This is probably going to be a somewhat unpopular opinion, BUT... I always thought that "Lightning" was a better translation choice than "Heaven" for the name of Crono's magic, even though it is the less technically-correct option. Why? Because of what we're discussing here.
Calling Crono's magic "Heaven" implies that it is the counterbalance to Shadow, much as one might infer that Fire and Water counterbalance. In terms of how magic actually works in the game, however, this isn't the case at all. As V_Translanka points out, all double and triple techs that combine magic types result in Shadow. Clearly Shadow, aside from being a unique element unto itself, is also what results from any other combination of elements. In other words, Lightning, Fire, and Water are equals and of a similar nature, while Shadow is something different from and perhaps greater than the others.
Personally, I like this arrangement. First of all, it keeps Lightning, Fire, and Water (Ice) on equal footing, as they have traditionallly been in Square games (re: Final Fantasy). It also keeps Crono, Lucca, and Marle equal partners in their magical triad. And, most importantly, it makes Magus extra cool for using Shadow magic.
So, yeah, it may be another Woolseyism-that-piggybacks-off-of-FF, ala calling the legendary sword "Masamune," but this sort of thing is the reason why I've always loved Woolsey.
OdF