This Image (or other media file) is in the public domain because its copyright has expired. This applies to the United States, where Works published prior to 1978 were copyright protected for a maximum of 75 years. See Circular 1 "COPYRIGHT BASICS" from the U.S. Copyright Office. Works published before 1924 are now in the public domain.
This file is also in the public domain in countries that figure copyright from the date of death of the artist (post mortem auctoris), in this case Galileo Galilei (1564 – 1642), and that most commonly runs for a period of 50 to 70 years from December 31 of that year.