Each year since 1963, Aldon D. Nielsen, 84, has taken pictures of the National Christmas Tree. He's trudged out there in the snow and sleet and rain, on frigid nights and perfectly clear star-kissed nights. He has photographed fat trees, skinny trees and one tree that had to be surgically repaired because the train carrying it to Washington derailed.

Over the years, he has become the nation's leading expert on the history of the tree. When the tree is lighted each year, it marks a moment in the country's history, the ceremony a reflection of what is going on in America and the world. Jimmy Carter, the sweater president, had energy-efficient lights. Richard Nixon pulled the switch to light the tree while being hooted by Vietnam War protesters. Ronald Reagan, after an assassination attempt, lighted the tree from the White House instead of the Ellipse for security reasons. During the Iran hostage crisis, the tree was dark except for a star on top out of respect for the captured Americans. On Dec. 18, 1980, Carter lighted the tree for only 417 seconds, one for each day the hostages had spent in captivity. On Inauguration Day in 1981 when the hostages were released, Reagan ordered the tree redecorated in time for their return home.

Throughout the years, presidents have moved in and out of the White House, but Nielsen has made the trip from his home in Northern Virginia to record the image of the tree. A self-trained photographer who worked for the Department of the Interior for most of his life, Nielsen is believed to be the only person in the country with such an extensive archive of photos of the National Christmas Tree. He has his collection on a slide show, compete with music and a historical narrative. He has gone from film to digital to video, from middle age to old age, from father to grandfather to great-grandfather, and all the while kept up his mission.