The answer to this riddle lies in a couplet of Bhai Nandlal jee, written in Farsi. Bhai Nandlal jee was a great mystical poet. It's said that he wrote Bandaginama, a collection of Ghazals written in Farsi, for Siri Guru Gobind Singh jee but when Guru Sahib read the Ghazals, He changed the name to Zindaginama. In other words, Guru Sahib called his collection an account of life. Indeed, reading Bhai Sahib's Zindaginama one feels one is alive.
Presented below is the couplet of Bhai Nandlal jee, that explain why the Sky bows down to Earth.
ਆਸਮਾਂ ਸਿਜਦਾ ਕੁਨਦ, ਪੇਸ਼ਿ ਜ਼ਮੀਂ, ਕਿ ਬਰ ਓ।
ਯਕ ਦੋ ਨਫ਼ਸ ਅਜ਼ ਪੈ ਜ਼ਿਕਰਿ ਖ਼ੁਦਾ, ਬਿ-ਨਸ਼ੀਨਦ।The sky does Sijda (Matha-tek) before the Earth because one or two Gurmukhs sitting on it, do Zikar-e-Khuda i.e. do Simran of Vaheguru.
Naam holds the ultimate weight in this universe. On whichever side Naam is, that side is heavier. Since Gurmukhs do Simran while sitting on Earth, the Sky has to bow before Earth.
Naam is the key to all powers and all greatness. The moral of the story is that we should engage in the true service of Vaheguru, by chanting the True Naam that is obtained from the True Guru, by giving our True Self to Him.
Bhul chuk dee maafi jee.
Daas,
Kulbir Singh