Origin of Modern
Astronomy
Early Astronomy
Ancient Greeks
 Astronomy is the science that studies the
universe. It includes the observation and
interpretation of celestial bodies and
phenomena.
 The Greeks used philosophical arguments
to explain natural phenomena.
 The Greeks also used some observational
data.
Early Astronomy
Ancient Greeks
 Geocentric Model = Ptolemy Greek Astronomer
• In the ancient Greeks’ geocentric model, the
moon, sun, and the known planets—Mercury,
Venus, Mars, and Jupiter—orbit Earth.
 Heliocentric Model = Nicolaus Copernicus
• In the heliocentric model, Earth and the other
planets orbit the sun.
Early Astronomy
Ancient Greeks
 Ptolemaic System
• Ptolemy created a model of the universe that
accounted for the movement of the planets.
• Retrograde motion is the apparent westward
motion of the planets with respect to the stars.
March
Feb.
Jan.
Dec.
April
Sept.
May
Aug.
East
June
July
Retrograde
motion of Mars
West
Retrograde Motion
99 Years of Astronomy
Early Astronomy
The Birth of Modern Astronomy
 Nicolaus Copernicus
• Copernicus concluded that Earth is a planet. He
proposed a model of the solar system with the sun at
the center. Heliocentric Model
This model explained the retrograde motion of
planets better than the geocentric model.
Early Astronomy
The Birth of Modern Astronomy
 Tycho Brahe
• Tycho Brahe designed and built instruments to
measure the locations of the heavenly bodies.
Brahe’s observations, especially of Mars, were far
more precise than any made previously.
 Johannes Kepler
• Kepler discovered three laws of planetary motion:
1. Orbits of the planets are elliptical.
2. Planets revolve around the sun at varying speed.
3. There is a proportional relationship between a planet’s
orbital period and its distance to the sun.
Early Astronomy
The Birth of Modern Astronomy
German astronomer
Johannes Kepler
(1571-1630) helped
establish the era of modern
astronomy by deriving
three laws of planetary
motion.
Johannes Kepler
• 1599 – Kepler hired by Tycho Brahe
– Work on the orbit of Mars
• 1609 – Kepler’s 1st and 2nd Laws
– Planets move on ellipses with the Sun at one focus
– The radius vector sweeps out equal areas in equal times
• 1618 – Kepler’s 3rd Law
– The square of a planet’s orbital period P is proportional
to the cube of its semi-major axis R.
Early Astronomy
Johannes Kepler used Tycho Brahe’s data to develop
three laws that explained the motions of the planets.
Earth’s orbit
June 15th
Equal areas
(30 days)
July 15th
January 15th
(30 days)
Sun
December 15th
KEPLER’S EQUAL AREA LAW states that a line connecting Earth to the sun will pass
over equal areas of space in equal times. Because Earth’s orbit is elliptical, Earth moves
faster when it is nearer to the sun.
Early Astronomy
Equal areas law
Faster
Slower
KEPLER’S EQUAL
AREA LAW states that a
line connecting Earth to
the sun will pass over
equal areas of space in
equal times. Because
Earth’s orbit is elliptical,
Earth moves faster when
it is nearer to the sun.
Early Astronomy
 Galileo Galilei
Italian scientist
Galileo Galilei (1564—1642)
used a new invention, the
telescope, to observe the Sun,
Moon, and planets in more
detail than ever before.
Early Astronomy
The Birth of Modern Astronomy
 Galileo Galilei
• Galileo’s most important contributions were his
descriptions of the behavior of moving objects.
• He developed his own telescope and made
important discoveries:
1. Four satellites, or moons, orbit Jupiter.
2. Planets are circular disks, not just points of light.
3. Venus has phases just like the moon.
4. The moon’s surface is not smooth.
5. The sun has sunspots, or dark regions.
Early Astronomy
 Sir Isaac Newton
English scientist
Sir Isaac Newton
(1642—1727)
explained gravity as
the force that holds
planets in orbit around
the Sun.
Early Astronomy
The Birth of Modern Astronomy
 Sir Isaac Newton
• Although others had theorized the existence of
gravitational force, Newton was the first to formulate and
test the law of universal gravitation. The universal law of
gravitation, helped explain the motions of planets in the
solar system.
