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Redacted years ago as an undergraduate in an astro class, there were jokes about the "Hubble not so constant", as it was difficult to pin it down at the time. I see that we're continuing the tradition.

(Astro was a bit of a rude awakening to an engineer -- anything that wasn't an order of magnitude could safely be shoveled into the "constant" part of the calculation.)

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Famously, Hubble's 1931 paper that detected the expansion of the universe found H0 ~ 500 km/s/Mpc (fig 5 in [1]). The distances that he was using were way off...

Through most of the late 1900s the uncertainty was between 50 and 100 km/s/Mpc.

Now we know it at least as well as most other things, but this history of uncertainty means it is treated differently. Most annoyingly, simulations often work in units of Distance/h (where h = H0/100). This causes anyone who uses them incredible annoyance as you need to get your factors of little h right. Someone even wrote a paper called "Damn you little h" [2]. It's a total pain...

[1] http://articles.adsabs.harvard.edu/pdf/1931ApJ....74...43H

[2] https://arxiv.org/abs/1308.4150

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