This week's parsha, and the first aliyah, begins that song. The song begins with heaven and earth being called to witness that the Jews were properly warned. Rashi says that Moshe's thoughts were that he was human, and not going to live until the time the Jews would need to be reminded about this warning, so who could argue if the Jews someday claimed that they never accepted upon themselves the covenant with G-d. Moshe therefore calls upon heaven and earth to be witnesses because they exist forever.