Transcript History of Astronomy

EARLY PLAYERS IN ASTRONOMY
The Really Early Days
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Astronomy began the first time a Cro Magnon
(early homo sapien) walked out at night, looked up
at the sky, and said “Wow, Dude.”
That person was promptly eaten by a nocturnal
predator because he should have been looking
around and not up.
The ancient Greeks were the first to make scientific
observations of the sky.
Aristarchus
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Greek astronomer from 3rd
century BCE.
Estimated the sizes of the Sun and
Moon relative to the Earth.
First proposed the Sun as the
center of the universe, since it was
larger than the Earth.
Heliocentric (Sun centered)
theory did not gain acceptance
until Copernicus about 1800
years later.
Aristotle
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Greek philosopher lived from 384 322 BCE
Devised a system where the Earth
was the immoveable center of the
universe.
The heavens were perfect and
unchanging.
Aristotle’s geocentric system was
used for centuries.
Hipparchus
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Lived during 2nd century BCE
Made a catalog of 850 stars
that was still in use by
astronomers during the Middle
Ages.
Divided these stars into 6
brightness categories called
magnitudes.
First magnitude stars are the
brightest and sixth magnitude
stars are the dimmest.
The current magnitude system
is based on this one.
Claudius Ptolemy
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Lived from c.100 - c.178 CE
Wrote a major astronomical
encyclopedia called the Almagest
- an attempt to summarize all
previous Greek astronomy
orbits of the Moon and planets
were circles whose centers were
offset from the Earth.
embraced the idea that the
heavens were perfect, only shape
sufficient for motions in the
heavens was a circle or a sphere.
The Dark Ages
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For nearly 1500 years after Ptolemy, Europe was in
the Dark Ages. No advances were made and
earlier knowledge was lost.
Greek knowledge passed into Arab hands, who
preserved it and passed it back into Europe when
the Moors conquered Spain.
As a result of astronomy advancing via Arab
cultures, many stars have Arabic names, like
Aldebaran, Altair, and Algol.
Nicolaus Copernicus
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A Polish monk who lived from
1473 - 1543, during the
Renaissance.
By this time errors in predictions of
the planets’ positions based on
Ptolemy’s model were obvious.
Copernicus returned to the
heliocentric (sun-centered) model
developed by Aristarchus.
The heliocentric model allows a
much simpler description of the
motions of the celestial bodies.
Copernicus on the Heliocentric Model
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Sun is the center of the universe.
A planet’s speed of movement through the sky depends
on its distance from the Sun.
Mercury and Venus are never far from the Sun in the
sky. This can be explained if they orbit closer to the Sun
than Earth does.
Retrograde motion of some of the planets (the reason
for some of the complexity in Aristotle’s model) could be
explained by them orbiting farther out and Earth
periodically catching up to, and passing them.
Retrograde Motion of Mars
Retrograde Motion
Copernicus
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Copernicus published his theory in the year he died
(1543) in a book called On the Revolutions of the
Celestial Spheres.
The model had a problem. It still assumed the orbits
were perfect circles.
Tycho Brahe
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Danish astronomer who lived from 1546 - 1601.
He realized that all previous theories of planetary
motion were inaccurate. He decided to make very
precise measurements of the motions of the planets
first, then to try to develop a theory of celestial
motion based on the data.
In doing so, he proved that many of the long-held
ideas about the heavens were incorrect.
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He proved that a new star that appeared in
Cassiopeia in 1572 was far off in space,
contradicting the idea that the heavens were
unchanging and any transient events were
actually in the Earth’s atmosphere.
The remains of Tycho’s Supernova are still
detectable.
He showed that the comet of 1577 moved
among the orbits of the planets, contradicting the
notion of crystalline spheres governing the
motions of celestial bodies.
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Tycho developed his own theory of celestial
motion in which the planets orbited the Sun, but
the Sun and the stars orbited the Earth.
Tycho took a German mathematician named
Johannes Kepler as his assistant. He hoped
Kepler would take his observations and use them
to refine and confirm his model of the heavens.
Kepler ended up reviving the Copernican model
of celestial motion.
Tycho Brahe
Johannes Kepler
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Lived from 1571 - 1630.
Inherited Tycho’s vast collection of astronomical
observations.
Kepler determined the planets travel around the
sun along simple elliptical curves, not the complex
combinations of circular motions Tycho and others
devised.
He later discovered a formula that links a planet’s
distance from the sun with the time it takes to
complete its orbit.
Kepler published his model in 1609.
Johannes Kepler
Kepler's Laws Explained
In the mean time:
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In 1609 the Italian scientist Galileo Galilei (1564 1642) heard about the telescope and decided to
build one.
A Dutch optician named Hans Lippershey (c.1570 1619) is credited with inventing the telescope.
Galileo’s refracting telescope had an objective lens
1.75 in. in diameter and magnified 33x.
Galileo Galilei
What Galileo Saw:
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The surface of the moon was pockmarked with
craters, further proof that the heavens were not
perfect.
The Milky Way, the band of light that crosses the
sky, was made up of countless stars that individually
were too faint to be seen with the naked eye. Since
the stars could not be seen as disks, they must be
very far away, as Copernicus had theorized.
More Discoveries
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The planet Venus went through phases similar to
those of the Moon. The only was this was possible
was if Venus orbited the Sun.
The planet Jupiter had four moons that orbited it.
Both of these observations put the final nails in the
coffin of the geocentric (Earth-centered) model of
the universe.
