Earth
Science, 10th edition
Chapter
20: Origin of Modern Astronomy
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 (planetai)
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 Sun-centered, 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.
Geocentric model
c. 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. 1500s and
1600s
2. Five noted
scientists
a. 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
b. 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
c. 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 orbital period and its
distance
to the Sun
(measured in astronomical units (AUs) – one AU averages about
150 million kilometers, or 93 million miles)
d. Galileo Galilei (1564-1642)
1. Supported Copernican theory
2. Used
experimental data
3.
Constructed an astronomical telescope in 1609
4. Galileo's discoveries using the
telescope
a.
Four large moons of Jupiter
b.
Planets appeared as disks
c.
Phases of Venus
d.
Features on the Moon
e.
Sunspots
5. Tried
and convicted by the Inquisition
e. 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
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
2. Sidereal
day – the time it takes for Earth to make one complete rotation (360 )
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 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 Moon orbits Earth in a counter-
clockwise
(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
292 days
b. Sidereal
month
1. True
period of the Moon's revolution around Earth
2. Takes
27 a 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,
27a 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
eclipse
1. Moon
moves in a line directly between Earth and the Sun
2. Can
only occur during the new-Moon phase
b. Lunar
eclipse
1. Moon
moves within the shadow of Earth
2. Only
occurs during the full-Moon 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