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Chapter 22: Origin of Modern Astronomy
Section 22.1 Early Astronomy
 Astronomy – the science that studies the universe.
 The “Golden Age” of astronomy was centered in
ancient Greece.
 Aristotle – (384-322 B.C.) a Greek philosopher
that concluded that the Earth was round because
it always cast a curved shadow on the Moon
during an eclipse.
Ancient Astronomy
 Eratosthenes – (276-194 B.C.) the first to
successfully establish the circumference of Earth.
He calculated 39,400 km. It is actually 40,075 km.
Models of the Solar System
 Geocentric Model – Aristotle and other ancient
Greeks believed the Earth was the center of the
Universe and that every thing revolved in orbit
around the Earth.
 Heliocentric Model - Earth and other planets orbit
the Sun, first proposed by Aristarchus (312-230
B.C.)
Models of the Solar System
 Ptolemy – Greek astronomer who believed in the
geocentric model, but theorized that each planet
moves in small circles called epicycles to explain
retrograde motion.
The Birth of Modern Astronomy
 Nicolaus Copernicus – (1473-1543) from Poland.
He proposed a model of the solar system with the
sun at the center.
The Birth of Modern Astronomy
 Tycho Brahe – (1546-1601) a Danish astronomer
who devoted his life to making detailed
observations of the positions of the stars and
planets.
Birth of Modern Astronomy
 Johannes Kepler – (1571-1630) a German
astronomer who assisted Brahe. He was able to
explain Brahe’s observations in mathematical
terms. He developed three laws that explained
planetary motion. He concluded that
planets orbit the Sun in paths called
ellipses – oval shaped.
Kepler’s Laws
 Law of Ellipses – states that each planet orbits the
sun in a path called an ellipse. An ellipse is an
oval shape. The point where an orbit of a planet is
closest to the sun is the perihelion. The point
where it is farthest from the sun is the aphelion.
The average distance between the Earth and the
sun is known as one astronomical unit, or AU.
 Law of Equal Areas – describes the speed at
which planets travel at different points in their
orbits. Planets travel faster when nearer the sun,
and they travel slower when farther away.
Kepler’s Laws
 Law of Periods – describes the relationship
between the average distance of the planet from
the sun and the orbit period of the planet. The
orbit period is the time required for the planet to
make one revolution around the sun.
 Astronomical Units – the average distance
between the Earth and the Sun, which is used to
measure the distance to all the planets.
Birth of Modern Astronomy
 Galileo Galilei – (1564-1642) an Italian scientist
that used telescopes to confirm the heliocentric
model. He discovered four satellites (moons)
orbiting Jupiter, which are called the Galilean
Moons. He lived the last years of his life under
house arrest.
Birth of Modern Astronomy
 Sir Isaac Newton – (1642-1727) Newton applied
Kepler’s laws to come up with the reason of why
the planets move in the way that Kepler described.
Newton identified a force that kept the planets in
orbit around the sun. This force was gravity, the
attractive force that exists between all objects in
the universe.
Section 22.2 The Earth-Moon-Sun System
Motions of the Earth
 Rotation – the turning, or spinning, of a body on its
axis. On Earth it takes ~ 24 hours and causes day
and night.
 Revolution – the motion of a body, such as a
planet or moon, along its orbit around some point
in space. On Earth it takes ~ 365 days – 1 year.
When the Earth is at perihelion, it is closest to the
Sun – about 147 million kilometers. When the
Earth is at aphelion, it is farthest from the Sun –
about 152 million kilometers.
Motions of the Earth
 Precession – the wobbling of Earth on its axis. It
takes 26,000 years to make one “wobble”. This
makes the Earth’s axis tilt between 21.5o and
24.5o. At the present the axis is tilted 23.5o.
The Moon
 Earth has one natural satellite, the moon.
 It takes 27 1/3 days for the moon to orbit the Earth
and 27 1/3 days for the moon to rotate once on its
axis. Since they take the same time, we only see
one side of the moon.
 Perigee – when the moon is closest to Earth.
 Apogee – when the moon is farthest from Earth.
 Lunar phases – caused by the changes in how
much of the sunlit side of the moon faces Earth.
Phases of the Moon
Eclipses
 Solar Eclipse – when the moon moves between
the Earth and the Sun, and the moon’s shadow is
cast upon the Earth. Occurs during new-moon.
Eclipses
 Lunar Eclipse – when the Earth is between the
Sun and the moon, and the Earth’s shadow is cast
upon the moon. Occurs during full-moon phase.
Section 22.3 Earth’s Moon
The Moon
 Apollo 11 – the mission that put the first man on
the moon, Neil Armstrong on July 20, 1969.
The Moon
 Craters – the most obvious features on the lunar
surface. Most were produced by the impact of
rapidly moving debris or meteoroids.
 Rays – elongated streaks that radiate outward for
hundreds of kilometers from younger craters.
 Tycho
The Moon
 Highlands – mountainous areas on the moon that
cover most of the lunar surface and are heavily
cratered. Most astronomers think they formed
when large asteroids struck the moon.
The Moon
 Maria – (singular: mare) the dark, relatively
smooth areas on the moon’s surface. Maria are
ancient beds of basaltic lava that originated when
asteroids punctured the lunar surface letting
magma “bleed” out.
The Moon
 Regolith – a layer of gray debris that covers the
lunar surface. It formed from billions of years of
bombardment from space debris. Thought to be
up to 20 meters thick.
Formation of the Moon
 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.
Formation of the Moon