However , it is difficult to determine the distance to astronomical objects .
In the 1910s , Vesto Slipher ( and later Carl Wilhelm Wirtz ) interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth .
It is difficult to determine the distance to astronomical objects .
In the 1910s , Vesto Slipher ( and later Carl Wilhelm Wirtz ) interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth .