Which part of speech do compass directions (north, south, etc.) belong to?

'North' and the other cardinal directions, can be used in multiple contexts, acting as different parts of speech.

• Noun: "I live in the north" (it must be a noun because you're using the article 'the' in front of it)
• Adjective: "I live on the north bank" (because it is modifying a noun)
• Adverb: "I live north of the river (because it is modifying the verb)

Words can have multiple meanings, and part of that meaning can be its part of speech.

'North' is not really a preposition but prepositions have an adverbial feel to them, so easy to use similar labels. Time periods like months, seasons, parts of day are similar having the possibility of being used in multiple ways "Winter is coming", "I'm arriving Tuesday night".

Yes, they are prepositions according to CaGEL (The Cambridge Grammar of the English Language). The justification is as follows (CaGEL p613-614),

(c) Spatial terms

Among the words that do not license NP complements there are a fair number belonging to the spatial domain that occur as goal complement with such verbs as come and go, and also, in most cases, as locative complement to be:

[29]

i a. They went ashore.

b. They are ashore.

ii a. I’ll take them downstairs.

b. They are downstairs.

iii

a. Kim is coming home.

b. Kim is home.

iv

a. Let’s put everything indoors.

b. Everything is indoors.

There are good grounds for putting words of this kind in the preposition category. In the clause, prototypical adverbs generally occur in adjunct rather than complement function. They are not entirely excluded from functioning as complement (cf. They treat us appallingly, etc.: see Ch. 8, §2.1), but this usage is relatively exceptional. No adverb in -ly could substitute as goal complement for the underlined words in the [a] examples in [29]. Leaving aside its specifying use, be does not license adverbs in -ly as complement, so none could substitute for the [bolded] words in the [b] examples either. Notice, moreover, that ashore, downstairs, etc., in these [b] examples cannot reasonably be said to ‘modify’ the verb. They no more modify the verb than does young in They are young. Thus although they are traditionally analysed as adverbs, it is arguable that they do not in fact satisfy the traditional definition.

These words are syntactically very like the prepositions in [27] except that they cannot take NP complements. Compare, for example, They went/are aboard and They went/are ashore. Some of them can be modified by right and straight, as in They are right downstairs or We went straight indoors. We accordingly include these too in the preposition class. They maybe contrasted with the adverb locally, which belongs semantically in the spatial domain but is syntactically quite different from these prepositions.

The main prepositions of this kind are as follows:

i abroad abreast adrift aground ahead aloft apart ashore aside away

ii here there where hence thence whence

iii east north south west

iv aft back forth home together

v downhill downstage downstairs downstream downwind uphill upstage upstairs upstream upwind

vi indoors outdoors overboard overhead overland overseas underfoot underground

vii backward(s) downward{s) eastward{s) forward(s) heavenward(s) homeward{s) imvard(s) leftward(s) northward(s) omvard(s) outward(s) rightward(s) seaward{s) skyward{s) southward(s) upward(s) westward(s)

Another property making it prefereable to analyze them as prepositions is that they can appear as a post-head modifier in an noun phrase whereas adverbs never do. CaGEL p446:

(d) PPs

[14]

i a woman [of great wisdom], a school [of this type], the man [with black hair], the church [near the river], friends [from Boston], Jill’s career [as a journalist]

ii the temperature [outside], the floor [below], the year [before]

iii his behaviour [after his wife left him], the car [as we know it today] A very great range of PPs can function as post-head modifier.

Those in [i] illustrate the most frequent pattern, with the preposition having an NP as complement. In the last example, with as, the oblique NP is interpreted predicatively: Jill was a journalist. We also find prepositions without complements, generally locative or temporal, as in [ii]; and in [iii] the prepositions have clauses as complement.

So we find NPs like the following with a prepositional phrase headed by north as post-head modifier.

The 72-year-old rabbi, who leads one of the largest Hasidic sects in the world with tens of thousands of followers, spent the Sabbath in isolation in a room at his home on Zanz Street in [the town north of New York City]. (Times of Israel)

[A town north of Quito] had been permanently closed to the gospel, despite the efforts of an evangelical mission to establish a religious work. (Preparing the Soil for Global Revival; ACAD: Church History; 2007)

This makes sense if we recognize one of the central properties of adverbs as being that they do not modify nouns, CaGEL p562:

Adverbs as modifiers of heads that are not nouns

We noted at the beginning of this chapter that the words that modify verbs are in general distinct from the words that modify nouns. Compare:

1 modifier of noun / modifier of verb

i a. old houses b. ∗They endured old.

ii a. ∗her quite enjoyment of it b. She quite enjoyed it.

iii a. a remarkable/ ∗remarkably change b. It changed remarkably/ ∗remarkable.

Old and remarkable are adjectives, modifying nouns but not verbs, while quite and remarkably are adverbs, modifying verbs (or VPs) but not nouns. In a great number of cases, there are morphologically related pairs of adjective and adverb, with the latter derived from the former by suffixation of -ly, as with remarkable and remarkably in [iii]. This provides the starting point for a definition of adverb: a grammatically distinct category of words whose members are characteristically used to modify verbs but not nouns. Broadly speaking, however, the words that can modify verbs can also modify adjectives and other adverbs – and many can also modify expressions of additional categories other than nouns (or nominals). Compare, for example:

2

i

a. They [almost suffocated ]. [verb]

b. The article was [almost incomprehensible]. [adjective]

c. She [almost always] gets it right. [adverb]

d. [Almost all ] the candidates failed. [determinative]

e. They are [almost without equal ]. [PP]

f. She read [almost the whole book] in one day. [NP]

ii

a. He [behaved annoyingly]. [verb]

b. We’d had enough of his [annoyingly unpredictable] behaviour. [adjective]

c. They are late [annoyingly often]. [adverb]

d. Annoyingly, they hadn’t left us any milk. [clause]

Almost is amongst the most versatile, occurring not just with verbs, adjectives, and adverbs, as in [ia–c], but also with determinatives, PPs, and NPs. Note that it is necessary to distinguish between nouns and NPs. Adverbs do not occur as attributive modifiers within a nominal, but many can occur as external modifier with an NP as head. Almost the whole book, for example, has the NP the whole book as head, and may be contrasted with ∗She congratulated him on his [almost success], where it is inadmissibly functioning as modifier of the noun success. In [ii] we see annoyingly in construction with not only a verb, adjective, and adverb, but also a whole clause.

The most important defining property of adverbs thus needs to be given in the form:

Adverbs characteristically modify verbs and other categories except nouns, especially adjectives and adverbs.