 Universal Gravitation
• Gravitational force decreases with distance.
• The greater the mass of an object, the greater is
its gravitational force.
Gravity’s Influence on Orbits
Newton’s Laws of Motion
• 1st Law
– A body at rest, or in uniform motion, will
remain so unless acted upon by an unbalanced
force.
• 2nd Law
– The change in motion (acceleration) is
proportional to the unbalanced force
• 3rd Law
– For every action there is an equal and opposite
reaction
Gravity
• Gravity is the force that
–
–
–
–
holds us to the Earth
causes a rock to fall towards the ground
causes the Earth to go around the Sun
causes the Sun to be pulled towards the center
of the Milky Way galaxy
• Gravity acts between any two objects even
if they are far apart.
– “action at a distance”
Summary
• Kepler’s and Galileo’s Laws provided Newton
with important clues that helped him formulate his
laws of motion
• Newton arrived at 3 laws that govern the motion
of objects
– The law of inertia
– The law of force
– The law of action and reaction
• Newton also arrived at a law of gravity
– But it seemed to require action at a distance!
Earth Science
Light and Astronomical
Observations
Important Astronomical Measurements
• An ellipse is an oval-shaped path.
An astronomical unit (AU) is the average distance between
Earth and the sun; it is about 150 million kilometers.
Light-year The distance that light travels in one year,
about 9.5 trillion kilometers.
Parsec: A unit of measurement used to describe
distances between celestial objects, equal to 3.258 lightyears.
The study of light
Electromagnetic radiation
•Visible light is only one small part of an array of
energy
•Electromagnetic radiation includes
•Gamma rays
•X-rays
•Ultraviolet light
•Visible light
•Infrared light
•Radio waves
*Energy radiated in
the form of a wave,
resulting from the
motion of electric
charges and the
magnetic fields they
produce.
The study of light
Electromagnetic radiation
•All forms of radiation travel at 300,000 kilometers
(186,000 miles) per second
Light (electromagnetic radiation) can be described in two
ways
Wave model
A continuum depicting the range of
electromagnetic radiation, with the
longest wavelength at one end and
the shortest at the other.
Wavelengths of radiation vary
Radio waves measure up to several kilometers long
Gamma ray waves are less than a billionth of a centimeter long
White light consists of several wavelengths corresponding to the colors of
the rainbow
Light (electromagnetic radiation) can be
described in two ways
•Particle model
•Particles called photons
•Exert a pressure, called radiation pressure, on matter
•Shorter wavelengths correspond to more energetic
photons
The study of light
Spectroscopy
•The study of the properties of light that depend
on wavelength
•The light pattern produced by passing light
through a prism, which spreads out the various
wavelengths, is called a spectrum (plural: spectra)
The study of light
A spectrum is produced when white light passes
through a prism
The study of light
Spectroscopy
•Types of spectra
•Continuous spectrum: A spectrum that contains all
colors or wavelengths.
•Produced by an incandescent solid, liquid, or high
pressure gas
•Uninterrupted band of color
•Dark-line (absorption) spectrum
•Produced when white light is passed through a
comparatively cool, low pressure gas
•Appears as a continuous spectrum but with
dark lines running through it
Formation of the three types of spectra
Emission spectrum of hydrogen
Emission Spectrum
Absorption Spectrum
A spectrum consisting of individual
lines at characteristic wavelengths
produced when light passes
through an incandescent gas; a
bright-line spectrum.
A continuous spectrum
crossed by dark lines
produced when light
passes through a
nonincandescent gas.
Absorption Spectrum of Hydrogen
The study of light
Doppler effect
•The apparent change in wavelength of radiation
caused by the relative motions of the source and
observer
•Used to determine
•Direction of motion
•Increasing distance – wavelength is longer
("stretches")
•Decreasing distance – makes wavelength shorter
("compresses")
•Velocity – larger Doppler shifts indicate higher
velocities
The Doppler effect
Originally discovered by the Austrian
mathematician and physicist, Christian Doppler
(1803-53), this change in pitch results from a
shift in the frequency of the sound waves.
