The last one was in 1982 -- and the next one won't happen until 2033.
On Sunday, millions of people around the world will be looking up into the night sky to watch a star-gazing double feature: a lunar eclipse that will occur at the same time there's a "supermoon."
Starting at 7:11 p.m. Pacific time, the moon will look larger and dimmer than normal, taking on a reddish tint. About an hour later, it will change back to a more typical full-moon appearance as the shadow of the Earth passes across it.
Because of the time and expected clear conditions, you won't have to stay up late -- or drive to the top of a mountain -- to watch it. And unlike with meteor showers and other celestial fireworks, children will be able to watch this one and still be in bed at a reasonable time for school the next morning.
"The moon is so easy to see, and the eclipse is so dramatic, people will ooh and ahh wherever they are in the Bay Area as long as they have a clear view to the east," said Andrew Fraknoi, chairman of the astronomy department at Foothill College in Los Altos Hills. "It's a free show."
A lunar eclipse happens when the moon passes directly behind the Earth. With the planet between the sun and the moon, Earth's shadow creeps across the moon's surface, temporarily blocking it out.
"It's like the Earth becomes a big dim flashlight, and the moon is passing through the shadow that the Earth casts into space," said Ben Burress, staff astronomer at Chabot Space and Science Center in the Oakland Hills.
"When the moon enters the total shadow, it should go completely dark," he said. "But because of Earth's atmosphere, which refracts sunlight, that light gets bent into the shadow and the moon appears red."
That's only half of the story this weekend, however.
A supermoon -- a term used more by the media and general public than by astronomers -- occurs when the moon is closest to the Earth in its monthly orbit, making it appear large in the night sky. Because the moon's orbit around Earth is elliptical, when it reaches the closest point to Earth each month, it appears about 14 percent larger than normal and up to 30 percent brighter.
The total eclipse will last 72 minutes, ending at 8:23 p.m. After that, it will be in partial eclipse until 9:27 p.m.
Unlike a solar eclipse, which occurs when the moon passes between the sun and the Earth, people watching it don't have to worry about damaging their eyes. Binoculars will help make the moon look larger, and there are viewing events around the Bay Area on Sunday night, including at Chabot Space and Science Center, where experts with telescopes will be helping the public view the event between 7 p.m. and 9:30 p.m. for a $10 admission.
The curved shadow of the Earth that creeps across the moon during lunar eclipses is key to humankind's understanding of our planet. More than 2,000 years ago, it provided evidence for ancient Greeks to believe the Earth was round, even though the telescope hadn't been invented yet.
"When you see the shadow of a tree, it looks like a tree," Fraknoi said. "When you see the shadow of your uncle, it looks like your uncle. Your senses tell you the Earth is flat. But the Greeks realized that when they saw the shadow of the Earth on the moon, that gave them hints about the shape of the Earth."
Sunday's eclipse will be visible in North America, South America, Africa, Europe and parts of Asia.
To explain the concepts to kids, Burress suggested, you can hold a basketball or soccer ball -- to represent the Earth -- in front of a light bulb, representing the sun. Close the drapes so the room is dimly lit. With your other hand, place a smaller ball, like a baseball, behind the basketball and watch the shadow that is cast from the basketball over the baseball. Then take the little ones outside to watch the real thing.
"An 8-year-old should be able to grasp it," Burress said. "Younger kids too. Amazing spectacles in the sky can inspire people to get really interested in science and what is going on in space. With kids, you never know what the spark is going to be."
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