When a total eclipse of the moon takes center stage tonight in the skies over Western Pennsylvania, it will be the third time in less than a year residents have been able to see the event.
But a cosmic drought is about to set in.
We won't have another total eclipse of the moon for nearly three years.
"On average, for any one location, you go about two to three years that you see them," Carnegie Science Center astronomer John Radzilowicz said. "We're coming off this stretch of being able to see three eclipses in the course of one year. That's a little unusual."
Weather permitting, tonight's eclipse will be visible to millions living in North and South America, Europe, Africa and in parts of the Pacific Ocean.
The moon is visible because it reflects light from the sun. A total lunar eclipse occurs when the moon is full and passes completely into the deepest part of Earth's shadow, called the umbra, cutting it off from the sun's light. Because the moon's orbit about the Earth is inclined slightly, an eclipse does not happen every month.
During tonight's eclipse, some skilled observers may notice some slight shading to the moon before 8:30. This shading happens when the moon moves into the lightest portion of Earth's shadow, called the penumbra.
Most eclipse watchers, however, will first notice a "bite" from the moon with the start of the partial phase about 8:43 p.m.
Totality begins seconds before 10 p.m. and lasts until 10:51 p.m. Sunlight refracted through Earth's atmosphere will keep the moon from disappearing completely. And because the southern limb of the moon will lie close to the edge of the Earth's umbra, it will not appear as dark.
In fact, the color of the eclipse could be noteworthy.
"The moon is moving through the shadow very close to the edge, and we also know that the atmosphere is relatively clear right now," Mr. Radzilowicz said. "We'd be looking for a bright eclipse this time, sort of an orangy-reddish color. Of course, they can vary quite a bit and we won't know until we actually see it."
During the eclipse, two other objects will seem to accompany the moon on its celestial voyage.
"We've got Saturn, which was always a beautiful bright object in the sky, that will be down to the left of the moon," Mr. Radzilowicz said. "But we also have the bright star Regulus that's in the picture" to the right of the moon.
The eclipse's partial phase ends at 12:09 a.m. tomorrow. The moon then will begin to emerge from the light shading of the penumbra and return to its brilliant self.
The last total lunar eclipses visible from Pittsburgh were at moonrise on March 3 and at moonset Aug. 28. The next will occur in the early morning hours of Dec. 21, 2010. During that eclipse, the moon's northern edge will appear brighter because it will be closer to the edge of the umbral shadow.
Those wishing to watch the eclipse with expert help can visit Wagman Observatory in Deer Lakes Park, Frazer (www.3ap.org; 724-224-2510) or the rooftop observatory at Carnegie Science Center (www.carnegiesciencecenter.org; 412-237-3400).
Pete Zapadka can be reached at email@example.com or 412-263-1857.