Learn Portuguese (Portugal): O professor afirmou que o globo gira no seu eixo. - The teacher stated that the globe spins on its axis.

This highlights the exception for universal truths. Usually, when the reporting verb is in the past tense (like afirmou), the reported action backshifts to the imperfect tense (girava). However, because "the globe spins on its axis" is an unchanging scientific fact, it resists backshifting and stays in the present tense (gira).

Would it be wrong to backshift and say girava here instead?

It wouldn't be strictly wrong. You might hear native speakers use girava in casual conversation, strictly following the standard reported speech rule. But keeping it in the present tense (gira) is stylistically better here because it emphasizes that the statement is an eternal truth, which perfectly fits a teacher explaining an academic fact.

Why use afirmou instead of the more common disse?

While disse (said) is grammatically fine, afirmou translates to "stated" or "asserted". It carries a sense of authority and confidence, showing that the teacher is declaring a hard fact rather than just casually mentioning something.

Why does the sentence use no seu eixo instead of just em seu eixo?