Variantly: "Listen, O heavens," Torah having been given from the heavens, viz. (Shemoth 20:19) "You have seen that from the heavens I spoke to you" — and "and hear, O earth, the words of My mouth," Israel standing upon the earth and saying (Ibid. 24:7) "All that the L-rd has spoken, we shall do and we shall hear."
Variantly: "Listen, O heavens": because they had not observed mitzvoth pertaining to the heavens, viz.: Intercalation of years and determinations of New Moons, viz. (Bereshith 1:19) "And let them (the luminaries) serve for signs, and for festivals, and for days, and for years." "and hear, O earth": because they had not observed mitzvoth pertaining to the earth, viz.: leket, shikchah, peah, terumoth, ma'aseroth, shemittim, and yovloth.