Jupiter is visible without a telescope and is the fourth-brightest object in the sky after the sun, the moon and the planet Venus. It is three times brighter than the brightest star, Sirius.
The first person to observe Jupiter through a telescope was the scientist Galileo Galilei. He also observed Jupiter's four largest moons orbiting the planet, and this helped refute the geocentric theory that everything revolved around the Earth, which was the belief held by the Catholic church at that time. Galileo came out in favor of the Copernican theory that the Earth, Jupiter and the other planets revolved around the sun. For this he was persecuted by the church, forced to recant this theory and sentenced to house arrest where he remained until he died.