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Date Created: 02/23/16
PHYS 104: Astronomy Week 1 Course Notes, 2/1 – 2/5 Definitions Celestial Sphere: The apparent sphere surrounding Earth upon which all celestial objects appear. Zenith: The point on the celestial sphere directly overhead. Meridian: An imaginary line running from North to South through the Zenith. Celestial Equator: Aprojection of the Earth's equator onto the celestial sphere. Celestial Hemisphere: The half of the sphere one can see from one's vantage point. The Ecliptic: The apparent path of the sun against the background stars over the course of a year. Solar Day: The amount of time it takes the sun to return to meridian. Sidereal Day: The amount of time it takes a background star to return to meridian. I. What do we see from Earth and why? A. The Model • Earth is a sphere • We are on the Earth's surface • Earth spins around its axis one time per day • The sphere orbits the sun one time per year • Earth's axis of rotation is tilted with respect to the sun and Earth's orbit • Other planets orbit the sun in the same plane B. How do stars change with respect to our position on earth? • The North Star – Astar which the north axis of Earth points directly towards. – This star is at zenith at the north pole. • The visibility of the celestial sphere changes with respect to one's position on earth: everyone's celestial hemisphere is slightly different. Along exposure photo of the stars can show the path each visible star travels in one night, or multiple nights. If a photograph showed the stars rising and setting perpendicular to the horizon, traveling in large arcs, this means that the photographer was on the equator. Everywhere else, the stars rise and set at an angle to the horizon. Our perspective of the stars behind the sun (if the were visible during the day) changes with respect to where the Earth is and what time of year it is. PHYS 104: Astronomy Week 2 Course Notes, 2/8 – 2/12 Definitions 1A.U. (Astronomical Unit): The distance from the Earth to the sun. Perihelion: The closest point to the sun on an elliptical orbit. Aphelion: The furthest point from the sun on an elliptical orbit. Scientific Hypothesis: Aconjecture that must be falsifiable in order to be valid. Scientific Theory: Ahypothesis shown to be valid by experimental evidence. Epicycle: The retrograde motion of a planet orbiting something else from view on Earth. Geocentric: Having Earth at the center. Heliocentric: Having the sun at the center. II. Epicycles (circles within circles) A. Asmall circle which moves on the circumference of a larger circle, at whose center is Earth. B. Most medieval models of the solar system contained epicycles to explain the odd movement of planets. They hadn't yet figured out that these planets orbit the sun. Geocentric Model Heliocentric Model C. Retrograde motion When viewed from Earth, a planet whose orbit is outside Earth's, such as Mars, will appear to move backwards in the sky for a certain period of time before continuing its predicted motion. This is called retrograde motion, and occurs whenever Earth moves past a particular planet.
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