A total eclipse of the sun will turn day into night across a swath of the United States tomorrow when the moon moves to block the sun’s face and create a coast-to-coast phenomenon that hasn’t occurred in nearly a century. Millions of people will be watching as the eclipse crosses 1,800 miles from Oregon to South Carolina, creating a band of darkness on the ground roughly 70 miles wide. Only the sun’s corona will be visible as a fiery halo of blazing gas. Never will a total solar eclipse be so heavily viewed and studied — or celebrated. Sure, full solar eclipses happen every one, two or three years, when the moon positions itself smack dab between the sun and Earth. But these take-your-breath-away eclipses usually occur in the middle of the ocean somewhere. This time, the United States is in the bull’s-eye.