Right now someone is taking a picture of a sunset. It could be on a beach in Barbados or above the Arctic Circle or on a motel balcony in Seattle. And that person is not alone. Thousands of people are taking exactly the same picture. No need to search for them; Penelope Umbrico has already done that. A photo-based artist, Umbrico has compiled thousands of images from the social-media site Flickr. She exhibits collections of them, each photograph tightly cropped to reveal only the sun. Seeing one of her wall-size installations forces us to confront the question: Why are…