1. Origin of Modern Astronomy CHAPTER 22

2. Early Astronomy  Ancient Greeks  Golden Age of early Astronomy (600 B.C. – 150 A.D.)  Aristotle conclude the Earth was round because it made a curved shadow as it passed the moon and sun (eclipse)

3. Early Astronomy  Ancient Greeks cont…  Eratosthenes used the angles of the noonday sun in two cities to measure the circumference of the Earth  The angles differed by 7 degrees which is 1/50 of a complete circle so he determine the distance between the two cities was 1/50 the circumference of the Earth  His estimate was about 39,400 kilometers – darn close to the correct answer of 40,075 kilometers

4. Early Astronomy  Ancient Greeks continued…  Hipparchus  Determined the location of over 850 stars  Divided them into 6 groups based on brightness  Measured the length of the year (got it to within minutes of our current calculation)  Developed a method to predict the time of lunar eclipses

5. Early Astronomy  Geocentric Model  Believed Earth was stationary and at the center of the universe  Sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn were known

6. Early Astronomy  Ancient Greeks continued…  Heliocentric Model  Aristarchus (321-230 B.C.)  Sun centered universe  Calculated distance between Earth and the Sun as well as Earth and the moon (he was far off though)

7. Early Astronomy  Ancient Greeks continued…  Ptolemaic System  Claudius Ptolemy  Calculated future movement of the planets but he had the planets moving around a stationary Earth  Planets look like they go east, stop, go west, then go east again – retrograde motion

8. Modern Astronomy  Nicolaus Copernicus  1473-1543Polish  Felt the movement of the planets were easier to explain if Earth also moved – making Earth a planet  Put the sun back at the center of the solar system  Had all the planets orbiting in circles

9. Modern Astronomy  Tyco Brahe  1546-1601  Danish  Measured location of planets, especially Mars

10. Modern Astronomy  Johannes Kepler  1571-1630 (Brahe’s student)  Three Laws of Planetary Movement  Planets move in an ellipse around the sun  Planets travel faster when closer to the sun and slower when further from the sun  Length of time it takes to orbit the sun is proportional to it’s distance from the sun  Astronomical unit – distance between Earth and sun (150 million kilometers)  Planet orbital period (P) squared = mean solar distance (a) cubed

11. Modern Astronomy  Galileo Galilei  1564-1642 – Italian - used a telescope to discover  Four moons around Jupiter  Planets were circular disks not points of light  Venus has phases just like the moon (so it orbits the sun)  The moon’s surface is not smooth  Mountains and plains which he thought were lakes  Sun had spots  Determined the sun rotates

12. Modern Astronomy  Sir Isaac Newton  1642-1727  Inertia – an object in motion will continue to move at a constant speed and in a straight line  Universal gravitation  Every body attracts every other body with a force that is directly proportional to their masses and inversely proportional to the square of the distance between them  That means gravitational force decreases with distance  Also means the larger the mass the greater the gravitational force  Gravity plus inertia give planets an elliptical movement

13. Motions of Earth  Rotation  Spinning on its axis (Day and night)  Revolution  Earth revolves around the sun (107,000 million k/hr)  Perihelion – closest point to the sun (147 million kilometers) – January 3rd  Aphelion – furthest point from the sun (152 million kilometers) - July 4th

14. Motions of Earth  Precession  Earth’s axis rotates over a 26,000 year period – tilt stays the same but the direction changes (so different stars act as the North star)  Entire solar system is moving towards Vega

15. Motions of the Earth-Moon System  Perigree – moons closest point to the Earth  Apogee – moons furthest point from Earth

16. Motions of the Earth-Moon System  Phases of the moon  New moon  Crescent  Waxes to a first quarter phase  Full moon  Wanes to a new moon again  Due to the sunlight reflected off the surface of the moon (half is lit all the time) and motion of the moon (when the moon is between the Earth and Sun we get the dark side)

17. Motions of the Earth-Moon System  Lunar Motions  Synodic month – the 29 ½ days it takes to get through the phases of the moon  It really only takes 27 1/3 days but Earth is also moving so it takes another couple days for the moon to get back to its starting position  Moon revolves around it’s axis in 27 1/3 days so the same side is always facing Earth

18. Eclipses  Solar Eclipse  Moon is between Earth and the sun and casts a shadow on Earth  Lasts less than 10 minutes

19. Eclipses  Lunar eclipse  Moon moves into Earth’s shadow  The moon is coppery red because Earth’s atmospshere bends and transmits red light  Can last 4 hours  Doesn’t happen every month because the moon is about 5 degrees off the plane between the Earth and Sun

20. Earth’s Moon  Diameter of 3,475 kilometers (1/4 the size of Earth)  1969-1972 – six Apollo space crafts landed on the moon  Less dense than Earth (less iron in the core)  Gravitational attraction is 1/6 of Earth

21. Earth’s Moon  Lack of atmosphere means space particle hit the moon  Craters – round depressions  Rays – splash mark radiating out from a crater  Highlands – pitted area with mountains  Maria – dark smooth areas  Where basaltic magma was released  Rilles are long channels that could be old lava flows  Regollith – grey debris from meteorites like makes a soil like cover

22. Lunar History  Earth was hit by something about the size of Mars which cause debris to fly off  The debris came to together to make the moon  That’s why the moon contains so little iron (it is made mostly of crust and mantle)  The highlands are the oldest parts of the moon – it has been hit by the most debris  The maria basins formed next – lava flowed over the highlands  The most recent formation are the rayed craters