Cfrtjryk

Did Sanskrit नक्ति (nákti) "night", PIE *nókʷts, not participate in the kentum-satem split? Why? Is it a loan? There are at least two synonyms, if that makes any difference.

I have no actual reason to assume it should have undergone Satemization, unless that's what affected अष्ट (aṣṭá) "eight", PIE *oḱtṓw. Is that wrong?

In other words, why did these two roots develop differently?

There are three different series of guttural sounds reconstructed for Proto-Indogermanic, that are usually represented by *k' (the k that gets satemised), *k (plain k that stays k), and *kʷ (that has many different developments in the different languages, the outcomes include p (Ancient Greek ποῖος (poîos) from *kʷoyo), kw or kv, or plain k.

As the reconstructed form shows, *nókʷts has a non-satemising *kʷ. This is consistent with other satem languages: Lithuanian: naktis, Latvian: nakts. Russian: ночь (nočʹ) seems to be inconsistent, but here a series of sound changes has taken place *kt -> *t' -> čʹ with complete elimination of the original k. Note that Spanish noche has undergone similar sound changes independently (and we know the Latin precedent nox, noctis in this case).

As commonly reconstructed, PIE had three different types of "velar-ish" plosives:

• "Palatal velars" (probably plain velar): *ḱ, *ǵ, *ǵʰ
  


• "Plain velars" (probably uvular): *k, *g, *gʰ
  


• "Labial velars" (probably labial-something): *kʷ, *gʷ, *gʷʰ
  

We're not sure exactly how they were pronounced; I believe the distinction was probably velar/uvular/labiovelar rather than palatal/velar/labiovelar, but the terminology has stuck, and there's no hard proof either way.

In the centum languages, the "palatal" and "plain" series merged together. For the classic example that gave the phenomenon its name, *ḱm̥tóm "hundred" > Latin centum, with a /k/. The "labial" series stayed distinct.

In the satem languages, the "plain" and "labial" series merged together, and the "palatal" series turned into various sibilants, so *ḱm̥tóm became Avestan satem (and Sanskrit śatam).

Since this root had a "labial" *kʷ, it merged into *k in the satem languages rather than becoming a sibilant. That's what you see in Sanskrit. (Slavic languages do seem to have satemized it, as jknappen points out, but that's a later development due to the consonant cluster.)

EDIT TO ADD: As TKR points out, when I say things like "probably uvular", that's very far from certain or uncontroversial. Many linguists think that the sounds were uvular, but many others think they weren't, and there are solid arguments on both sides. Such are the perils of reconstructing!