Mahas is the sun. All the worlds are increased by the sun. Bhu is Agni (fire), Bhuvas is Vayu (air), Suvas is Aditya (sun). Mahas is the moon. All the heavenly lights are increased by the moon. Bhu is the Rik-verses, Bhuvas is the Saman-verses, Suvas is the Yagus-verses.