Galileo Galilei was an Italian physicist and astronomer in the 17th century. He was a strong proponent of heliocentrism, or the belief that the Earth revolves around the sun and not vice-versa. This went against Roman Catholic teaching at the time and lead to Galileo being tried and convicted by the Church on the grounds of heresy. One argument that would have helped him in his trial is how the position of the sun, planets, and stars in the sky change over the course of a year. Not only that, but they change with a regular pattern and return to the same starting position again after exactly one year. This is much more readily explained with orbital movement of the Earth than with a stationary Earth. Also, one argument used against Galileo was the lack of observable stellar parallax. Stellar parallax is the apparent shift in the observational position of stars relative to one another due to the movement of the Earth through space. But this was not observable in the 17th century due to the relatively crude telescopes at the time. The distances between stars are so great and the relative angles of observation so small that more powerful telescopes coupled with film photography of the sky were required many years later to properly observe stellar parallax.