Discoveries and Opinions of Galileo

Galileo Galilei

Nonfiction | Book | Adult | Published in 1957

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Index of Terms

Apogee

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.

Copernican System/Heliocentrism

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

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 atmosphere but rather on or near the surface of the sun.

Perigee

Perigee is the point in the orbit of a heavenly body (e.g., the moon) at which it is nearest to the earth. Galileo uses the term (along with its opposite, apogee) in “Letters on Sunspots.”

Ptolemaic System

The Ptolemaic System refers to an outmoded conception of the universe based on the teachings of ancient Greek mathematician and astronomer Ptolemy (fl. AD 127-151). Ptolemy believed that the earth was the stationary center of the universe and that the sun and moon revolved around it in epicyclic orbits. The Ptolemaic system is thus a geocentric (earth-centered) model of the universe. This model prevailed until the discoveries of Copernicus displaced it.