That union takes place through Vayu (air). So much with regard to the worlds. Next, with regard to the heavenly lights. Agni (fire) is the former element, Aditya (the sun) the latter, water their union. That union takes place through lightning. So much with regard to the heavenly lights. Next, with regard to knowledge. The teacher is the former element,

In plain words: Earth and heaven are joined through the air. Fire and the sun are joined by water through lightning. In learning, the teacher is the first part of the joining.