Let's start with the ending '-ar'. Anyone with a reasonable command of English knows that this ending is the common marker of an adjective, derived usually from a Latin-based word. So circle => circular but not round => roundar, column => columnar but not prop => propar... The less Latinate words often (not always - 'round' doesn't, for example) take the suffix '-like' instead: prop-like.... But '-like' is a very catholic device, and can be suffixed to almost any noun, as in column-like.
Next, '-ul-' . Latin had the diminutive suffix -ulus/a, which we still see in quite a few words we use in English: 'cumulus', 'homunculus', 'tumulus' [that's one for the map-readers in the audience - 'a little swelling' - think of the word 'tumor'], 'Ursula' (which means 'little bear' - don't mess with an ursa protecting her ursulam!), 'formula'... We can also see it in a word like 'regular' (regula 'little stick', cf our 'yardstick'). Another adjective - one of my favourite derivations and demonstrating again Guy Deutcher's 'reef of dead metaphors' idea (mentioned in another post) - is 'muscular', from mus ('mouse')/musculus ('little mouse'), which is the way muscles looked to Early Romans - at least the ones who didn't get within gawping distance of auspice-reading: a little metaphorical mouse scampering about under a carpet.
Finally, '-sc-'. This leads us to the concept of the infix. Most English speakers know about suffixes and prefixes; so even if you haven't met the word until now you can probably guess what an infix does. David Crystal in The Cambridge Encyclopedia of Language cites, in English, only - not entirely relevantly, I think (as what is introduced is a complete word rather than a syntactical nut or bolt) - the possibility of introducing an emphatic word into another word - tmesis. He gives the rather strait-laced example 'abso-blooming-lutely' (at least, he did in the first edition; he might have changed it by now to my favourite: 'un-f*cking-believable').
In Vulgar Latin there was -ESC/ISC- , known as an 'inchoative infix' (although more influential as a basis for the formation of Romance language verbs [Fr. finissons/finissez/finissent/etc from finir, etc - where there is no sense of 'inchoateness', and the infix just introduces this 'regular irregularity' to French -ir verbs; in Spanish and Portuguese they didn't use it as an infix at all, and used -ESCERE as a rather long suffix. to create verbs such as aparecer - 'appear' - presumably distantly related to Latin aperire - 'to open' (as in French, careful readers will notice that it happens only to -IRE verbs). In fact Elcock, in The Romance Languages says 'of all the innovations in the active verb of Vulgar Latin, perhaps the most noteworthy is the extension of the -ESC/ISC infix'. But I digress: 'Life is one damn thing after another'? - one digression after another, more like.
In many word-pairs, though, it can be seen as a truly inchoative infix: 'adult/adolescent', 'pubic/pubescent', 'native/nascent'.... other -sc- words have only one half of a potential pair: the image of a crescent moon is growing, but if the moon's full it's not crent. And if a fluorescent light comes on the moment you flick the light-chance'dBeAFineThing-switch (hmm, there are limits to this tmesis thing), instead of flickering for a few seconds, it's not fluorent.
Returning to 'crepuscular'.... Knowing as much I've said so far, but not what creper means (in fact that Webster's link points to 'crepuscule' for a derivation, which goes straight to Fr crépuscule - but I have a feeling that when I first looked at it, about 10 years ago, it mentioned creper) we can work out that it means 'getting (inchoative infix) a little bit (diminutive suffix) <something>-er'. It doesn't take too wild a guess, knowing what 'crepuscular' means, to conclude that the <something> must be 'dark'. My failure to guess at 'dark' all those years ago rankles to this day. I coulda been a contender.
And while we're on the subject of early evening, a man on Radio 3 the other day said Gershwin had written a piece after 'finding himself being serenaded at 4.00 in the morning by a band of street musicians in Havana' [or somewhere like that] Shouldn't that have been aubaded, or has the word not been invented yet? Well it has now.
Update: 2018.04.22.18:00 – Deleted old footer.