“Give ear, ye heavens, and I will speak; and let the earth hear the words of my mouth. My doctrine shall drop as the rain, my speech shall distil as the dew; as the small rain upon the tender grass, and as the showers upon the herb.” – Deuteronomy 32:1-2, JPS 1917 Tanach
Moses taught the Children of Israel a song that would serve to remind them, at some point in the future, of their failures, hopes, and redemption. Both Heaven and earth were called upon as witnesses to the words of Moses. Rashi adds that both heaven and earth would also serve to carry out the chastisement of Israel when they turned away from H’Shem: Heaven would withhold its rain, and the earth would withhold its produce.
Incidentally, almost as a sidenote, there is a reciprocal relationship between heaven and earth: “as above, so below.” Whatever we do on earth, causes a response in heaven. For example, when we pray, G-d will respond in a manner concomitant with our faith, and the nature of our prayer. Additionally, when we show kindness to others, we will find that in some unexpected way, we are rewarded for our kindnesses in due time, according to G-d’s wisdom.
This principle can also be found in the haftorah: “With the merciful Thou dost show Thyself merciful, with the upright man Thou dost show Thyself upright, with the pure Thou dost show Thyself pure; and with the crooked Thou dost show Thyself subtle” (2 Samuel 22:26-27, JPS 1917 Tanach). The principle is otherwise known as middah k’neged middah, “measure for measure.”