Leaving the title of the piece aside, it sounds evocative of the night, though compared to what I've listened to from Chopin's nocturnes for example, it sounds more descriptive than contemplative.
Is the reason the fact that it's included in a larger suite and wasn't called a nocturne by its author, or is there a more technical reason?
Composers can give any title they want to their pieces. There is not a set definition for "nocturne", though Chopin used a very specific form for all of his. Chopin's nocturnes are in ABA form, and the B section is in a different key from the A section. His nocturnes are also relatively short and stand-alone pieces.
There could be a number of reasons why Debussy did not name his piece a nocturne. On the flip side, I'm sure there are reasons why Chopin did not give his nocturnes descriptive names. There really aren't any rules for this.
These days, people call multi-movement pieces for piano or solo instrument with piano accompaniment "sonata" regardless of whether or not they use sonata form in any movement.
Nocturnes typically invoke the imagery of the night, nocturnes are also typically single character pieces with no overlapping theme. Clair de Lune is actually not a single piece but actually only one part of the Suite bergamasque. Why this suite is almost never played in its completeness is also something that is lost to the ages. It is also worth noting that Debussy was actually not all that happy with the publication of this suite as it was composed before his style matured.
Also, the style of the two is markedly different. The Nocturne style was a decidedly romantic era composition, and Chopin is one of its main proponents did nocturnes in a style that was wholly alien to Debussy.
Debussy had a great reimagining of the harmony that the romantic era simply never had. Debussy wanted his music to invoke the imagery that a Monet painting did and a lot of his music was with this theme in mind.
Interesting enough Debussy did write Nocturnes although not specifically for piano.
Debussy wrote an "introductory note" to Nocturnes as follows:
The title Nocturnes is to be interpreted here in a general and, more particularly, in a decorative sense. Therefore, it is not meant to designate the usual form of the Nocturne, but rather all the various impressions and the special effects of light that the word suggests. 'Nuages' renders the immutable aspect of the sky and the slow, solemn motion of the clouds, fading away in grey tones lightly tinged with white. 'Fêtes' gives us the vibrating, dancing rhythm of the atmosphere with sudden flashes of light. There is also the episode of the procession (a dazzling fantastic vision), which passes through the festive scene and becomes merged in it. But the background remains resistantly the same: the festival with its blending of music and luminous dust participating in the cosmic rhythm. 'Sirènes' depicts the sea and its countless rhythms and presently, amongst the waves silvered by the moonlight, is heard the mysterious song of the Sirens as they laugh and pass on.[4]
'Nocturne' originally referred to a piece, or set of pieces, not particularly descriptive of the night, intended to be performed at an evening gathering. Then it became applied to a short, descriptive piano piece.
Generally, it's a Nocturne if the composer says it is! It isn't policed.
It is just a name. It is not descriptive of the FORM or necessarily the style.
From a marketing standpoint, though, I think it is much more saleable as Clair de Lune as opposed to just Nocturne.
If he were smart, he would have named it CHRISTMAS STARS or even Christmas Nocturne which would guarantee it a ubiquitous spot on the radio every single year in December. Bing, Frank, Nat or Dean would have put holiday words to it and Ka-Ching!!!!!!
For now, the only airtime it gets is when they talk about the moon.
