Variantly: "Listen, O heavens": Because Moses was close to the heavens, he said "Listen, O heavens"; and because he was far from the earth, he said "and hear, O earth, the words of my mouth." Isaiah came and followed suit, saying (Isaiah 1:2) "Hear, O heavens, and listen, earth," being far from the heavens and close to the earth.
Variantly: Because the heavens are many, he (Moses) opened in the plural ("ha'azinu"); and because the earth is one, he opened with the singular ("tishma"). Isaiah came and followed suit, saying "Hear ('shimu'), O heavens, and listen ('ha'azini'), earth," matching the many with the plural and the one, with the singular. The sages say: This is not so; but when the witnesses testify, if their testimony is consistent, one with the other, it stands; if not, it does not stand. Therefore, if Moses had said "Listen, O heavens" (and nothing more), the heavens (in their defense) could say "We heard only by 'listening'" (and not by 'hearing'). And if he had said "and hear, O earth" (and nothing more), the earth (in its defense) could say "I heard only by 'hearing'" (and not by 'listening') — wherefore Isaiah came and followed suit, saying "Hear, O heavens," and "Listen, O earth," (in sum) attributing (both) listening and hearing to heaven, and listening and hearing to earth.