Why "agoraphobia" not "agorophobia"?

Question

Greek compounds are usually made by combining the roots of two nouns and inserting a linking thematic vowel when it would result in a sequence of two consonants. The vowel is usually -o-, so we have e.g.

claustr-o-phob-ia, electr-o-magnet-ism, ec-o-nom-y, hor-o-scope

When the second root starts with a vowel, the linking vowel -o- is not needed, so we have

mis-o-gyn-y

but

mis-anthrop-y, mis-andr-y.

But why is "agoraphobia" with an -a-? It is made from agora and phobia, so one would expect

*agor-o-phob-ia. (but this is not the case)

And, are there more such words that use -a- not -o-?

Answer 1

The connecting vowel in Ancient Greek compounds depends on the declension of the first noun: http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0007%3Apart%3D3%3Achapter%3D24

• If the first noun is first declension, the connecting vowel was originally -ā- : agor-ā-nomos 'market clerk', nik-ē-phoros 'bringing victory' (-ē- is the Ionic for -ā-)
• If the first noun is second declension, the connecting vowel was originally -o-: log-o-graphos 'speech-writer'
• If the first noun is third declension, and the stem already ends in a vowel, leave it alone, e.g. ichty-bolos 'catching fish'; otherwise, add -o-, e.g. sōmat-o-phylax 'bodyguard'

The selection of vowels is not random of course: first declension nouns have inflections based on -ā-, and second declension nouns have inflections based on -o-.

This rule has been massively messed up by analogy though: in Ancient Greek, there are plenty of first declension nouns with -o- (dik-o-graphos 'writer of legal speeches'), and second and third declension nouns with -ā- (elaphē-bolos 'deer-shooting'). The -o- linking vowel eventually prevailed, and is the only one used in Modern Greek.

In the case of agoraphobia, the precedent in Ancient Greek was that agora compounds, like agorānomos, always had -ā- as the connecting vowel. The new coinage preserved that precedent.

Answer 2

I believe that the vowel used to link the two words depends on the first one. For example, claustrophobia uses an -o- because it comes from the Latin "claustrum", same with electromagnetism, from Latin "electrum". In the case of misogyny, the Greek word involved is "misos", so an -o- is used, but since the root for agoraphobia is "agora", the -a- is preserved. Another example is triskaidekaphobia, the fear of the number thirteen, which comes from the Greek "triskaideka", which ends in "a". Basically, if it already ends in "a", there's no reason to change it to an "o" when connecting it to another word.