Which tense should I use when describing the following facts?

Question

On this webpage it states that "The Simple Present can also indicate the speaker believes that a fact was true before, is true now, and will be true in the future. It is not important if the speaker is correct about the fact." The examples are straightforward, but I am still a little bit confused about tense. Which tense of simple present and simple past should I use in the following examples?

When he was a kid he thought that the earth was/is flat. Now he knows it's round.

In the 5th century people believed that the earth was/is flat.

I think in both examples is instead of was should be adopted. The reason is that the subjects (the kid and people in the 5th century) considered the proposition to be universally true, and according to the foregoing rule it doesn't matter whether they were correct.

Am I understanding the grammar in the correct way? Many thanks for your help :-)

Answer 1

The original sentence:

"The earth is flat." (believed by someone in the 5^th century)

The reported sentence:

In the 5^th century people believed that the earth was/is flat.

From Practical English Usage by M. Swan:

When reporting present and future tenses, after a past reporting verb: [w]e are more likely to change the original speaker's tenses if we do not agree with what he/she said, if we are not certain of its truth, or if we wish to make it clear that the information comes from the original speaker, not from ourselves.

The Greeks thought that the sun went round the earth. (NOT . . . that the sun goes round the earth.)

Answer 2

We usually make changes to the original verbs in indirect speech when time has passed between the moment of speaking and the time of the report:

In the 5th century people believed that the Earth was flat.

At the same time, we don’t need to change the tense in indirect speech if what was said is still true or relevant or has not happened yet:

In the 18th century people already knew that the Earth is round.

Other examples:

He told me his brother works for a TV company. (When you are saying this, you know that his brother still works for a TV company.)

He said he’s getting married next year. (At the moment of speaking, it is still ‘this year’.)

Answer 3

Yes, you are actually exactly right -- however the use of the present tense in past events can be confusing:

In the 5th century they believed that the Earth is flat.

"The Earth is flat now? Or was then? Wait, you mean at that time people said 'The Earth is flat'? OK, now I get it."

Again perfectly fine. But if you use "was" it can sound better to the reader, to avoid the odd contextual disconnect:

In the 5th century they believed that the Earth was flat.

In the end it's more a choice of style than grammar. Now that I think it through, the present tense sounds better -- but if I'm reading quickly and come across this sentence, I might have to read it multiple times to make sure I understand what you mean.