correct phrasing

The teacher said, "The world is round" can be reported as The teacher said that the world is round because the world is round is a universal truth.

Is the bold part correctly used? I don't think so. If so, please rephrase it for me.

Many thanks.

the earth is flat at the poles and bulges at the equator

It's difficult to rephrase without changing it quite a lot.....

"because this is accepted as a universal truth"
"because this is recognised everwhere as a fact"

or a variety of similar phrases.

"...because the roundness of the world is accepted everywhere as a fact" is a bit awkward, maybe.

Hi Contrex

I agree with you that it could be reported using 'was'. However, I think many people will disagree. I've read that 'was' could be used in one English usage book. All the other books state that 'is' should be used.

This again proves that your English knowledge is vast.

Regards

Friedman's by now famous discovery of the world's flatness came to him when he was talking to Nandan Nilekani, CEO of one of India's leading new high-technology companies, Infosys Technologies, at its campus in Bangalore. The Indian entrepreneur remarked to Friedman: "Tom, the playing field is being leveled." The observation is commonplace, but it hit Friedman with the force of a revelation. "What Nandan is saying, I thought, is that the playing field is being flattened.... Flattened? Flattened? My God, he's telling me the world is flat!" Five hundred years ago, Columbus "returned safely to prove definitively that the world was round." As a matter of fact it was not Columbus who provided the proof but the Portuguese navigator Ferdinand Magellan, whose ship circled the globe in a three-year voyage from 1519 to 1522. Regardless, Friedman sees himself as a latter-day Columbus who has discovered that the world is no longer round: "I scribbled four words down in my notebook: 'The world is flat.'"