Unicode: "Měsíčku na nebi hlubokém"
English titles: "Rusalka's song to the moon," "O silvery moon"
An aria from Act 1 of Antonín Dvořák's opera Rusalka (libretto by Jaroslav Kvapil). Sung by the title character, Rusalka (soprano), addressed to the moon (měsíčku). If you know only one Czech aria, this is probably it.
The plot of Rusalka is very much like that of Hans Christian Andersen's "The Little Mermaid," only in fresh water. (Bohemia is, after all, landlocked, despite what Shakespeare says in The Winter's Tale.) In this aria, the water nymph Rusalka expresses her desire to become human so that she can be with the prince with whom she has fallen in love.
Here's an approximation of the melody, in abc notation:
T:Mesicku na nebi hlubokem
BBA |BdA |B/G/EG |z3 |\
AAB |AAB |c/A2z/ |z3 |
BBA |c2 B |e/c/ d2 |z3 |\
dcB |Be F/G/ |B/ A2 z/ |z3 |
DDC |E2D |F/D/ E2 |z3 |\
edB |d/c/B A/D/ |A/ G2 z/ |z3 |
G, G3/2G/ |F/D/ F2 |C//D//E3/2 D/C/ |B,3 |\
B,3/2A,/B, |DCB,/A,/ |G,//A,// B,2 z/|A,/F,/ G,2 |
GA3/2G/ |(3F/E/D/ F2 |C//D//E3/2 D//E//D//C// |B,3 |\
B,3/2A,/B, |DCB,/A,/ |G,//A,// B,2 z/ |A, G,2 |]
If you're looking for a recording, I'd recommend one with Gabriela Beňačková. She's got a good voice, and has the distinct advantage of being a native speaker of Czech.