One way to judge Full Moon is simply to assess how much it's out-of-round. When night falls on Saturday night, the Moon will look very round, but you might detect a slight defect on its lower left side. Sunday night the out-of-round defect will be much greater - this time on the top part. Result: Saturday wins.
Another way to tell an exact Full Moon is that it always rises precisely at sunset. A Moon that's already up in the east even though the Sun hasn't yet set is not quite full at that point, and still has a way to go. The fact that Saturday's Moon comes up a half-hour before sunset signals that it's close to full at that point, but still has a little way to go - seven hours, actually.
The Harvest Moon injects other interesting factors into all this. Always defined as the Full Moon closest to the Autumnal Equinox, the date was a bit of a horserace this year. September's Full Moon on the fourth fell 18 days before the Equinox, while this Saturday's Full Moon is 12 days after, making this one the closest Full Moon and thus the winner. The result is an unusually late Harvest Moon.
As you'll see this weekend, the Harvest Moon looks no different from any other Full Moon. It's not bigger, redder, lower, higher or anything else. Yet it isn't merely a meaningless label for the equinoctial Full Moon.
Hmm, not just a name, and yet it doesn't look special: What's left? Behavior, that's what. The Harvest Moon acts different from other Full Moons.
The moon normally comes up about an hour later each night. But only at the Harvest Moon do we have a Full Moon that rises only 20 or 25 minutes later on successive evenings. Result? From this coming Friday through Monday, we'll see a series of Full or nearly-Full Moons rising at around sunset each evening. During this whole extended weekend, the Moon will appear just a few minutes later than it did the evening before.
Tying this into the harvest is easy: Farmers trying to finish harvesting work by nightfall are given a helping hand with the light of the Full Moon conveniently coming up at sunset. (Of course, alternatively, they could turn on their tractor lights.) That this moonrise-at-nightfall business goes on for several nights in a row is the only thing that makes the Harvest Moon a special Full Moon.
By contrast, the Full Moon of early spring is extreme in the opposite way: It also rises at sunset, but then the next night it doesn't come up until 90 minutes later, and another 90 minutes the following night. So in spring we get just a single evening of full moonlight, and then the moon goes AWOL. A few nights later and it doesn't make an appearance until midnight. It's here and then, wham! It's gone.
Not now. Now the bright fat Moon lingers and lingers, Friday through Monday, up each night when full darkness falls. The nightly delays in moonrise starting Friday are 22 minutes, then 24, then 28. If you go north the effect is enhanced. In Quebec City the Moon rises just 15 minutes later from Friday to Saturday. From Fairbanks the Moon rises earlier on successive nights this weekend: spooky.
Don't bother using a telescope. The Harvest Moon is a dud. The even illumination on its surface washes out all craters and mountain ranges. And its bright light spoils the rest of the sky, too, so the Milky Way will be entirely gone. Planets can withstand such a brightness onslaught, and one glorious planet is out: Jupiter, the night's brightest "star." Telescopically, that's the place to get your best harvest.