Apogee is the point in the orbit of a heavenly body (e.g., the moon) at which it is farthest from the earth. Galileo uses the term (along with its opposite, perigee) toward the end of “Letters on Sunspots” in explaining why Jupiter’s stars sometimes look bigger and sometimes look smaller. He states that the reason cannot be because of the stars’ distance from the earth at perigee or apogee.

The Copernican System, or heliocentrism, refers to a conception of the universe based on the ideas of Nicolaus Copernicus (1472-1542). In contradiction to the earlier Ptolemaic system, Copernicus theorized that the sun was the center of the universe and that the earth and other planets revolved around it. It is thus also known as the heliocentric (sun-centered) model of the universe. Galileo’s belief in and advocacy for the Copernican system underlies much of his writing in Discoveries and Opinions of Galileo.

Parallax is the apparent displacement of an observed object due to a change in the position of the observer. Galileo uses the term in “Letters on Sunspots” to refer to the location of sunspots. He states that the fact that sunspots exhibit no parallax shows that they are not in the earth’s