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Chapter 19 Earth's Place in the Universe

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I.  Early history of astronomy

A. Ancient Greeks

1. Used philosophical arguments to explain natural phenomena

2. Also used some observational data

3. Most ancient Greeks held a geocentric (Earth-centered) view of the universe

a. "Earth-centered" view

1. Earth was a motionless sphere at the center of the universe

2. Stars were on the celestial sphere

a. Transparent, hollow sphere

b. Celestial sphere turns daily around Earth

b. Seven heavenly bodies ( planentai )

    1. Changed position in sky

     2. The seven wanderers included the

          a. Sun

          b. Moon

          c. Mercury through Saturn ( excluding Earth )

4. Aristarchus ( 312 - 230 B.C. ) was the first Greek to profess a suncentered, or heliocentric, universe

5. Planets exhibit an apparent westward drift

a. Called retrograde motion

b. Occurs as Earth, with its faster orbital speed, overtakes another planet

6. Ptolemaic system

a. A.D. 141

b. To explain retrograde motion, Ptolemy used two motions for the planets

1. Large orbital circles, called deferents, and

2. Small circles, called epicycles

B. Birth of modern astronomy

1. 1500’s and 1600’s

2. Five noted scientists

a. Nicolaus Copernicus

b. Tycho Brahe

c. Johannes Kepler

d. Galileo Galilei

e. Sir Isaac Newton


Nicolaus Copernicus ( 1473-1543)

1. Concluded Earth was a planet

2. Constructed a model of the solar system that put the sun at the center, but he used circular orbits for the planets

3. Ushered out old astronomy


Tycho Brahe ( 1546-1601)

1. Precise observer

2. Tried to find stellar parallax - the apparent shift in a star’s position due to the revolution of Earth

3. Did not believe in the Copernican system because he was unable to observe stellar parallax


Johannes Kepler (1571-1630)

1. Ushered in new astronomy

2. Planets revolve around the sun

3. Three laws of planetary motion

a. Orbits of the planets are elliptical

b. Planets revolve around the sun at varying speed

c. There is a proportional relation between a planet’s orbit period and its distance to the sun ( measured in astronomical units ( Aus ) - one AU averages about 150 million kilometers, or 93 million miles )


Galileo Galilei (1564-1642)

1. Supported Copernican theory

2. Used experimental data

3. Constructed an astronomical telescope in 16094. Galileo’s discoveries using the telescope

4. Galileo’s discoveries using the telescope

a. Four large moons of Jupiter

b. Planet’s appeared as disks

c. Phases of Venus

d. Features on the moon

e. Sunspots

5. Tried and convicted by the Inquisition


Sir Isaac Newton (1643-1727)

1. Law of universal gravitation

2. Proved that the force of gravity, combined with the tendency of a planet to remain in straight-line motion, results in the elliptical orbits discovered by Kepler


II. Constellations

A. Configuration of stars named in honor of mythological characters or great heroes

B. Today 88 constellations are recognized

C. Constellations divide the sky into units, like state boundaries in the U.S.

D. The brightest stars in a constellation are identified in order of their brightness by the letters of the Greek alphabet - alpha, beta, and so on


III. Positions in the sky

A. Stars appear to be fixed on a spherical shell ( the celestial sphere) that surrounds Earth

B. Equatorial system of location

1. A coordinate system that divides the celestial sphere

2. Similar to the latitude-longitude system that is used on Earth’s surface

3. Two locational components

a. Declination - the angular distance north or south of the celestial equator

b. Right ascension - the angular distance measured eastward along the celestial equator from the position of the vernal equinox


IV Earth motions

A. Two primary motions

1. Rotation

a.Turning, or spinning, of a body on its axis

b. Two measurements for rotation

1. Mean solar day - the time interval from one noon to the next, about 24 hours

2. Sidereal day - the time it takes for Earth to make one complete rotation (360 0) with respect to a star other than the sun - 23 hours, 56 minutes, 4 seconds

2. Revolution

a. The motion of a body, such as a planet or moon, along a path around some point in space

b. Earth’s orbit is elliptical

1. Earth is closest to the sun (perihelion) in January

2. Earth is farthest from the sun (aphelion) in July

c. The plane of the ecliptic is an imaginary plane that connects Earth’s orbit with the celestial sphere


    B. Other Earth motions

1. Precession

a. Very slow Earth movement

b. Direction in which Earth’s axis points continually changes

2. Movement with the solar system in the direction of the star Vega

3. Revolution with the sun around around the galaxy

4. Movement with the galaxy within the universe


V. Motions of the Earth - moon system

A. Phases of the moon

1. When viewed from above the North Pole, the moo orbits Earth in a counterclockwise (eastward) direction

2. The relative positions of the sun, Earth, and moon constantly change

3. Lunar phases are a consequence of the motion of the moon and the sunlight that is reflected from its surface


    B. Lunar motions

1. Earth-moon

a. Synodic month

1. Cycle of the phases

2. Takes 29 1/2 days

b. Sidereal month

1. True period of the moon’s revolution around Earth

2. Takes 27 1/3 days

c. The difference of two days between the synodic and sidereal cycles is due to the Earth-moon system also moving in an orbit around the sun

2. Moon’s period of rotation about its axis and its revolution around Earth are the same, 27 1/3 days

a. Causes the same lunar hemisphere to always face Earth

b. Causes high surface temperature on the day side of the moon

    C. Eclipses

1. Simply shadow effects that were first understood by the early Greeks

2. Two types of eclipses

a. Solar eclipses

1. Moon moves in a line directly between Earth and the sun

2. Can only occur during the new-moon phase

b. Lunar eclipses

1. Moon moves within the shadow of Earth

2. Only occurs during the fullmoon phase

3. For any eclipse to take place, the moon must be in the plane of the ecliptic at the time of new-or full-moon

4. Because the moon’s orbit is inclined about 5 degrees to the plane of the ecliptic, during most of the times of new-and full-moon the moon is above or below the plane, and no eclipse can occur

5. The usual number of eclipses is four per year

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