Chandogya Upanishad 2.2.2

As Sāma is everywhere, it is also in the five worlds most familiar to us. Starting from the top, these five worlds are dyauḥ (heaven), āditya (the sun), antarikṣa (the sky), agni (fire), and pṛthivī (the earth). To meditate on Sāma in these worlds, then, we can use the five symbols, which are respectively: hiṃkāra, prastāva, udgītha, pratihāra, and nidhana.

When we use the term symbol, we understand that there is always a connection between the symbol and the thing symbolized. In this case, hiṃkāra is the first among the symbols, and heaven is the highest among the worlds. This is why hiṃkāra stands for heaven. Prastāva stands for the sun, for when the sun rises everyone gets ready to work. The word prastāva means ‘getting ready.’ The sky is gagana. It begins with ga, and the word udgītha also contains ga. So udgītha stands for gagana, the sky. Pratihāra stands for fire, because fire makes people ‘scatter’ (pratiharim). The earth is nidhana (extinction), for all things fall from above and finally disappear on the earth.