Understanding the Sky: Phases of the Moon and Eclipses - Prof. Joshua Ridley, Study notes of Astronomy

Download Understanding the Sky: Phases of the Moon and Eclipses - Prof. Joshua Ridley and more Study notes Astronomy in PDF only on Docsity! © 2010 Pearson Education, Inc. Chapter 2 Understanding the Sky A quick review To Polaris » \ \ 23,5° \ a \ \ > Precession I -recession (©2007 Thamsan Hiner Ecveaton © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. Why do we see phases of the Moon? • Lunar phases are a consequence of the Moon’s 27.3-day orbit around Earth. © 2010 Pearson Education, Inc. Phases of the Moon • Half of Moon is illuminated by Sun and half is dark. • We see a changing combination of the bright and dark faces as Moon orbits. Moon Rise/Set by Phase ae a — _ Show Horizon Show time of day © 2010 Pearson Education, Inc. © 2010 Pearson Education, Inc. A. first quarter B. waxing gibbous C. third quarter D. half moon It’s 9 a.m. You look up in the sky and see a moon with half its face bright and half dark. What phase is it? Thought Question © 2010 Pearson Education, Inc. The Moon and the Earth are always half lit, except.... © 2010 Pearson Education, Inc. What causes eclipses? • The Earth and Moon cast shadows. • When either passes through the other’s shadow, we have an eclipse. © 2010 Pearson Education, Inc. Lunar Eclipses Solar Eclipse Simulation of a Total Eclipse of the Sun ¢ View from Earth View from space (not drawn to scale) © 2010 Pearson Education, Inc.  © 2010 Pearson Education, Inc. Solar Eclipses Total Eclipse Annular Eclipse Partial Eclipse © 2010 Pearson Education, Inc. Summary: Two conditions must be met to have an eclipse: 1. It must be full moon (for a lunar eclipse) or new moon (for a solar eclipse). AND 2. The Moon must be at or near one of the two points in its orbit where it crosses the ecliptic plane (its nodes). © 2010 Pearson Education, Inc. Predicting Eclipses • Eclipses recur with the 18-year, 11 1/3-day saros cycle, but type (e.g., partial, total) and location may vary. © 2010 Pearson Education, Inc. What have we learned? • Why do we see phases of the Moon? – Half the Moon is lit by the Sun; half is in shadow, and its appearance to us is determined by the relative positions of Sun, Moon, and Earth. • What causes eclipses? – Lunar eclipse: Earth’s shadow on the Moon – Solar eclipse: Moon’s shadow on Earth – Tilt of Moon’s orbit means eclipses occur during two periods each year. © 2010 Pearson Education, Inc. What was once so mysterious about planetary motion in our sky? • Planets usually move slightly eastward from night to night relative to the stars. • But sometimes they go westward relative to the stars for a few weeks: apparent retrograde motion. © 2010 Pearson Education, Inc. We see apparent retrograde motion when we pass by a planet in its orbit. © 2010 Pearson Education, Inc. Explaining Apparent Retrograde Motion • Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus laps us). • But very difficult to explain if you think that Earth is the center of the universe! • In fact, ancients considered but rejected the correct explanation. © 2010 Pearson Education, Inc. What have we learned? • What was so mysterious about planetary motion in our sky? – Like the Sun and Moon, planets usually drift eastward relative to the stars from night to night, but sometimes, for a few weeks or few months, a planet turns westward in its apparent retrograde motion. • Why did the ancient Greeks reject the real explanation for planetary motion? – Most Greeks concluded that Earth must be stationary, because they thought the stars could not be so far away as to make parallax undetectable. © 2010 Pearson Education, Inc. Homework #2 - Due Tuesday, January 25th, 2011