Can "crepuscular" and/or "twilight" apply to morning half-light as well as in the evening

I know that's "sorta" two questions in one, but I'm stuck in an argument with a guy who says both words can apply to morning half-light. I disagree and think both only apply in the evening.

I think both can only be used about dim light in the evening, but I have to admit I don't know any equivalent words for dim light in the morning.

EDIT: More votes needed! I get the feeling I'm losing here!

Sorry, FF, but twilight is well defined as a technical term strictly relating to degrees of solar elevation. In other words equally as applicable to near sunrise as to near sunset. And if anything, crepuscular refers more likely to morning twilight than it does evening, though it more formally means both.

Twilight can apply to morning or evening. Etymology is confused, it seems, but twi- either means "half-" or "two". That is, it's either half-light or a light which occurs twice a day. Either way, it's morning and evening.

Crepuscular is more interesting. Etymonline says both and particularly morning:

crepuscular fig. use from 1660s; lit. use from 1755, from L. crepusculum "twilight." Especially of morning twilight.

Collins nearly agrees:

crepuscular (krɪˈpʌskjʊlə) — adj
1. of or like twilight; dim
2. (of certain insects, birds, and other animals) active at twilight or just before dawn
[C17: from Latin crepusculum dusk, from creper dark]

...but implies that "twilight" isn't "just before dawn", and translates crepusculum as dusk, which is definitely evening twilight.

I had always thought crepuscular was related to dusk rather than dawn, but I'd be stumped to find another accepted word to describe animals active in morning twilight! Matutinal could mean "of the dawn" but is more likely simply to mean "of the morning" and auroral is more likely to be associated with polar aurorae.

It does look like your protagonist is right.

Some basic definitions:

crepuscular, adj. : active in the twilight

twilight, n. : the light from the sky between full night and sunrise or between sunset and full night

So crepuscular absolutely applies to the periods before sunrise or after sunset when it's not full night.

Note also this article:

When an animal is said to be crepuscular, it is active during the twilight hours at dawn and dusk... Rabbits and cats are both famously crepuscular.

I found this University of Illinois source that mentions both "morning twilight" and "evening twilight." So, I would say that it can be used accurately for both the time between evening and full dark and full dark and dawn. This source is very specific about when twilight ends in the evening: "Perfect darkness is achieved when the Sun sinks to 18 degrees below the horizon, at which time no light reaches even the upper air, and twilight is over."

That being said, I think most people think of twilight as being after the day, before full darkness sets in. I personally would use dawn for the morning light.

As for crepuscular, this article makes a further distinction:

Special classes of crepuscular behaviour include matutinal (or "matinal") and vespertine, denoting species active only in the dawn or only in the dusk, respectively. Those that are active mainly during both morning and evening twilight are said to have a bimodal activity pattern. (emphasis mine)

• EDIT: You left out the related gloaming, now included below. All three of these terms — twilight, crepuscule, and gloaming — seem usable for either half-light period, although during some historical periods occasionally one or the other of the two ends of that was the more common. Old English had a disambiguating variant, ǽfen-glommung, as today we might use evening twilight. Personally, I find dawn and dusk the simplest and most direct of all these, although perhaps twilight places an emphasis on the lighting.

The answer is that both crepuscular and twilight refer to the the half-darkness of the dawn (that is, before sunrise) and of the dusk (that is, after sunset). Neither is more of one nor the other, at least in modern use. I’m most familiar with characterizing felines as crepuscular critters, where no bias towards the morning nor towards the evening is either meant or implied.

The OED gives perhaps a more complete view of these, including several similar words with the same ultimate ancestry and which entered English nearabouts the same time as crepuscular did. Citations omitted for the sake of brevity.

twilight n. [1412–20]

1. • a. Generally.
   • b. spec. Most commonly applied to the evening twilight, from sunset to dark night. second twilight n. see quot. 1883.
   • c. Morning twilight, which lasts from daybreak to sunrise.
2. transf. A dim light resembling twilight; partial illumination.
3. fig.
   • a. An intermediate condition or period; a condition before or after full development.
     ^{_{twilight of the gods [transl. of Icelandic ragna rökkr, altered from the original ragna rök, the history or judgement of the gods], in Scandinavian Mythol. the destruction of the gods and of the world in conflict with the powers of evil; also transf. Cf. Götterdämmerung n., Ragnarök n.}}
   • b. esp. in reference to imperfect mental illumination or perception.
4. • a. Of, pertaining to, or resembling twilight; seen or done in the twilight.
     ^{_{twilight arc (also twilight arch), twilight curve, the outline of the earth's shadow, which rises in the east as the sun sets, forming an arch which divides the twilight or shaded portion of the sky from that which is lighted by the direct rays of the sun. twilight glow, a diffuse glow in the sky at twilight; spec. in Meteorol., that caused by spectroscopic emission in the upper atmosphere from atoms excited by solar radiation. twilight parallel, the small circle of the celestial sphere, parallel to and 18 degrees below the horizon, at the sun's crossing which evening twilight ceases or morning twilight begins (Webster, 1911). twilight vision, vision in which colours are hardly perceptible owing to the dimness of the light; scotopic vision.}}
   • b. fig. Having an intermediate character.
   • c. Lighted as by twilight; dim, obscure, shadowy; also fig. of early times.
   • d. fig. Of the nature of or pertaining to imperfect mental light.

