Why is the letter *z* customarily used to denote redshift?

What events led to the nearly universal acceptance of the letter "z" as the denotation of redshift? What did the letter originally stand for?

• This stack exchange might help on this. My wild guess is that z stands for a depth coordinate on the 2D sky. I think z is for redshift due to expansion of space, effectively the distance, and not for redshift in general. I may very well be wrong. – LocalFluff Dec 11 '16 at 8:29
• A quick scan of online literature suggests that this use is postwar. The older papers on cosmological redshift express it as parts per million, or just converting to radial velocity (Slipher uses km, which I take to be km/s) It is remarkable, reading papers like Silpher – James K Dec 11 '16 at 9:37

It may be that someone will come along with a better answer, but the first example I can find for the use of "z" explicitly for redshift is is William de Sitter's 1934 paper.

I'm tempted to think this is one of the earliest uses if not the first use, certainly looking at de Sitter's 1933 book where he touches on this briefly, he does not use the letter "z". However he did use "z" in a 1930 paper to represent the ratio between the radius of the Universe and initial radius, which in this context is actually the same quantity as 1 + z and so why he would've used "z" in his 1934 paper.

If I had to hazard a guess, there is no particular reason for the choice of "z" for redshift, it was just a convenient letter to use for the variable.

• That was a good find. I spent some time looking through papers by Huggins, who was the first to measure a redshift in the 1860's but he never used $z$ to refer to redshift in any of the papers I saw. – zephyr Dec 11 '16 at 22:57