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Download Which of the following represent the best explanation we currently
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Which of the following represent the best explanation we currently have? a) Einstein’s Theory of Relativity b) Newton’s Law of Gravitation The concept of theory, as used in science, is often misunderstood. ! A theory consists of a set of basic principles. ! These principles are often widely accepted. Early models of the universe attempted to explain the motion of the five visible planets against the background of “ fixed” fixed” stars. The main problem was that the planets do not move uniformly against the background of stars, but instead appear to stop, move backward, then move forward again. This backward motion is referred to as retrograde motion. The Path of Mars in 2005-2006 and 2009-2010 Ptolemy explained this motion using a geocentric (Earthcentered) model of the solar system in which the planets orbited the Earth indirectly, by moving on epicycles which in turn orbited the Earth. Geocentric Explanation of Retrograde Motion 1 Nicolaus Copernicus developed the first heliocentric (sun-centered) model of the solar system. In this model, the retrograde motion of Mars is seen when the Earth passes Mars in its orbit around the Sun. A Heliocentric Explanation of Planetary Motion We define special positions of the planets in their orbits depending where they appear in our sky. For example, while at a conjunction, a planet will appear in the same part of the sky as the Sun, while at opposition, a planet will appear opposite the Sun in our sky. However, the cycle of these positions (a synodic period) is different from the actual orbital period of the planet around the Sun (a sidereal period) because both the Earth and the planet orbit around the Sun. 2 When a new “star” appeared in the sky during the 16th century, a Danish astronomer named Tycho Brahe reasoned that the distance of the object may be determined by measuring the amount of parallax. The apparent change in the location of an object due to the difference in location of the observer is called parallax. Because the parallax of the “star” was too small to measure, Tycho knew that it had to be among the other stars, thus disproving the ancient belief that the “heavens” were fixed and unchangeable. After Tycho Brahe’ Brahe’s death, Johannes Kepler (pictured here with Tycho in the background) used Tycho’ Tycho’s observations to deduce the three laws of planetary motion. 3 KEPLER’S THREE LAWS OF PLANETARY MOTION LAW #1. The orbit of a planet around the Sun is an ellipse with the Sun at one focus. The amount of elongation in a planet’ planet’s orbit is defined as its orbital eccentricity. An orbital eccentricity of 0 is a perfect circle while an eccentricity close to 1.0 is nearly a straight line. In an elliptical orbit, the distance from a planet to the Sun varies. The point in a planet’ planet’s orbit closest to the Sun is called perihelion, and the point farthest from the Sun is called aphelion. KEPLER’S THREE LAWS OF PLANETARY MOTION LAW #2: A line joining the planet and the Sun sweeps out equal areas in equal intervals of time. Planet moves Planet moves faster in its orbit when closer to the Sun. Kepler's First and Second Laws slower in its orbit when farther away from the Sun. 4 KEPLER’S THREE LAWS OF PLANETARY MOTION LAW #3: The square of a planet’ planet’s sidereal period around the Sun is directly proportional to the cube of its semi-major axis. This law relates the amount of time for the planet to complete one orbit around the Sun to the planet’s average distance from the Sun. If we measure the orbital periods (P) in years and distances (a) in astronomical units, then the law mathematically can be written as P2 = a3 . Planetary Orbits Galileo was the first to use a telescope to examine celestial objects. His discoveries supported a heliocentric model of the solar system. Galileo discovered that Venus, like the Moon, undergoes a series of phases as seen from Earth. In the Ptolemaic (geocentric) model, Venus would be seen in only new or crescent phases. However, as Galileo observed, Venus is seen in all phases, which agrees with the Copernican model as shown. Galileo also discovered moons in orbit around the planet Jupiter. This was further evidence that the Earth was not the center of the universe. 5