- 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.
- 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.
- transf. A dim light resembling twilight; partial illumination.
- 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.
- 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:
- Of or pertaining to twilight.
- 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.
- Zool. Appearing or active in the twilight.
A. adj Pertaining to twilight; illuminated by twilight, dim, dusky.
†B. n. The (morning) twilight. Obs.
Of the nature of twilight; dim, dusky, indistinct. (lit. and fig.)
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.
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.
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.