Why does the moon turn yellow on a harvest moon?

The color of the moon as we see it from the ground depends upon how much atmosphere the moon's light passes through and how much dust, pollution and water vapor are in the air the light passes through. Normally, the sun and moon will appear redder when they are near the horizon and whiter when they are overhead.

At the time of the harvest moon rises, the moon is low on the horizon and looks orange or red ( an orange faced "man in the moon" leading to the idea of carving pumpkins). It's called the harvest moon because it's the full moon closest to the autumnal equinox in late September. For several days before and after full moon, the moon rises at close to the same time (due to the angle that its orbit makes with the horizon). It's light allows farmers to spend more time in their fields harvesting the crops, thus the name.

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