Why is it a Corn Moon?
I believe that these names for lunar names come from North American folklore and were greatly popularised in print by North American farmer’s almanacs. I also believe that at least some of them may derive from Native American traditions.
The name itself makes sense in the Northern Hemisphere with September’s being a time of harvesting. In Celtic languages, there is the Scots Gaelic for September, which is An t-Sultain – the “fattening month”, whereas Irish has Mí Meán Fomhair. Welsh has Medi, the “month of reaping” and Cornish and Breton have Mis Gwynngala and Gwengolo – the “white hay/straw month” – respectively. All of these names are obviously connected to the seasons and agriculture. Interestingly, the Coligny Calendar may indicate Anagantio for September (roughly) and it has been suggested that this means “non-travelling” month, which would make sense if people were busy with the harvest and gathering in the crops before winter.