Chapter 22
Origin of Modern
Astronomy
Section 1
Early Astronomy
Notes 22-1
Vocabulary
 Astronomy
 Geocentric
 Orbit
 Heliocentric
 Retrograde motion
 Ellipse
 Astronomical unit (AU)
Ancient Greek
 Golden Age (600 BC – AD 150)
 Used philosophical arguments to explain the heavens
 Developed the basics of geometry and trigonometry
 Mathematics helped them measure sizes and
distances to the sun and moon
Ancient Greek
 Aristotle (384 – 322 BC)
 Said that Earth is round because it casts a curved
shadow on the moon when it passes between the
sun and the moon
 Idea was ignored in the Middle Ages
 Eratosthenes (276 – 194 BC)
 First attempt to determine the Earth’s size
 Observed the angle of the sun at noon at two
Egyptian cities (Syene {now Aswan} and Alexandria)
Aristotle
Eratosthenes
 Because the sun angles
were different by 7
degrees (1/50 of a circle),
Earth must be 50 times
the distance between the
two cities.
 He came up with a
distance of 39,400 km.
 Actual measure of Earth’s
dimension is 40,075 km.
Ancient Greek
 Hipparchus (second century BC)
 Greatest of Greek astronomers
 Best know for his star catalog
 Determined the location of 850 stars
 Divided them into six groups according to
their brightness
Geocentric Model
 Greeks believed in an earth centered solar system.
 Geocentric believes Earth is unmoving in the center and
all bodies move around it.
 Planets are called wanderers because they move frequently
 Beyond the planets is the celestial sphere where all the
stars travel
 This helped explain the movements of all the celestial
bodies
 Not correct
Heliocentric Model
 Aristarchus (312 – 230 BC)
 Was the first to propose a sun-centered model of the
solar system
 He used geometry to calculate the distance to the
sun and moon relative to the earth
 His true measurements were too small but learned
the sun was larger than the earth and the moon was
smaller than the earth.
 This model was largely ignored for 2000 years
Ptolemaic System
 In AD 141, presented his model of the solar
system
 Used a geocentric model and explained the
motion of the planets.
 Retrograde motion is when a planet appears to stop
in its path and reverse direction.
 Used a epicycle to explain this motion
 Small circular paths on the overall path around the
earth
 Not correct
Epicycles
Modern Astronomy
 Nicolaus Copernicus: (1473 – 1543)
 Polish astronomer
 Claimed Earth was a planet
 Used a heliocentric view of the solar system
 Used perfect circles to describe the path of the
planets
 This was not complete because the planets didn’t
appear to stay in their proposed paths
Modern Astronomy
 Tycho Brahe: (1546 – 1601)
 Danish Astronomer
 Used precise measurements made by his devices to
determine the path of the planets
 Measured planets for 20 years
 Johannes Kepler: (1571 – 1630)
 German Astronomer
 Brahe’s assistant
 Took Brahe’s observations and created three
mathematical laws of planetary motion
Law of Ellipses
 Kepler’s first law:
 Each planet orbits the sun in a path called an
ellipse
 Oval shape
 Determined by two points in the oval
 Points are called foci (singular: focus)
 If you draw a line from any point on the oval to the
foci; the length of the lines will be the same
 Circle is a special ellipse where the two foci are at
the same spot
Law of Ellipses
 Planet is therefore not always the same distance
from the sun
 Perihelion: closest to the sun
 Aphelion: farthest from the sun
 Distance from the sun averages the two
distances together
 Average distance from the earth to the sun is 149.5
million km
 Called an astronomical unit (au)
 Used to measure the distance from the sun to other
planets
Circle where both foci
are at the same point.
Ellipse where the
sun is at one focus.
Law of Equal Areas
 Kepler’s second law:
 Area of a long, thin sector is the same as an area of a
short, wide sector
 Describes the speed planets travel at different
spots in their orbits
 Earth moves fastest when closest to the sun
 Earth moves slower when farther from the sun
 The sun is off center in Earth’s orbit
 The triangles, made by the sun and two points on the
orbit of Earth, will have the same areas
Equal Area
SUN
Equal Area
Law of Periods
 Kepler’s third law:
 Describes the relations between the average
distance of a planet to the sun to its orbital
period
 Orbital period: time it takes for one trip around the
sun
 K x d3 = T2
 K is a constant equal to 1 when AU’s are used
Law of Periods
 Example:
 Jupiter is 5.2 AU’s from the sun (radius) = d
 Its period is 11.9 years (distance) = T
 K x d3 = T2
 1 x (5.2)3 = (11.86)2
 140.6 = 140.6
Modern Astronomy
 Galileo Galilei: (1564 – 1642)
 Using his telescope, he made important discoveries to
support the heliocentric model
1. Discovery of 4 satellites orbiting Jupiter

Disproved that Earth was the only center of motion in the
universe
2. Planets are circular disks

Not point of light as previously thought
3. Venus has phases just like the moon

Therefore it also moves around the sun
4. Moon’s surface is not smooth

Thought objects were smooth and perfect
5. Sun has sunspots

Sun is not perfect and shows sun rotates too
Galileo
Modern Astronomy
 Sir Isaac Newton (1642 – 1727)
 Inertia: a moving object will stay in its path and at a
constant motion
 Gravity is the force that changes speed and path
 Law of Universal Gravitation
 Every body attracts every other body with a force that is
directly proportional to their mass and inversely
proportional to the square of the distance between their
centers
 The greater the mass of the object, the greater the
gravitational force
 Weight is not the same as mass
 Weight varies with gravity and mass stays the same
Sir Isaac Newton
 Force of gravity, combined with the tendency of a
planet to move in a straight line results in an elliptical
orbits.
 Combination of Earth’s forward motion and its falling
motion that defines its orbit
 If gravity stopped, Earth would move out in a straight line into
space.
 If forward motion stopped, Earth would fall into the sun.
Newton
Homework
Assessment Section 22-1
1-7
Complete Sentences