I’ve not seen an explanation why “forenoon” has become archaic while “afternoon” is vigorously alive. But undoubtedly “Good morning” trips off the tongue rather more smoothly than “Good forenoon”.
I stumbled across “forenoon” in Steve Roud’s excellent book on “The English Year. The nation’s customs and festivals from May Day to Mischief Night”. According to the Shorter Oxford Dictionary, forenoon is “chiefly archaic except in nautical contexts. The day until noon: morning”. Webster has “the part of the day ending with noon; usually the time between daylight or breakfast and noon”. Webster has “daybreak” as one of the meanings of “daylight” and that is presumably what he means here. According to the Concise OD, forenoon is North American or nautical and means “the morning”.
And for morning, the Shorter Oxford has “the process or fact of the approach of dawn, the time about sunrise; daybreak. And as a second meaning “the beginning or early part of the day, esp. from sunrise to noon”. According to the Concise OD, the morning is “the period of time between midnight and noon, especially from sunrise to noon”. Webster has “the early hours of the light; the time from rising to noon”.
Not a great deal of difference, both forenoon and morning are used in the longer (daybreak to noon) and the shorter (sunrise to noon) sense.
For me, the order of events is daybreak, the first glimmers of light in the sky. This is the dawn or the beginning of dawn (depending on whether you regard it as a fixed point or a process). Dawn as a process continues through the twilight period until the sun rises.
Checking on the net for definitions of the difference between “forenoon” and “morning” gives both variants, as for which (morning or forenoon) starts at daybreak and which starts at sunrise. I didn’t find anything that felt convincing – it just felt that the set framework, the need to distinguish x from y was triggering an oversimplified distinction.
I did also learn that the word “forenoon” is apparently used by the Amish. The Internet has its uses.
It’s a bit of a muddle but it probably reflects our weaker focus on natural events than in an agricultural society (and we get up later…). Daybreak, dawn, sunrise – it’s all just horribly early, end of debate. A variant on the discussion on the large number of words the Sami have to describe various forms of snow. Language loses its fine distinctions when describing things we’re not that interested in.
None of these dictionary definitions offer an explanation of why morning more or less completely replaced forenoon. The usual etymological explanation of “morning” is that it originates from the Middle English “morwening”, which became “morne” and “morrow” and eventually “morning”.
According to my Anglo-Saxon dictionary “morgen” was used extensively for “morning” (including “morningspell” for news published at morn and “morgentidlic” defined as matitudinal). Morning has in other words been knocking around for a good while and its history didn’t start with Middle English.
There’s also “forenight” but I’ll leave that for another time…