Transcript Chapter 22 Origin of modern astronomy

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 (384-322 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 (276-194 B.C.)
• First successful attempt to establish the size of the Earth
• Hipparchus (2nd 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 (312-230 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 (1473-1543 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 (1546-1601 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 (1571-1630 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 (1564-1642 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 (1642-1727 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