Astronomers calculate the ellipsoid which has the Earth and a supernova remnant at its focal points in order to locate clouds of dust and gas at its boundary. Identification can be done using laborious comparisons of photos taken months or years apart, and spotting changes in the light rippling across the interstellar medium. By analyzing the spectra of reflected light, astronomers can discern chemical signatures of supernovae whose light reached Earth long before the invention of the telescope and compare the explosion with its remnants, which may be centuries or millennia old.