The Doppler effect
The electromagnetic radiation emitted by a
moving object also exhibits the Doppler effect.
•Redshift, a phenomenon of electromagnetic
waves such as light in which spectral lines
are shifted to the red end of the spectrum.
The Doppler effect
Blueshift: This spectrum shows hydrogen shifted to the blue end of
the spectrum. This star is moving toward Earth.
The radiation emitted by an object moving
toward an observer is squeezed; its
frequency appears to increase and is
therefore said to be blueshifted. In
contrast, the radiation emitted by an object
moving away is stretched or redshifted.
Blueshifts and redshifts exhibited by stars,
galaxies and gas clouds also indicate their
motions with respect to the observer.
Redshift: This spectrum shows hydrogen shifted to the red end of the
spectrum. This star is moving away from Earth.
Astronomical tools
Optical (visible light) telescopes
•Two basic types (1) Refracting telescope
•Uses a lens (called the objective) to bend (refract)
the light to produce an image
•Light converges at an area called the focus
•Distance between the lens and the focus is called the
focal length
•The eyepiece is a second lens used to examine the
image directly
•Have an optical defect called chromatic aberration
(color distortion)
A simple refracting telescope
Astronomical tools
Optical (visible light) telescopes
•Two basic types (2) Reflecting telescope
•Uses a concave mirror to gather the light
•No color distortion
•Nearly all large telescopes are of this type
A prime focus reflecting telescope
Cassegrain focus reflecting telescope
Newtonian focus reflecting telescope
The 200" (5m) Hale Reflector of Palomar Observatory is
shown above. Until recently it was the world's largest
optical/infrared telescope.
Astronomical tools
Optical (visible light) telescopes
•Properties of optical telescopes
•Light-gathering power
•Larger lens (or mirror) intercepts more light
•Determines the brightness
•Resolving power
•The ability to separate close objects
•Allows for a sharper image and finer detail
Astronomical tools
Optical (visible light) telescopes
•Properties of optical telescopes
•Magnifying power
•The ability to make an image larger
•Calculated by dividing the focal length of the
objective by the focal length of the eyepiece
•Can be changed by changing the eyepiece
•Limited by atmospheric conditions and the
resolving power of the telescope
•Even with the largest telescopes, stars (other than
the Sun) appear only as points of light
Astronomical tools
Detecting invisible radiation
•Radio radiation
•Gathered by "big dishes" called radio telescopes
•Large because radio waves are about 100,000 times
longer than visible radiation
•Often made of a wire mesh
•Have rather poor resolution
•Can be wired together into a network called a radio
interferometer
Radio Telescope
A steerable radio telescope at Green Bank, West
Virginia
Astronomical tools
Detecting invisible radiation
•Radio radiation
•Gathered by "big dishes" called radio telescopes
•Advantages over optical telescopes
•Less affected by weather
•Less expensive
•Can be used 24 hours a day
•Detects material that does not emit visible
radiation
•Can "see" through interstellar dust clouds
Radio Telescope
The 300-meter
radio telescope at
Arecibo, Puerto
Rico
The Big Bang Theory
The theory holding that the universe originated from
the instant expansion of an extremely small
agglomeration of matter of extremely high density
and temperature.
Photons converted
into particleantiparticle pairs and
vice-versa
E = mc2
Early universe was
full of particles and
radiation because of
its high temperature
The Big Band Theory
•Evidence for Big Bang
•This is the theory of the universe’s earliest
moments.
•It presumes that the universe began from a tiny,
hot, and dense collection of matter and radiation.
•It describes how expansion and cooling of
particles could have led to the present universe of
stars and galaxies.
•It explains several aspects of today’s universe
with a very good accuracy.
The Big Band Theory
The Big Bang theory is a model, which explains
some facts (observations).
It should be able to make predictions that can be
verified through observations or experiments.
Two important predictions:
1. Cosmic microwave background radiation.
2. Fusion of original hydrogen into helium.
Evidence for the Big Bang
The Cosmic Background Radiation (Microwaves)
Penzias & Wilson (1962) discovered an isotropic background
microwave signal during testing a microwave antenna at
Bell Labs in 1965. The noise was found to be coming from
every direction.
