Sun spots are the parts on the solar surface which has temperature nearly 4500 K. this temperature is 1500 K less than the temperature of the surrounding photosphere. So these regions are somewhat cool then the other parts. These sun spots are 150 kilometers away from the earth or we can say 93 million miles away. They will only appear dark. If we remove these spots from the sun we would be able to see complete glowing disc.
Sun spots were noticed earlier also but almost all the records kept by the ancient people were lost. Galileo says that sun spots were not any different body but they are the region of sun itself. They can be a cloud or a spot. But Scheimer not agreed with Galileo he says that these spots may be some planets or moon revolving around the sun. Schiemer was a Jesuit priest and believed that sun is a perfect creation of god so how can it has spots on the surface.
Now Galileo noticed after few days that the sun spots became small in size as they reach to the edges. So he came to a conclusion that they are surely on the surface of sun. he was not sure that what exactly they are but he says that these spots are not any planets or moons.
The largest sun spot on the sun is 50,000 kilometers in its diameter which can be seen from naked eyes. Generally sun spots remain in groups. They remain at a place for months, weeks, days or hours and they move away by sun rotation. Sun is not a solid body so it is rotating and cannot be actually measured. A sun spot has a dark region in the centre which is called the umbra and less darker on the sides is called the penumbra.