SB 5.21.2

etena hi divo maṇḍala-mānaṁ tad-vida upadiśanti yathā dvi-dalayor niṣpāvādīnāṁ te antareṇāntarikṣaṁ tad-ubhaya-sandhitam.
etena — by this estimation; hi — indeed; divaḥ — of the upper planetary system; maṇḍala-mānam — the measurement of the globe; tat-vidaḥ — the experts who know about it; upadiśanti — instruct; yathā — just as; dvi-dalayoḥ — in the two halves; niṣpāva-ādīnām — of grain such as wheat; te — of the two divisions; antareṇa — in the intervening space; antarikṣam — the sky or outer space; tat — by the two; ubhaya — on both sides; sandhitam — where the two parts join.
As a grain of wheat is divided into two parts and one can estimate the size of the upper part by knowing that of the lower, so, expert geographers instruct, one can understand the measurements of the upper part of the universe by knowing those of the lower part. The sky between the earthly sphere and heavenly sphere is called antarikṣa, or outer space. It adjoins the top of the sphere of earth and the bottom of that of heaven.