"That's where sunlight is filtering through the Earth's atmosphere and being refracted into the moon's direction to illuminate the moon," he said.
For example, cloudy weather where the sun is setting will block more light from reaching the moon, thus making the moon appear darker.
And since Earth is moving during the eclipse, the sun's position behind Earth changes, making the light bending around the edges change in brightness as well.
In addition, dust in Earth's atmosphere, such as from recent volcanic activity, can dim the moon's brightness.
No major eruptions have occurred recently, however, so dust shouldn't be a factor Wednesday, Espenak noted.
Wednesday's eclipse officially gets underway at 8:43 p.m. ET, when the moon's eastern edge slips into Earth's shadow.
The 50-minute totality begins at 10:01 p.m. The eclipse ends at 12:09 am Thursday.
Up to three total lunar eclipses occur a year, but some years have none. The next one won't occur until December 21, 2010.
Local weather depending, Espenak encourages eclipse enthusiasts to take advantage of Wednesday's event, especially given the relatively convenient viewing hours in the Americas.
"Even though there might be a chance to see one every two, three, or four years, a good percentage of those are clouded out, so you don't get to see them," he said.
Unfortunately, in Seattle, where this writer lives, Wednesday's forecast is for clouds and rain likely.
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