Question: What is the difference between "Serenity" and "Tranquility"?

They were the names of two lunar mare, which among all of them are the only pair that have got names with very similar meaning.

Answer: 
(from <http://www.etymonline.com/>)

**serene** (adj.)
mid-15c., "clear, calm," from Latin serenus "peaceful, calm, clear" (of weather), figuratively "cheerful, glad, tranquil," of uncertain origin; perhaps from a suffixed variant of PIE \*ksero- "dry," source of Greek xeros "dry" (see xerasia). In English, *applied to persons since 1630s* (italics added).

**tranquil** (adj.) from tranquility (n.), also tranquillity, late 14c., from Old French tranquilite "peace, happiness" (12c.), from Latin tranquillitatem (nominative tranquillitas) "quietness, stillness; serenity," from tranquillus "quiet, calm, still," perhaps from trans- "over" (here in its intensive sense of "exceedingly") + a root possibly related to quies "rest".

Although clearly similar, I would usually use **serene** to describe untroubled states of mind (i.e. mental calmness), and **tranquil** to describe physical stillness.
Meaning of Serenity:

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> *noun*
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> the state of being calm, peaceful and untroubled.
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Meaning of Tranquility:

> 
> *noun*
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> 
> the quality or state of being tranquil, calm.
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> 
> 

There isn't much difference between the two words. In fact, if you go to Thesaurus and search for [serenity](http://www.thesaurus.com/browse/serenity?s=t), tranquility comes up as a synonym and vice versa.