For a while I ponder on some words that I have not been able to find.
I always wondered why, since English is so huge compared to my native Danish where we *do* have the following two words:

 1. A word for a 24-hour period. I found the Greek one: [*nychthemeron*][1]. "It is sometimes used, especially in technical literature, to avoid the ambiguity inherent in the term *day*."  
I of course know the word *day* which we have in Danish too, but we also (along with other languages) have a specific word for a 24-hour period.
    
 2. The adjective of *solidarity* — so we have *sympathetic* but not *solidaric* — can anyone tell me why we can be sympathetic to a cause but have to show solidarity?   
Is *solidary* to *solidarity* what *sympathetic* is to *sympathy*? Any usage examples?  
I am interested in any etymologic reason why it is not *solidaric* (Greek root versus French root?) 

In Danish I can say *I am solidaric to your cause*.   
I cannot hear myself saying *I am solidary to your cause*.

  [1]: http://en.wikipedia.org/wiki/Nychthemeron