Śvetāśvatara Upaniṣad · 6..1

सविता यस्तवीमहान् सूर्यो यस्तवीमहान् चन्द्रमा यस्तवीमहान् विद्युत् यस्तवीमहान् स य एषोऽन्तर्हृदय आकाशः तस्मिन् चायं विश्वमिदं स्थितम्

savitā yastavīmahān sūryo yastavīmahān candramā yastavīmahān vidyut yastavīmahān sa ya eṣo'ntarhṛdaya ākāśaḥ tasmin cāyaṃ viśvamidaṃ sthitam

The Savitar who is greater, the Sun who is greater, the Moon who is greater, the Lightning who is greater —and He who is this inner Space of the heart, in Him is established this universe.

A hierarchy of greatness: savitā (the sun god, vital force), sūrya (the physical sun), candra (the moon), vidyut (lightning) —each “greater” than the previous in some sense. But the greatest is the ākāśa (space) antarhṛdaya (interior of the heart). In our yoga practice, this reminds us that the entire universe (viśvam idam) is not “out there” but established in the space of the heart. The heart is the microcosm that contains the macrocosm, the hṛdaya that is vaster than any outer sky.