1. History of Astronomy

2. Our Universe Earth is one of nine planets that orbit the sun The sun is one star in 100 billion stars that make up our galaxy- The Milky Way Our galaxy is one of billions of galaxies in the universe Views of our galaxy and Universe have changed tremendously throughout time

3. Early Astronomy Astronomy- science that studies the universe Astronomy- science that studies the universe Deals with properties of objects in space and the laws through which the universe operates Deals with properties of objects in space and the laws through which the universe operates

4. Early Astronomy- Ancient Greeks Aristotle (384-322 B.C.) Aristotle (384-322 B.C.) – Concluded that Earth was round from observations of the curved shadow on the moon when Earth passes between sun and moon Eratosthenes (276-194 B.C.) Eratosthenes (276-194 B.C.) – Calculated circumference of the Earth Used angles of noonday sun in two locations on the planet Used angles of noonday sun in two locations on the planet

5. Early Astronomy- Ancient Greeks Hipparchus (2 nd century B.C.) – Determined the location of 850 stars and categorized them according to brightness – He measured the length of a year and developed a method for predicting lunar eclipses

6. Models of the Universe Greeks believed in the Geocentric model of the universe Earth is the center and planets, sun, and moon orbit around Earth Aristarcus (312-230 B.C.) – First to propose a heliocentric model- Earth and other planets orbit the sun Still- the Geocentric model dominated for nearly 2,000 years

7. Ptolemaic System Claudius Ptolemy published the Ptolemaic system in 141 A.D. This system accounted for the movements of the planets, but was still a Geocentric model which was determined to be inaccurate Ptolemy discovered retrograde motion

8. Retrograde Motion Planets appear to stop, reverse direction, and resume normal motion

9. Modern Astronomy Nicolaus Copernicus (1473-1543) – Concluded that Earth was a planet and proposed a model of the solar system with the sun at the center (Heliocentric) – Tycho Brahe (1546-1601) Used instruments to measure the locations of “heavenly bodies” His observations were far more precise than any made before Johannes Kepler started his astronomy career as an assistant to Brahe

10. Modern Astronomy Johannes Kepler (1571-1630) 3 laws of planetary motion – Discovered that planetary orbit around the sun is not in a circle but an ellipse (oval shape) – Also discovered that speed of planets changes as revolve around the sun Faster as they get closer to the sun (perihelion) slower as it gets further from the sun (aphelion)

11. Kepler’s 3 Laws of Planetary Motion Law 1 – The path of each planet around the sun is an ellipse, with the sun at one focus, and other focus symmetrically opposite

12. Kepler’s 3 Laws of Planetary Motion Law 2 – Each planet revolves around the sun so that it sweeps over equal areas in equal time intervals- therefore planets travel faster as they near the sun

13. Kepler’s 3 Laws of Planetary Motion Solar distances of the planets can be calculated when their periods of revolution are known Distances are expressed in astronomical units (AU)

14. Galileo Galilei (1564-1642) Created the first telescope in 1609- previous to this invention all astronomical discoveries were made without aid of a telescope – Telescope magnified objects 3X

15. Galileo’s Discoveries Four moons orbiting Jupiter – Disproved that Earth was center of universe Planets are circular disks and Earthlike – Disproved that planets were points of light Venus has phases like the moon – Shows that Venus orbits its light source (the sun) Moons surface is not smooth Sun has sunspots, or dark regions – Helped to calculate the rotational period of the sun

16. Sir Isaac Newton (1642-1727) 1.Was the first to formulate and test the law of universal gravitation – This helps to explain why planets move in ellipses and not in a straight line Gravity is directly proportional to mass of an object and inversely proportional to distance Greater mass = Greater gravitational attraction Greater distance = less gravitational attraction – Gravity Pulls objects towards the sun = elliptical orbits