CHAPTER 22 ORIGIN OF MODERN ASTRONOMY

EARLY ASTRONOMY The Earth is one of the planets and many smaller bodies that orbit the sun The sun is part of a much larger family of perhaps 400 billion stars that make up our galaxy, the Milky Way There are billions of galaxies in the universe

ANCIENT GREEKS Astronomy  the science that studies the universe It deals with the properties of objects in space and the laws under which the universe operates Aristotle ( B.C.) Concluded the Earth was round because of the curved shadow it cast on the moon when it passed between the sun and moon Eratosthenes ( B.C.) First successful attempt to establish the size of the Earth Hipparchus (2 nd century B.C.) Determined the location of almost 850 stars

GEOCENTRIC MODEL Geocentric  Earth is a sphere that stays motionless at the center of the universe In the geocentric model, the moon, sun, and the known planets – Mercury, Venus, Mars, and Jupiter – go around the Earth Orbit  the path of an object as it goes around another object in space

HELIOCENTRIC MODEL Heliocentric  the sun is the center of the universe First proposed by Aristarchus ( B.C.) Used geometry to calculate the size of the sun and planets that led to this theory In this model, Earth and the other planets orbit the sun

PTOLEMAIC SYSTEM Claudius Ptolemy Much of our knowledge of Greek astronomy derives from his work He noticed that the movement of other planets seems to stop and reverse direction when compared to the stars Retrograde Motion  the apparent backwards drift of the planets as they move This results from the combination of the motion of Earth and the planet’s own motion around the sun

THE BIRTH OF MODERN ASTRONOMY Nicolaus Copernicus ( A.D.) Polish scientist considered the first great astronomer to emerge after the Middle Ages Copernicus concluded that Earth is a planet He proposed a model of the solar system with the sun at the center He used circles to represent the orbits of planets

THE BIRTH OF MODERN ASTRONOMY Tycho Brahe ( A.D.) Danish nobility that became interested in astronomy while viewing a solar eclipse Had an observatory built before the telescope had even been invented Designed and built instruments the measure the locations of the heavenly bodies Brahe’s observations, especially of Mars, were far more precise than any made previously

THE BIRTH OF MODERN ASTRONOMY Johannes Kepler ( A.D.) Discovered three laws of planetary motion 1. The path of each planet around the sun is an ellipse (oval-shaped path), with the sun at one focus 2. Each planet revolves so that an imaginary line connecting it to the sun sweeps over equal areas in equal time intervals 3. The square of the length of time it takes a planet to orbit the sun is proportional to the cube of its mean distance to the sun Astronomical Unit (AU)  the average distance between Earth and the sun (about 150 million km)

THE BIRTH OF MODERN ASTRONOMY Galileo Galilei ( A.D.) The greatest Italian scientist of the Renaissance Galileo’s most important contributions were his descriptions of the behavior of moving objects Galileo constructed his own telescope and used it to study the sky Discovery of four satellites (moons) orbiting Jupiter Discovery that planets are circular discs Discovery that Venus has phases like the moon Discovery that the moon’s surface was not smooth Discovery that the sun had sunspots (dark regions)

THE BIRTH OF MODERN ASTRONOMY Sir Isaac Newton ( A.D.) Born the year of Galileo’s death Many scientists had tried to explain the forces involved in planetary motion Galileo proposed inertia  a moving object will continue to move at a constant speed and in a straight line Although others had theorized the existence of an unknown force, Newton was the first to formulate and test the law of universal gravitation

UNIVERSAL GRAVITATION Universal Gravitation Every body in the universe attracts every other body with a force that is directly proportional to their masses and inversely proportional to the square of the distance between their centers of mass Mass  a measure of the total amount of matter an object contains Weight  the force of gravity acting upon an object Gravitational force decreases with distance

MOTIONS OF EARTH The two main motions of Earth are rotation and revolution Rotation  the turning, or spinning, of a body on its axis Revolution  the motion of a body, such as a planet or moon, along its orbit around some point in space Earth also has another very slow motion Precession  the slight movement, over a period of 26,000 years, of Earth’s axis

REVOLUTION Earth revolves around the sun in an elliptical orbit at an average speed of 107,000 km per hour Its average distance from the sun is 150 million km, but since the orbit is an ellipse, the distance varies Perihelion  Earth is closest to the sun (about 147 million km); occurs about Jan 3 every year Aphelion  Earth is farthest from the sun (about 152 million km); occurs about July 4 every year

MOTIONS OF THE EARTH-MOON SYSTEM The Earth has one satellite, the moon, which also follows an elliptical revolution around the Earth Average distance of 384,401 km from Earth Perigee  the moon is closest to Earth Apogee  the moon is farthest from Earth

PHASES OF THE MOON Phases of the Moon  a change in the amount of the moon that appears lit New moon – no moon lit Crescent – only small sliver present Quarter – half moon lit Gibbous (waxing and waning) – more than quarter but less than a full moon Full – entire moon lit Lunar phases are caused by the changes in how much of the sunlit side of the moon faces Earth

ECLIPSES Solar Eclipse  when the moon moves in a line directly between earth and the sun, it casts a dark shadow on Earth Lunar Eclipse  the moon is eclipsed when it moves within Earth’s shadow During a new-moon or full-moon phase, the moon’s orbit must cross the plane of the ecliptic for an eclipse to take place

EARTH’S MOON The Lunar Surface Craters  round depressions in the surface of the moon The largest are about 250 km in diameter, about the width of Indiana Most craters were produced by the impact of rapidly moving debris or meteoroids A 3m meteoroid can blast out a 150m wide crater Rays  splash marks that radiate outward for hundreds of km

EARTH’S MOON Lunar Surface Mare  a dark, relatively smooth area on the moon’s surface (plural: maria) Maria, ancient beds of basaltic lava, originated when asteroids punctured the lunar surface, letting magma “bleed” out Rilles  long channels that look similar to valleys or trenches; may be the remnants of ancient lava flows

EARTH’S MOON Lunar Surface Lunar Regolith  layer of gray debris derived from a few billion years of bombardment from meteorites Soil-like layer composed of igneous rocks, glass beads, and fine lunar dust Just over 3 meters thick

LUNAR HISTORY The most widely accepted model for the origin of the moon is that when the solar system was forming, a body the size of Mars impacted Earth Huge quantities of mantle rock were ejected into Earth’s orbit, and eventually coalesced into the moon due to the immense gravitational force of the Earth’s mass