Every event in the past is recorded by each own number of photons. If we take the entire earth as an event or world-point, this world-point radiates continuous number of photons into outer space. If we can intercept some of these escaping photons then we can look back (by extracting the infos out of the photons) into the past of earth just like viewing a prerecorded program on TV.
Let's try to follow the path of one photon as it escape from the event it originated from on planet earth. In one second, this photon will be almost at the moon. In 4 hours, this photon will be at the orbit of Pluto. In 8.6 years, this photon will be at the brightest star Sirius. In 24,000 years, this photon will be at the center of the Milky Way. Whatever object intercepted these photons, it is capable of reflecting (assuming perfect blackbody) these photons back to earth. And if we can filter all the photons we detected, we can basically extract the informations of the past.