What is a "three-part moon"?

Question

This is from Book 2, Chapter 23 of Salamandastron. Samkin, Arula, and Spriggat are sneaking at night into a vermin camp, planning to steal back the sword of Martin the Warrior.

Under a burgeoning three-part moon they set off through the woodland, slipping silently along amid the shadowed tree-trunks and undergrowth.

I'm not sure what a "three-part moon" is. Searching brings up "three part moon phase cards" for teaching the phases of the moon, and well as everything from a "Triple Goddess" which includes the moon as an aspect to a three-part moon mission. Safely discarding the latter two results, I also doubt that the first one is relevant, since the characters are looking at a single moon - not multiple phases.

Having exhausted my admittedly limited search skills, I now turn to Literature SE. What is a "three-part moon"?

Answer 1

“Three-part” means “three-quarter”:

three-part, adj. b. = three parts, n.

three parts, n. Three out of four equal parts, three quarters.

_{Oxford English Dictionary.}

“Three-quarter moon” means “gibbous moon”, that is, a moon that is more than half but less than fully illuminated. A citation for this sense:

the greater part of the bright side is seen, and it is called “gibbous” and appears as a “three-quarter moon”

_{George Dalziel & Edward Dalziel (1862). A Handy Book to the Sky, Air, Earth, and Waters, p. 14. London: Ward and Lock.}

Jacques’ use of “burgeoning” instead of the more usual term “waxing” for describing a growing moon suggests that he might have been going for a sense of estrangement through unusual word choices, or maybe it was a case of elegant variation.

Concern was expressed in comments that this sense of “three-part” is ambiguous: how do you know that it means three parts out of four, rather than three out of five or however many? Well, language is often ambiguous, and people have to use context to disambiguate. The examples given by the OED include

1843   G. Borrow Bible in Spain I. vii. 129   He was half-intoxicated, and soon became three parts so.
1871   M. Collins Marquis & Merchant x   He rides a three-parts thorough-bred.
1878   R. Browning La Saisiaz 72   There's the stoppage at the inn Three-parts up the mountain.
1887   R. L. Stevenson Memories & Portraits xv. 250   Conduct is three parts of life, they say; but I think they put it high.

In “three-parts thorough-bred” the listener has to deduce that it is three of the horse’s four grand-parents that are referenced.

Answer 2

A three-part moon is, apparently, just the moon. As the Wikipedia page about the Triple Goddess that the question links to says, modern Pagan usage such as Wicca considers the moon's waxing, full, and waning phases to be three parts of a single deity, usually identified with Hecate. Another reference to the three-part moon goddess is in a post on the confusingly-named and -written blog called thebible.net, which bills itself as "the most comprehensive collection of non-faith Biblical materials ever assembled":

The Lilim (ללים) were nocturnal spectres, equivalent to the Greek Empusae, and Lilit (Lilith in English) herself the equivalent of Empusa, or Hecate, was their mother; indeed the mother of what we now call witchcraft but in the ancient world was simply the rites and ceremonies of the cult of the three-part moon goddess: new moon (Sleeping Beauty, Red Riding Hood, Cinderella, Maid Marian, Mary Magdalene), full moon (Madonna), waning moon (wicked step-mother, Hecate, Snow Queen).

^{Prashker, David. "Lilit, Lilim." TheBibleNet. thebiblenet.com. Accessed March 23, 2021.}

A poet named Keenan Kelly uses the same phrase in a poem entitled "I'm here where":

I'm now where the three part moon glows
The rise of my love now hides behind the large pine tree
And sadness visits me because you are so far from me
The setting of the moon makes distance sing your name     (ll. 1–4)

^{Breathless: A Journal from the Heart. Lincoln, NE: iUniverse, 2005. p. 71. Google Books. Accessed March 23, 2021.}

Kelly's poem shows that the phrase "three-part moon" is not restricted to neo-pagan contexts any more but is found in literary contexts as a fancy way of saying "moon."