I'm trying to come up with a better translation of Aratanaru Chikai. Some of the lines have strange translations ("burn like tea"!?).
(1) Does anyone know what "Chiheisen ga oshiete kureteru yo" means?
地平線(ちへいせん) = horizon, 教(おし)える = teach/inform, but I'm not familiar with kureru constructions. Is the speaker asking the horizon to tell him something? Assuming the romanization is correct, the horizon is the one who does the informing.
(2) I have trouble with this verse:
"Madamada nakama hairu hazu sa
Suiheisen miteru to kanjiru yo
Madamada yaru koto ippai tte
Omoi to ni ureshii ze!"
Maybe something like:
"I expect that my companions haven't gone there yet.
When I see the horizon, I sense that
it's full of things still to do
??? [he's happy about something or other]"
Any suggestions anyone? Note that there could be errors in the romanization.
(1) Does anyone know what "Chiheisen ga oshiete kureteru yo" means?
地平線(ちへいせん) = horizon, 教(おし)える = teach/inform, but I'm not familiar with kureru constructions. Is the speaker asking the horizon to tell him something? Assuming the romanization is correct, the horizon is the one who does the informing.
(2) I have trouble with this verse:
"Madamada nakama hairu hazu sa
Suiheisen miteru to kanjiru yo
Madamada yaru koto ippai tte
Omoi to ni ureshii ze!"
Maybe something like:
"I expect that my companions haven't gone there yet.
When I see the horizon, I sense that
it's full of things still to do
??? [he's happy about something or other]"
Any suggestions anyone? Note that there could be errors in the romanization.

some examples: 思い、重い、想い。 But I think your translation (which was nearly like) "I feel happy with this thought" is a correct possibility, and the "ni" is either wrong or very artificial.