A partial eclipse of the full moon tomorrow may get an added boost of red color from an Iceland volcano's ash, astronomers say.

Earth's shadow will be cast across half of the moon's surface over the course of the three-hour event, which begins 3:17 a.m. PDT on June 26. The lunar eclipse will be seen throughout most of the Americas, Australia, and Asia, astronomers say.

A lunar eclipse occurs when the sun, Earth, and moon align. Unlike during total lunar eclipses, when the entire moon is engulfed in Earth's darkest shadow, the moon never completely dims during a partial lunar eclipse. (Take a quiz on moon mysteries and myths.)

"The dark part of the Earth's shadow will clip the moon, and you will see the part that lies within it will get much darker than the rest," said Geza Gyuk, astronomer at the Adler Planetarium in Chicago, Illinois.

"Skywatchers will see the moon decrease in brightness and maybe even a change [in] color."

The first stages of the eclipse will be visible across central and western sections of the North American continent at dawn. Eastern parts of the continent will not see the event. For those in the Pacific Basin and Asia, the eclipse will occur late Saturday night.

Skywatchers in the U.S. central and mountain time zones will catch most of the eclipse, while those within the Pacific time zone will see it in its entirety until the moon exits the shadow completely at 6:00 am PDT.

Volcano Ash May Add Flash to Sky Show

The deepest and most interesting part of the eclipse—when Earth's shadow will fall on 54 percent of the moon's disk—occurs at 4:38 am PDT.

At this point, when the moon is at its darkest, its face may turn red from ash thrown into the upper atmosphere by recent eruptions of the Iceland volcano Eyjafjallajokull. (See pictures of lava exploding from the Iceland volcano's ice cap.)

During the height of the eclipse, sunlight refracting off the scattered dust in Earth's atmosphere is projected onto the moon—the same effect that's in play during red sunsets.