Why is v almost always written in lowercase and D in uppercase in $v=H_0D$?

OK, v is in lowercase, as usual, but then why is D in uppercase? What's so different/special about it?

In my physics school text/book and Wikipedia it is written as v=H0D, and I always assumed that was the correct form. Yes I have seen it written as v=H0d, but I always thought that was a mistake. I think I assumed that it was D because both my sources pointed out that it was a proper distance, as opposed to a co-moving distance, I suppose. I cannot believe it is just arbitrary. Physics symbols are case sensitive. Was v=Hod considered too confusing!?

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    $\begingroup$ It's only that way becasue that if how you typed it. If you typed V= H0d then the V is uppercase and the d is lowercase. $\endgroup$
    – James K
    Jan 31 at 5:53
  • $\begingroup$ @James K: No it isn't! It is almost always written as v=H0D. Just Google it. $\endgroup$ Jan 31 at 8:09
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    $\begingroup$ However your (English) words are written like this and not "Hauewwer juhr wörds ahr not written laik sis" - it's all a matter of convention and making communication easy. A decent source ALWAYS tells the reader what each symbol in an equation is supposed to mean. Just google it. v as in velocity, D as in distance, H as in Hubble. capitalization to not confuse with V for volume or d for ... also distance. why ever. $\endgroup$ Jan 31 at 8:14
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    $\begingroup$ related: astronomy.stackexchange.com/questions/36298/… $\endgroup$ Jan 31 at 8:28
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    $\begingroup$ It's moderately common to use d for diameter and D for distance, but this is merely a common (not universal) convention. Someone using d for distance is not making a "mistake". $\endgroup$ Feb 10 at 16:45

1 Answer 1


The question is built on a false premise and $v = H_0 D$ is not a universal convention.

If a child asked you, you could point them first to the BBC bitesize revision notes on Hubble's law, which uses $v = H_0 d$.

Then hyperphysics notes, which uses $v = H_0 r$.

(These were the two of the first three non-pdf hits I got when I googled "Hubble's law")

I would then explain to the child that any algebraic symbol can have any meaning you choose to attach to them.

  • $\begingroup$ Ye's and no. Ye's I just Googled it and I was surprised by how many hits had it as v=H0d but I always assumed that was a mistake, coz my school physics text/book and Wikipedia have it as v=H0D. I think I assumed it was D because it was a proper distance, as opposed to a comoving distance, which should be d? I cannot believe it's just arbitrary. $\endgroup$ Jan 31 at 14:59
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    $\begingroup$ @CuriousCat - well, convention is not the same as arbitrary. And there is no entity that dictates symbol usage and holds people to it. $\endgroup$
    – Jon Custer
    Jan 31 at 17:20
  • $\begingroup$ @Jon Custer: Really!? Oh, come on. Physics symbols are case sensitive, and I have only ever seen F=ma, for example, written as such. Never as f=MA. $\endgroup$ Feb 1 at 14:20

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