Why didn't the Buddha, who knew there were 31 realms of existence, not talk about the roundness of the earth and planets?
The basic project of Buddhism is solving the problem of suffering (or sometimes some other variation of the fundamental problem), usually through a realization about who we really are.
In addition to that, the Buddha didn't bother to challenge many of the other ideas of his day regarding the realms, or karma. (He did take on caste, the economic system, to some degree the inequalities of men and women) He didn't challenge the world's thoughts on a lot of other issues, like medicine, astronomy, geography and so on.
The sutras were written 100s of years later, and accumulated all sorts of unrelated material. By 700 AD, when Vajrayana was well established, the Buddhist canon included books on medicine, astrology and more.
There are many varieties of Buddhism, the secular version dispenses with the cosmologies as instructive myths and the core project of Buddhism remains.
On the other hand, traditional Buddhism still holds the realms to be literally true, necessary for the Buddhist project and proponents like to call doubters "scientist", "materialist" or dismissive of "faith".
If the basic project is to stop the cycle of rebirth and enter a sort of heaven (nirvana), then the realms are essential to Buddhism, without them the basic goal becomes incoherent. If the basic project to end suffering, that is a goal that can imaginably be achieved in this lifetime and it's validity is unaffected by what may happen after we die.
The Buddha who knew there were 31 realm but didn't mention anything about a round earth?
Perhaps because it had nothing to do with being free from suffering.I think the only reason the Buddha mentioned the 31 planes of existence was to help us understand more about samsara and that there are mind states that correlates with these 31 planes of existence.
How come? if he didn't know about the basic fact that all kids now know, instead he said it was flat.
He may not have been referring to the physical earth but the metaphysical or spiritual universe.And i can't find any reference of him saying the earth is flat.can you?
Doesn't it mean that what is said in the buddhist text is unreliable?
That depends on each person.If the person has developed the jhanas and the ability to travel to other realms then the text is reliable to him.If the person has faith in the Buddha the text is reliable for him,for people who completely ignore any metaphysical aspect and practice insight meditation in the here and now,the text is reliable for them.For people who need to see scientific proof for everything then the text is unreliable for them.The Dhamma invites you to practice and see for yourself.No one is forcing you to believe.You decide what's best for yourself.
On the other hand he did mention about 31 realm of existence which is more difficult to comprehend and cannot be proved.
Yes he had to mention the 31 planes of existence because it was related to his teaching of suffering and the cessation of suffering.He didn't mention the earth is round,the chemical composition of the sun,how to build a space ship,how many rings there are in jupiter,what my cats name is because it was irrelevant.
The 31 planes of existence is only difficult to comprehend if you do not see cause and effect.Everyday our mind states changes.One minute we are happy (in the deva realm) the next we are angry (hell realms).Really you don't even have to believe in a place this in itself is enough.
That said some of the Buddhist Universe truth are proven false by the today scientific discovery already.
Fortunately all Buddhists only have to worry about freeing themselves from suffering.
The Buddha who knew there were 31 realm but didn't mention anything about a round earth
The Buddha always talked about spherical shapes throughout the suttas. Spherical shapes when referring to other worlds and also realms. You may also be confused when it is talked about the eight cardinal directions, confusing it for a flat plane. Instead the saying "anything in the eight cardinal directions" refers to the entire unvierse outside of yourself.
Also, according to this blog entry he describes the earth very perfectly including its radius.
Also, in the Surangama sutra or some other sutra Buddha describes the roundness of planets and "sun-systems." I can look for the exact source but try googling it and comment back first.
You may find Mr Piya Tans commentaries about the Aggañña Sutta and the Satta Suriya sutta to give some examples where the Buddha used cosmology . As to why he declared some knowledge and not others read the Simsapa Sutta.
According to my knowledge, there are six realms if we are talking about the Living Buddha. He mentioned it during his teachings on Bhavachakra, known as wheel of life. These realms are taught based on the lives of all sentient beings in earth.