What instrument is used on Earth to study parts of the universe not visible to the naked eye?
It was that the sun was the center of our universe (Heliocentric Theory) and that the earth travels on three axis; revolves around the sun, spins on its axis creating day and night, and up and down on its axis causing all of our seasons. Also that the order of the planets was; Mercury, Venus, Earth and moon, Mars, Jupiter, and Saturn. all planets visible to the naked eye.
There are other galaxies visible from Earth. Among them are the SMC (Small Magellanic Cloud) and LMC (Large Magellanic Cloud), and the Andromeda Galaxy (M31). These are just the ones visible to the naked eye at nighttime, of course. There are over 100 billion visible by other means, including telescopes used on ground or orbiting Earth.
There are many more people on earth than there are stars in the sky that are visible to the naked eye, even if you include all the stars that are ever visible from any spot on earth and not just the ones that you can see. However, if you include all the stars that we know about that are not visible to the naked eye, there are many more stars, by many orders of magnitude…
To the ancients the earth was the centre of the universe and the sun and other stars appeared to rotate around it. In contrast, the five planets that are visible to the naked eye seemed to 'wander' about the sky and were assigned Greek names amounting to wandering stars which have been corrupted to 'planet' in English. Please see the link.
All of the planets except Pluto, which is no longer officially a planet, can be seen with the naked eye from Earth. Depending on their position in orbit, all the planets, except Neptune, are visible to the naked eye with proper viewing conditions. Neptune can be seen with telescopes or even strong binoculars.