LESSON 2: MODELS OF THE UNIVERSE

• Astronomy a branch of natural science concern with the study of celestial objects and events.
• Astronomer is a space scientist whose primary responsibility is to do study on the universe and outer space using earth based obsevatories.
• Astronauts is a professional space traveler who has been trained to do certain missions in space.
• Geocentric concept - earth is the center of the universe.
• Heliocentric concept - sun is the center of the universe.
• Eudoxus' Model - Eudoxus of Cnidus, a Greek astronomer and mathematician, was the first to propose a model of the cosmos based on geometry (c. 395-390 B.C.). His model is made up of 27 concentric spheres, with the Earth at its core. The fixed stars, the Sun, the Moon, and the planets all have spheres. Each sphere has a pole that connects it to a bigger sphere. The daily rotation of the heavens is accounted for by the rotation of the spheres on their poles once every 24 hours.
• What are the 4 planets? Mercury, venus, mars, and earth.
• Retrograde motion is the forward and backward movement of a planet proposed by eudoxus.
• Aristotle's model - The model suggested by Eudoxus was examined by Aristotle (born c. 384 B.C.), a Greek philosopher and astronomer, but he saw these spheres as physical things. He believed that the heavenly and everlasting "ether" that caused the spheres to move was inside them. He introduced the Prime Mover as the driving force behind the spheres' movement. His model consisted of 56 spheres that led the Sun, Moon, and five known planets through their orbits. The spheres kept the same distance from the Earth as they moved. They also travelled at a consistent speed.
• Prime mover caused the outermost sphere to rotate at constant angular velocity and this motion was imported from sphere to sphere.
• Aristarchus' model - Aristarchus of Samos, a Greek astronomer and mathematician, was the first to propose that the Sun is the center of the universe (about 310 B.C.). He imagined the Moon orbiting a spherical Earth, which then rotated around the Sun. The lack of stellar parallax - that is, the stars do not change positions relative to each other as the Earth revolves around the Sun led him to conclude that the stars are extremely far away from the Earth.
• Ptolemy's model - Claudius Ptolemy, a Greco-Egyptian astronomer and mathematician, created his own geocentric (Earth-centered) model of the cosmos to explain "Imperfect movements" of celestial bodies. He explained the apparent movements of the planets around the Earth by believing that each planet revolved around an epicycle, which is a sphere. The epicycles center then shifted to a bigger sphere known as a deferent.
• Copernicus' Model - Nicolaus Copernicus, a Renaissance mathematician and astronomer born in Poland, he established that the Sun, not the Earth, is the center of the universe. He reaffirmed the old Greek idea that the motion of spherical celestial bodies is constant, everlasting, and circular in his work. Because the Earth is spherical, he reasoned, its velocity must be circular. He also believed that fixed stars cannot move. The rotation of the Earth causes their apparent movement. Because these stars are so far away from the Earth, there is no discernible parallax.
• Using galileo Galilei' telescope Copernicus said that the sun is the center of the universe.
• Tycho Brahe - Brahe proposed a new model for the universe as a result of his discoveries. All of the planets orbited the sun in Brahe's system, while the sun and moon orbited the Earth. His model permitted the path of the planet Mars to intersect the path of the sun, in accordance with his observations of the new star and the comet.