What did people of the early civilizations believe was the center of the universe?

One popular notion was that the stars were "fixed" on the dome of heaven. The entire universe would have been considerably smaller than one light year in diameter.

The ancient Greeks coined the Word for "planets" which means "wanderers." They knew of Mercury, Venus, Mars, Jupiter, and Saturn. Uranus is visible on very dark nights to the naked eye, but no one was aware of it until well after the invention of the telescope.

The ancient Greeks (400 BCE) also cleverly measured the distance to our moon, determining it was about 60 earth diameters. What they did not know, until Eratosthenes figured it out about 200 BCE, was just how big the earth was. Impressively big, but not nearly as big as Jupiter, or the sun. Our sun is huge and vast, and very far away, though not nearly as far as any other star in our galaxy, and the myriads of galaxies far beyond.