Johannes Franz Hartmann (January 11, 1865 – September 13, 1936) was a German physicist and astronomer. In 1904, while studying the spectroscopy of Delta Orionis he noticed that most of the spectrum had a shift, except the calcium lines, which he interpreted as indicating the presence of interstellar medium. His doctorate was from the University of Leipzig in 1891 on lunar eclipses. The lunar crater Hartmann on the far side of the Moon was named in his honor. Note, however, that the asteroid 3341 Hartmann was named after William K. Hartmann and is unrelated to him.