Jambūdvīpa is not an account of a physical reality.
The purāṇas contain a bewildering array of information about the material world that does not match with pratyakśa or direct perception. A typical example is the account in the purāṇas of a lotus-like Bhū whose pericarp is the Meru mountain. North, south, east and west of Meru are the varśas collectively called Jambūdvīpa. Jambūdvīpa itself is surrounded by seas of different liquids- of sugarcane juice, liquor, ghee, curds, milk and so on. Further surrounding them is a land of gold which is further surrounded by the lokāloka mountain. Below Bhū are present seven more lokas, and above are present the bhuvar-lokas extending upto the sun. Beyond them and the sun is the moon (!) and so on followed by svar-loka, and finally seven more lokas (mahar, jana, tapa upto satya).
Now of course nobody can point to the sky and identify these lokas, oceans and so on. What is one to make of such fantastical descriptions? There have been two extreme approaches in some modern sects of Caitanya Vaisṇavism. In one approach, faithful bhaktas insist that these descriptions are real and that astronomers are clueless about the real structure of the universe. For this reason, they reject that humans went to the moon because it is much farther away from the earth than the sun! Other bhaktas have tried to interpret the purāṇic distances in such a way that they can be reconciled with astronomical descriptions.
The first approach is mere fanaticism or andha bhakti. The second approach misses the point- the descriptions of Bhū are not of the physical universe at all.
As Śrī Babaji explains it, śāstra describes the experiences of yogīs in trance of things beyond the material world (adhidaivika level), but an untrained reader may try to interpret the information as literally referring to a physical reality. Before the account of the universe in the Bhāgavatam, Śukadeva Goswami mentions how what he is about to narrate is beyond the ability of the mind to grasp, even if one thought about it for the lifetime of Brahmā. And yet, there are those who try to make sense of these accounts. What a waste of effort!
Are these accounts false then? They are clearly untrue as far as the adhibhautika level is concerned. But these stories may not be completely false, in that they could be experiences of the adhidaivika type of great yogīs.
What is the need for these narratives in the scriptures? There is no one answer to this question of course, because such questions can crop up for many different narratives in the scriptures. For the case of Jambūdvīpa, one simple reason could be that the audience understand their own insignificance in light of the magnificence of the world. The stories are embellished by interesting portrayals – an ocean of sugarcane, a land of gold and so on – to engage the reader. It is an example of pedagogy, and not to be taken literally.
Grappling with material mechanisms in the darśanas
Now it is undeniable that the darśanas contain accounts of material mechanisms that are mistaken. As an example, in refuting advaitavāda, Śrī Vaisṇavism posits that there is no such thing as bhrama or illusion. Advaitavāda considers everything perceptible to be ultimately a projection of thought and not really real. A favorite example of advaitavāda is that one is liable to see silver in an oyster shell but it is not actually present there. Similarly the world is not present in a real sense but only appears to exist being superimposed by māyā on Brahman. Śrī Vaisṇavas refute this by positing that there is no such thing as bhrama or illusion. When one sees silver in an oyster shell, it is because there actually is silver there! They justify this by the process of pañcikaraṇa. Simply put, the five mahabhūtas are present in each other, and therefore silver can be present even in the oyster shell. Of course, we know well that the silver on the oyster shell is a trick of light reflecting from its surface at an angle, and not due to the literal presence of silver there.
Advaitavādis posit that an object is seen when the antaḥ-karaṇa or subtle sensory instrument flows out through the eye to envelope the object. Of course, we now know that light from the object enters the eye and gets decoded in the brain.
What is one to make of these accounts of material mechanisms? It is useful to keep in mind that the goal of the darśanas is not to teach about material mechanisms but to teach difference between matter and the ātman, or prakṛti and puruṣa. All darśanas, whether sāṅkhya, advaita vedānta, or vaisṇava vedānta accept that prakṛti is distinct from the puruṣa. The goal of these darśanas is not only mokṣa or prema but it is also to make one happy here and now. So the Sāṅkhya-kārikā begins with duḥkha traya abhighātāt jijñāsa – because of the problem of the three fold misery, there is the desire for inquiry (into the means of alleviating misery). It is a fact that happiness is not to be found by learning about material mechanisms.
The goal of the dārśanikas was not to understand material nature and indeed, they were ill-equipped to do so, lacking as they were in the expertise and instruments of modern science. Their goal was teaching about non-material reality as they had experienced it, and indeed that was their singular purpose. To the extent that they were obliged to engage with accounts of material nature, whether to refute ideas contrary to the realization of their experiences, or simply because those ideas existed in the times in which they were present, they engaged with them.
These great yogis were unparalleled, however, in their understanding of human psychology, of the origin of emotions, and of the reasons for why humans behave the way they do. They taught techniques and concepts that helped their followers to transcend material emotions and the material mind and experience that which cannot be experienced by material implements like the senses and the mind. This experience, then, is the only goal of the scriptures. Because understanding how light works or the brain functions was not their focus, the dārśanikas’ understanding in these matters was limited.
Frequently the accounts of material nature in the scriptures do not refer to physical reality. Accounts of material mechanisms by the dārśanikas can also be mistaken. The interest of these dārśanikas was not in understanding material mechanisms, but in experience of a reality beyond material nature. Because that was their singular purpose, their descriptions of spiritual reality are reliable and should be the focus for a sādhaka. Those whose goal is to understand material nature are called scientists. Their accounts of the material world are reliable to the extent that their instruments and theoretical concepts allow them to understand it.