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Supermoon RiverOn Saturday the moon (pictured over Lisbon, Portugal's Tagus River) made its closest approach to Earth in 18 years—making the so-called supermoon the biggest full moon in years. (Get the full story of the supermoon.)The monthly full moon always looks like a big disk, but because its orbit is egg-shaped, there are times when the moon is at perigee—its shortest distance from Earth in the roughly monthlong lunar cycle—or at apogee, its farthest distance from Earth.Likewise, because the size of the moon's orbit varies slightly, each perigee is not always the same distance away from Earth.Saturday's supermoon was just 221,566 miles (356,577 kilometers) away from Earth—making the supermoon about 20 percent brighter and 15 percent bigger than a regular full moon, said Anthony Cook, astronomical observer for the Griffith Observatory in Los Angeles.Before the supermoon, astronomer Geza Gyuk said, "Look for the full moon as it rises above the eastern horizon as the sun sets below the western horizon—it will be a beautiful and inspiring sight."That's advice a lot of photographers, including National Geographic fans, one of whom contributed this picture, apparently took to heart.(See more readers' supermoon pictures.)More Moon Pictures, Facts, and News