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The full "super moon" rises over Mt. Diablo in Walnut Creek, Calif. in this view from Dinosaur Hill in Pleasant Hill on Saturday, May 5, 2012. The moon is at its perigee, or closest point to earth on its elliptical orbit for the year. (Jane Tyska/Staff)

A special full moon hangs in the weekend's night skies, adding an unusually large and bright splash of silver to our longest days.

The full moon -- appearing at 4:32 a.m. Sunday -- peaks within 20 minutes of the moon's nearest encounter with Earth for all of 2013, making it the largest full moon of the year. Dubbed a "supermoon," it will appear 15 percent larger and 20 percent brighter than when it's at its farthest point -- and will not be so close again until August 2014.

It is part of an astronomical trifecta -- a close-in "supermoon" that is perfectly full, just after the summer solstice, the longest day of the year -- that lifts the hearts of skywatchers.

The convergence of the three events means we will be bathed in more light this weekend than any other time of the year, said astronomy instructor Jonathan Braidman of Oakland's Chabot Space and Science Center, which will offer to the public its three telescopes -- for free -- from sunset to about 10:30 p.m. Friday and Saturday.

"After a very long day, we'll have a nearby full moon, reflecting the sun," he said. "There will be lots of vitamin D!"

Three astronomical facts create the happy coincidence:

  • The Earth, moon and sun are all in a line, creating a "full" moon.

  • The moon's orbit brings it closest to Earth. At this perigee, it lies only 221,824 miles away. In two weeks, on July 7, the moon will swing out to the apogee -- its farthest point for the year -- 252,581 miles distant. That's because the moon orbits in an oval, rather than a circle.

  • The tilt of the Earth's axis means the sun's rays hit us at the most direct angle, creating the longest day.

    "Enjoy the evening," Braidman said. "We'll have a long day, a really nice sunset and a perfect time to go on a moonlit hike."

    Contact Lisa M. Krieger at 650-492-4098.

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    EYE THE SKY
    Chabot Space and Science Center will have its telescopes available to the public for moon viewing from sunset to about 10:30 p.m. Saturday at 1000 Skyline Blvd., Oakland. Information is available at 510-336-7300 or www.chabotspace.org.