Sir Isaac Newton
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English, lived from 1642 - 1727
Published his theory of gravity in 1687, which
explained why the planets orbit the sun.
This was the final evidence that the heliocentric
theory was the correct one.
Using the 1663 idea of Scotsman James Gregory,
in 1668 Newton built a telescope that used mirrors
instead of lenses to collect and focus light.
Isaac Newton
2.2 The Geocentric Universe
Sun, Moon, and stars all have simple movements in the sky
Planets:
• Move with respect to
fixed stars
• Change in brightness
• Change speed
• Undergo retrograde
motion
2.2 The Geocentric Universe
• Inferior planets: Mercury, Venus
• Superior planets: Mars, Jupiter, Saturn
Now know:
Inferior planets have
orbits closer to Sun
than Earth’s
Superior planets’
orbits are farther away
2.2 The Geocentric Universe
Early observations:
• Inferior planets never too far from Sun
• Superior planets not tied to Sun; exhibit retrograde
motion
• Superior planets brightest at opposition
• Inferior planets brightest near inferior conjunction
2.2 The Geocentric Universe
Earliest models had Earth at center of solar system
Needed lots of
complications to
accurately track planetary
motions
2.3 The Heliocentric Model of the
Solar System
Sun is at center of solar system. Only Moon orbits around
Earth; planets orbit around Sun.
This figure shows
retrograde motion
of Mars.
Discovery 2-1: The Foundations of the
Copernican Revolution
1. Earth is not at the center of everything.
2. Center of earth is the center of moon’s orbit.
3. All planets revolve around the Sun.
4. The stars are very much farther away than the Sun.
5. The apparent movement of the stars around the
Earth is due to the Earth’s rotation.
6. The apparent movement of the Sun around the
Earth is due to the Earth’s rotation.
7. Retrograde motion of planets is due to Earth’s
motion around the Sun.
2.4 The Birth of Modern Astronomy
Telescope invented around 1600
Galileo built his own, made
observations:
• Moon has mountains and valleys
• Sun has sunspots, and rotates
• Jupiter has moons (shown):
• Venus has phases
2.4 The Birth of Modern Astronomy
Phases of Venus
cannot be explained
by geocentric model
2.5 The Laws of Planetary Motion
Kepler’s laws were derived
using observations made by
Tycho Brahe
2.5 The Laws of Planetary Motion
1. Planetary orbits are ellipses, Sun at one focus
2.5 The Laws of Planetary Motion
2. Imaginary line connecting Sun and planet sweeps out
equal areas in equal times
2.5 The Laws of Planetary Motion
3. Square of period of planet’s orbital motion is
proportional to cube of semimajor axis
More Precisely 2-1: Some Properties
of Planetary Orbits
Semimajor axis and eccentricity of orbit completely
describe it
Perihelion: closest approach to Sun
Aphelion: farthest distance from Sun
2.6 The Dimensions of the Solar System
Astronomical unit: mean distance from Earth to Sun
First measured during transits of Mercury and Venus,
using triangulation
2.6 The Dimensions of the Solar System
Now measured using radar:
Ratio of mean radius of
Venus’s orbit to that of
Earth very well known
2.7 Newton’s Laws
Newton’s laws of motion
explain how objects interact
with the world and with each
other.
2.7 Newton’s Laws
Newton’s First Law:
An object at rest will remain at rest, and an object moving in a
straight line at constant speed will not change its motion,
unless an external force acts on it.
2.7 Newton’s Laws
Newton’s second law:
When a force is exerted on an object, its acceleration is
inversely proportional to its mass:
a = F/m
Newton’s third law:
When object A exerts a force on object B, object B exerts an
equal and opposite force on object A.
2.7 Newton’s Laws
Gravity
On the Earth’s surface,
acceleration of gravity is
approximately constant,
and directed toward the
center of Earth
2.7 Newton’s Laws
Gravity
For two massive objects,
gravitational force is
proportional to the product of
their masses divided by the
square of the distance
between them
2.7 Newton’s Laws
Gravity
The constant G is called the gravitational constant; it is
measured experimentally and found to be:
G = 6.67 x 10-11 N m2/kg2
More Precisely 2-2: The Moon is Falling!
Newton’s insight: same force causes apple to fall and
keeps Moon in orbit; decreases as square of distance, as
does centripetal acceleration: a = v2/r
2.8 Newtonian Mechanics
Kepler’s laws are a
consequence of Newton’s
laws; first law needs to be
modified: The orbit of a
planet around the Sun is an
ellipse, with the center of
mass of the planet–Sun
system at one focus.
More Precisely 2-3: Weighing the Sun
Newtonian mechanics tells us that the force keeping the
planets in orbit around the Sun is the gravitational force due to
the masses of the planet and Sun.
This allows us to calculate the mass of the Sun, knowing the
orbit of the Earth:
M = rv2/G
The result is M = 2.0 x 1030 kg (!)
2.8 Newtonian Mechanics
Escape speed: the speed
necessary for a projectile
to completely escape a
planet’s gravitational field.
With a lesser speed, the
projectile either returns to
the planet or stays in
orbit.
Summary of Chapter 2
First models of solar system were geocentric but couldn't
easily explain retrograde motion
Heliocentric model does; also explains brightness
variations
Galileo's observations supported heliocentric model
Kepler found three empirical laws of planetary motion
from observations
Summary of Chapter 2, continued
Laws of Newtonian mechanics explained Kepler’s
observations.
Gravitational force between two masses is proportional
to the product of the masses, divided by the square of
the distance between them.