Why did scientists previously believe that the sun revolved around the Earth? What helped them discover this was not the truth?
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Aristarchus found a Sun distance of 40 times that of the distance to the Moon. This would make the Sun also 40 times the size of the moon. His calculation was way off, but he still suggested that the Earth orbits the Sun because the Sun was so HUGE.

The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. ... They eventually lost their cell wall and much of their DNA because they were not of benefit within the host cell. Mitochondria and chloroplasts cannot grow outside their host cell.