In the hours just before sunset on Sunday, May 20, the first solar eclipse in the USA in nearly two decades will dazzle viewers along a swath from the Pacific Coast to Texas.

And some of the best places to see the annular eclipse - one in which all but the outermost rim of the sun is blocked by the moon, leaving a "bull's eye" ring of sunlight - are national parks, where rangers and astronomers offer "solar parties" and other special programs.

Weather permitting , visitors at 33 national parks along the eclipse path will see the disc of the moon within the disc of the sun, with six parks - Redwoods and Lassen Volcanic in California; Zion in Utah, Glen Canyon National Recreation Area in Arizona and Canyon De Chelly National Monument and Petroglyph National Monument, both in New Mexico – at the center of the eclipse path.