What's the meaning of the phrase 'Harvest moon'?
The full moon closest to the autumn equinox.
What's the origin of the phrase 'Harvest moon'?
Let's start with the equinox, which involves the sun rather than the moon. The equinox is the time when the Earth's equatorial plane is directly in line with the sun. At that time the length of the day and of the night are equal. This happens twice a year; once in the spring, around the end of March and again in autumn, around the end of September.
The date of the Harvest moon isn't the same each year, as it is fixed by the appearance of the nearest full moon, which can fall on any date.
For those interested, a fuller definition of Harvest moon in the Northern Hemisphere is "The moon which is full within a fortnight of the autumnal equinox (22nd or 23rd September), and which rises for several nights nearly at the same hour, at points successively further north on the eastern horizon.".
That's the science bit over. As far as the linguistics of 'Harvest moon' go we know that it has been used as a name since at least the 18th century. The English poet Isaac Watts was an early user of it in the lyric poem The Celebrated Victory of the Poles, 1706:
Where flows the fruitful Danube; seventy springs
Smiled on his seed, seventy harvest-moons
Fill'd his wide granaries with autumnal joy.
Of course, the Harvest moon isn't all about the moon; it is also about the harvest. Farmers have used it as a marker in the annual cycle telling them when the growing season has come to an end and when to harvest their crops. The added benefit of a bright moon lengthened the working day at the busiest time of the farming year.
If you happen to see a Harvest moon, (this year (2017) it will appear on 5th October), you may see it as having an orange tinge. That's because Harvest moons are low in the sky and we have to look through more of the Earth's atmosphere to see them. This also has the effect of making them appear very large, although this is an optical illusion rather than an atmospheric effect. The nature of the illusion (which, of course, happens in our minds rather than in the sky) isn't fully understood.