Adding Lunar Phases to Emacs' Org Agenda

Posted on 19th of February 2023 | 911 words

As some of you may know, I’ve been a practising Buddhist for some time, focusing mainly on the teachings of Early Buddhism and Theravada. One aspect of this practice is keeping up with Uposatha, which is also present in other schools of Buddhism. Wikipedia describes Uposatha as follows :

The Uposatha (Sanskrit: Upavasatha) is a Buddhist day of observance, in existence from the Buddha’s time (600 BCE), and still being kept today by Buddhist practitioners. The Buddha taught that the Uposatha day is for “the cleansing of the defiled mind,” resulting in inner calm and joy. On this day, both lay and ordained members of the sangha intensify their practice, deepen their knowledge and express communal commitment through millennia-old acts of lay-monastic reciprocity. On these days, the lay followers make a conscious effort to keep the Five Precepts or (as the tradition suggests) the ten precepts. It is a day for practicing the Buddha’s teachings and meditation.

Uposatha days change from lunar month to lunar month, so there are no set days for those. But generally speaking, Uposatha is observed about once a week in accordance to the lunar phases. Which lunar phases are observed depends on the culture, but there are four phases to this: the new moon, the full moon and two quarter moons between those. Theravada cultures generally observe all four, but for example, in Sri Lanka, they tend to only observe the new and the full moon.

How these Uposatha days are calculated is quite interesting, and there is a paper published on GitHub by Gambhiro Bhikkhu about that.

Personally, I have followed these moon days by adding those to my Apple Calendar via the iCal link provided in the repository above, and I’ve been quite happy with that. Lately, though, I noticed that I tend to easily miss these days since I don’t often have notifications on or my calendar open when these days happen, which made me easily miss those. That got me thinking that it would be pretty cool to have those lunar phases on my emacs since that is most likely always open for me and I’m already mainly doing my time management in org-mode.

So I started digging around on how these could be easily added to my org-agenda, and then I found a built-in command from emacs that is at least the first step there called M-x lunar-phases, which basically prints lunar phases of the last, current and next month. So I started hacking around to try to get these to my Agenda view.

Not long after starting my hacking in this, I found a nice doc piece about some ad-hoc tweaks to be made to the agenda from org-mode, which actually made exactly what I wanted. What it involves is that I need to build an agenda file with these lunar phases so that the Agenda view in emacs picks those up.

So first, I needed to add some arbitrary file to my org-agenda-files. With use-package:

(use-package org
  (org-agenda-files '(<your other agenda files> "/path/to/")))

Or normally:

(setq org-agenda-files '(<your other agenda files> "/path/to/"))

I happen to have all my agenda files in my Documents folder on my macOS, so they just get synced across my devices, but naturally, you can use any path you want.

The file itself is pretty simple. Basically, we add a header to it with a single diary sexp calling a function, org-lunar-phases, which we soon define:

* Lunar phase

The org-lunar-phases itself either isn’t too complicated. It involves that we pass in the current day to it, which gets passed in in a relatively odd way via the diary sexp we used above, and after that we just parse the lunar-phase-list with the current day, or month and year in this case, since lunar-phase-list returns a list of lunar phases for the next three months.

(require 'cl-lib)

;; Pass current day to `org-lunar-phases', which is annoyingly in a stupid
:: format, (MM DD YYYY).
(with-no-warnings (defvar date))
(defun org-lunar-phases ()
  "Show lunar phase in Agenda buffer."
  (require 'lunar)
  (let* ((phase-list (lunar-phase-list (nth 0 date) (nth 2 date)))
         (phase (cl-find-if (lambda (phase) (equal (car phase) date))
         (lunar-phase-names '("● New Moon"
                              "☽ First Quarter Moon"
                              "○ Full Moon"
                              "☾ Last Quarter Moon")))
    (when phase
      ;; Return the phase to the agenda file.
      (setq ret (concat (lunar-phase-name (nth 2 phase)))))))

Naturally, you can get all fancy with those lunar-phase-names. Maybe adding emojis and whatnot if you’re into it. But in all simplicity, that is how you can add lunar phases to your agenda, and it shows as following in there:

10 days-agenda (W08-W09):
Monday     20 February 2023 W08
  Lunar:      ● New Moon
Tuesday    21 February 2023
Wednesday  22 February 2023
Thursday   23 February 2023
Friday     24 February 2023
Saturday   25 February 2023
Sunday     26 February 2023
Monday     27 February 2023 W09
  Lunar:      ☽ First Quarter Moon
Tuesday    28 February 2023
Wednesday   1 March 2023

One thing that I noticed from this was the fact the GitHub link above that I mentioned. That happens to produce a little bit different results for these phases. Every once in a while, some phases differ just a tiny bit. There is a mention that the calculation method used in that paper was related to how these days are calculated in Mahānikāya in Thailand, so there might be some variance compared to the M-x lunar-phases. But personally, I feel it’s close enough and quite beneficial to me.