The etymology provided for twilight is:

Middle English, < twi- comb. form + light n¹, corresponding to West Frisian twieljocht, Dutch tweelicht (from 16th cent.), Low German twilecht, German zwielicht. The rare form twilighting n. is recorded a little earlier. The exact force of twi- here is doubtful: compare in same sense Middle High German zwischenliecht ‘'tweenlight’, and Low German twêdustern, twêdunkern, lit. ‘twi-dark’.

And here are the various crepusc- headwords:

crepuscle n. [1665]

Twilight.

crepuscular adj. [1668]

1. Of or pertaining to twilight.
2. • a. fig. Resembling or likened to twilight; dim, indistinct.
   • b. esp. Resembling or likened to the morning twilight as preceding the full light of day; characterized by (as yet) imperfect enlightenment.
3. Zool. Appearing or active in the twilight.

crepuscule n. [c1400]

Now rare.
Twilight.

crepusculine, adj. and n. [c1550]

rare
A. adj Pertaining to twilight; illuminated by twilight, dim, dusky.
†B. n. The (morning) twilight. Obs.

crepusculous, adj. [1646]

Of the nature of twilight; dim, dusky, indistinct. (lit. and fig.)

crepusculum, n. [1398]

Twilight, dusk.

The etymology given for the very last of those, which is clearly both the earliest and the least assimilated into English, is:

Latin = twilight, a diminutive formation, related to creper dusky, dark, creperum darkness.

Here is the entry for gloaming. This time I will give its citations, because they include mentions of the two preceding terms.

gloaming, n. [c1000]

Pronunciation: /ˈgləʊmɪŋ/
Forms: OE glómung, (ǽfen-)glommung, ME glomyng, ME–16 gloming, 17– gloaming.
Etymology: repr. Old English glómung strong feminine, < (on the analogy of ǽfning evening n.¹) glóm twilight, probably < the Germanic root ∗glô- (see glow n.); the etymological sense would thus seem to be the ‘glow’ of sunset or sunrise (compare gloom n.²), whence the passage to the recorded sense is not difficult.

The vowel of the mod. gloaming is anomalous, as Old English glómung should normally become glooming. The explanation probably is that the ó was shortened in the compound ǽfen-glommung (as the spelling seems to show was actually the case), and that from this compound there was evolved a new n. glŏmung, which by normal phonetic development became Middle English glǭming, modern English gloaming. In the literary language the word is a comparatively recent adoption from Scottish writers; but it is found in the dialect of Mid. Yorks.

• a. Evening twilight.

  • c1000 in T. Wright & R. P. Wülcker Anglo-Saxon & Old Eng. Vocab. (1884) I. 117/7 Crepusculum, glomung.
  • c1000 in J. Stevenson Lat. Hymns Anglo-Saxon Church (1851) 16 Crepusculum mens nesciat, æfen glommunge mod nyte.
  • c1425 Wyntoun Cron. ɪᴠ. vii. 827 Fra the glomyng off the nycht.
  • c1540 J. Bellenden tr. H. Boece Hyst. & Cron. Scotl. ɪx. xxv. f. 128ᵛ/2, He‥efter supper past furth in yᵉ glomyng.
  • c1610 in R. Pitcairn Criminal Trials Scotl. III. 3 This fell furth in the gloming.
  • 1786 R. Burns Twa Dogs xxxv, in Poems 21 By this, the sun was out o' sight, An' darker gloamin brought the night.
  • c1800 Hogg Song, 'Tween the gloaming and the mirk, When the kye comes hame.
  • 1807 Byron Elegy Newstead Abbey ix, Soon as the gloaming spreads her waving shade.
  • 1830 Tennyson Leonine Elegiacs, Lowflowing breezes are roaming the broad valley dimmed in the gloaming.
  • 1866 ‘G. Eliot’ Felix Holt I. Introd. 2 The happy outside passenger seated on the box from the dawn to the gloaming [etc.].
    fig.
    
  • 1786 R. Burns Poems 73 When ance life's day draws near the gloamin.
  • 1889 J. M. Barrie Window in Thrums xvi. 144 The help she and Hendry needed in the gloaming of their lives.

• b. Said occas. of morning twilight.

  • 1873 H. B. Tristram Land of Moab iii. 38 The sun had scarcely cast the gloaming of approaching dawn over the eastern peaks.
  • 1894 S. R. Crockett Raiders 21, I rowed home in the gloaming of the morning.

• c. Shade, dusky light.

  • 1832 W. Motherwell Jeanie Morrison vii, And in the gloamin o' the wood, The throssil whusslit sweet.

So it seems that gloaming is more often the evening twilight of dusk, but sometimes the morning twilight of dawn. Amongst its compounds are a gloaming sight, a special kind of sight used for shooting guns and rifles in the evening hours.