If there are broken skies tonight, you will see . . . the moon
Full-orbed, and breaking through the scattered clouds,
Shew her broad visage in the crimsoned east,
And shed a softer day.
The poet, James Thomson, was referring to the Harvest Moon, the romantic name given to the full moon nearest to the autumnal equinox.
The equinox, you may recall, occurred last Thursday, so tonight's full moon will be this year's Harvest Moon. On the other hand, if you wished to be pedantic, you could point out that the exact instant of the full moon occurred somewhere around 6 a.m. today, so maybe last night's moon should be the Harvest Moon? But let us settle for a rough approximation.
The moon's orbit is such that for several days at this time of year it rises above the horizon at about the same time each evening, just when the sun is setting, and seems, as it were, to take over the illuminating duties of the sun. It was the custom in days gone by for farmers to take advantage of this succession of bright moonlit evenings to gather in the last of their crops.
Because of its position low in the sky near the horizon, the early-evening Harvest Moon appears much larger than usual, much bigger than later in the night when it is high up in the heavens, but this bigness is merely an optical illusion.
It is well established by instrumental measurements that, in fact, the moon's angular size does not vary significantly whether viewed near the horizon or approaching its zenith; what varies is our perception of its size.
Our perception of the size of distant objects is known to be closely related to how far away the object is. Or to be more precise, to how far away we think the object is. Our impression of a newly-risen full moon is influenced by the angular proximity of familiar objects on the horizon; the resulting illusion that the moon is much larger than usual results from an impression that the moon near the horizon is further away from us than it seems to be when it is high in the sky.
It is an illusion from which no amount of astronomical knowledge can free us.
There are, of course, real differences in the appearance of the Harvest Moon as it moves across the sky; as, for example, when it gradually changes colour from yellow to white. These, however, can be explained easily by the effect of the Earth's atmosphere on the moon's reflected light.