While other astronomers had observed the phenomenon earlier, Slipher made significant contributions to understanding its meaning and its connection to the distance of galaxies.

Here's why Slipher is credited:

* Extensive observations: Slipher systematically measured the redshifts of numerous spiral nebulae (what we now know as galaxies) between 1912 and 1925.

* Revealed a pattern: He noticed that most spiral nebulae showed a redshift, and the amount of redshift was proportional to the distance of the nebulae.

* Foundation for Hubble's Law: Slipher's work laid the foundation for Edwin Hubble's later discovery of the relationship between redshift and distance, now known as Hubble's Law.

While Slipher's work was crucial in understanding redshift, it's important to note that other astronomers like William Huggins and James Keeler had also observed redshift in celestial objects earlier.