a lot of other ephemera of that era. However, there is another plain TeX macro file, https://ctan.org/pkg/dayofweek which includes a macro which calculates the phase of the moon.
I found this by going to https://ctan.org and entering “moon” into the searchbox. Too bad there's no longer any trace of that old `phoon.tex` file. I'm pretty sure that was the first solution of the problem.