Bhashya
स्वात्मनि तस्मै संप्राप्ताय विजिहीते स्वात्मावयवान् विगम-यति छिद्रीकरोत्यात्मानमित्यर्थः ।किंपरिमाणं छिद्रमित्यु-च्यते— यथा रथचक्रस्य खं छिद्रं प्रसिद्धपरिमाणम् ; तेनछिद्रेण स विद्वान् ऊर्ध्वः आक्रमते ऊर्ध्वः सन् गच्छति ।स आदित्यमागच्छति ; आदित्यः ब्रह्मलोकं जिगमिषोर्मा-र्गनिरोधं कृत्वा स्थितः ; सोऽपि एवंविदे उपासकायद्वारं प्रयच्छति ; तस्मै स तत्र विजिहीते ; यथा लम्बरस्यखं वादित्रविशेषस्य छिद्रपरिमाणम् , तेन स ऊर्ध्व आक्र-मते ।स चन्द्रमसम् आगच्छति ; सोऽपि तस्मै तत्र विजि-हीते ; यथा दुन्दुभेः खं प्रसिद्धम् ; तेन स ऊर्ध्व आक्रमते ।स लोकं प्रजापतिलोकम् आगच्छति ; किंविशिष्टम् ? अ-शोकं मानसेन दुःखेन विवर्जितमित्येतत् ; अहिमं हिमव-र्जितं शारीरदुःखवर्जितमित्यर्थः ; तं प्राप्य तस्मिन् वसतिशाश्वतीः नित्याः समाः संवत्सरानित्यर्थः ; ब्रह्मणो बहून्क-ल्पान् वसतीत्येतत् ॥
In plain words
Air opens up there, in its own being, once the traveler arrives — which simply means it parts its own substance and makes itself passable, like a hole opening in something solid. How large is that opening? Just the familiar size of a chariot wheel's hub. Through it, the one who truly knows climbs upward, rising as he goes. He reaches the sun — and the sun, which normally blocks the path of anyone trying to reach Brahmā's world, makes an exception here: for someone who has meditated and knows this way, it opens a door, a passage sized like the hollow of a lambara drum, and through it he climbs higher. He reaches the moon, and the moon opens for him too, this time like the hollow of a great kettle-drum, and through it he rises further still. At last he reaches the world of Prajāpati itself. What marks that world out? It is free of grief — no mental suffering reaches there — and free of cold, meaning free of every bodily pain as well. Once he arrives, he lives there for years the text calls 'everlasting' — which is to say, for kalpa after kalpa of Brahmā's own vast lifetime.
Close to the Sanskrit
In its own self, for him having arrived, it opens up — [meaning] it disperses the parts of its own being, renders itself perforated; that is the meaning. What is the size of this opening? It is asked: like the hole of a chariot-wheel, a well-known size. Through that opening, that knower ascends, going while ascending. He comes to the sun; the sun, having blocked the path of one wishing to go to the world of Brahman, stands so; that one too, for one who thus knows, the worshipper, grants a door; for him, that one opens up there; like the hole of a lambara, a particular instrument, is the size of its opening; through that he ascends. He comes to the moon; that one too, for him, there, opens up; like the hole of a kettle-drum, a well-known size; through that he ascends. He comes to a world, the world of Prajāpati. Distinguished by what? Griefless — devoid of mental suffering, that is this; free from cold — devoid of cold, that is, devoid of bodily suffering; that is the meaning. Having attained that, he dwells there for 'everlasting years' — meaning perpetual years, that is, years; that is the meaning — meaning he dwells for many kalpas of Brahmā; that is this.