At the same time, physicists from Princeton calculated the
expected radiation from the initially hot universe.They
suggested that this radiation could be detected with a
microwave antenna.
The result was a Nobel Prize in physics for 1978.
The Cosmic Microwave Background
The Cosmic Background Radiation (Microwaves)
Background
radiation from Big
Bang has been
freely streaming
across universe
since atoms
formed at
temperature ~
3,000 K: visible/IR
The Cosmic Microwave Background
The background consists of photons (radiation)
arriving at Earth directly from the end of the era of
nuclei (when the Universe was about 380,000 years
old).
Neutral atoms captured most of the electrons.
Photons were released and have flown freely
through the universe ever since.
This background radiation can be detected with a small TV
antenna as part (1%) of static “snow”. The redshifted
spectrum of the background radiation has now a
temperature of 2.73 K.
Cosmic Background Explorer
The first satellite built dedicated to cosmology. Its goals were to
investigate the cosmic microwave background radiation (CMB) of
the universe and provide measurements that would help shape
our understanding of the cosmos.
Cosmic Background Explorer
This work helped cement the big-bang theory
of the universe. According to the Nobel Prize
committee, "the COBE-project can also be
regarded as the starting point for cosmology
as a precision science". Two of COBE's
principal investigators, George Smoot and John
Mather, received the Nobel Prize in Physics in
2006.
Cosmic Background Explorer
Cosmic Background Explorer
The "famous" map of the CMB anisotropy
formed from data taken by the COBE
spacecraft.
Evidence for the Big Bang
In 1927, the
Belgian priest
Georges
Lemaître was
the first to
propose that
the universe
began with the
explosion of a
primeval atom.
Evidence for the Big Bang
Edwin Hubble found experimental evidence to help
justify Lemaître's theory. He found that distant
galaxies in every direction are going away from us
with speeds proportional to their distance (the
redshift).
The big bang was initially suggested because it explains why
distant galaxies are traveling away from us at great speeds.
The theory also predicts the existence of cosmic background
radiation (the glow left over from the explosion itself). The
Big Bang Theory received its strongest confirmation when
this radiation was discovered in 1964 by Arno Penzias and
Robert Wilson, who later won the Nobel Prize for this
discovery.
Hubble’s Evidence
• Doppler shifting - wavelength emitted by something moving away
from us is shifted to a lower frequency
• Sound of a fire truck siren - pitch of the siren is higher as the fire truck
moves towards you, and lower as it moves away from you
• Visible wavelengths emitted by objects moving away from us are
shifted towards the red part of the visible spectrum
• The faster they move away from us, the more they are redshifted.
Thus, redshift is a reasonable way to measure the speed of an object
(this, by the way, is the principal by which radar guns measure the
speed of a car or baseball)
• When we observe the redshift of galaxies outside our local group,
every galaxy appears to be moving away from us - universe is
expanding.
Expansion of universe has redshifted thermal
radiation from that time to ~1000 times longer
wavelength: microwaves
Evidence for the Big Bang
Big Bang Theory - Evidence for the Theory
What are the major evidences which support the Big Bang theory?
•First of all, we are reasonably certain that the
universe had a beginning.
•Second, galaxies appear to be moving away from us
at speeds proportional to their distance. This is called
"Hubble's Law," named after Edwin Hubble (18891953) who discovered this phenomenon in 1929. This
observation supports the expansion of the universe
and suggests that the universe was once compacted.
Evidence for the Big Bang
•Third, if the universe was initially very, very hot as
the Big Bang suggests, we should be able to find
some remnant of this heat. In 1965,
Radioastronomers Arno Penzias and Robert Wilson
discovered a 2.725 degree Kelvin (-454.765 degree
Fahrenheit, -270.425 degree Celsius) Cosmic
Microwave Background radiation (CMB) which
pervades the observable universe. This is thought to
be the remnant which scientists were looking for.
Penzias and Wilson shared in the 1978 Nobel Prize for
Physics for their discovery.
Evidence for the Big Bang
•Finally, the abundance of the "light elements"
Hydrogen and Helium found in the observable
universe are thought to support the Big Bang model of
origins.
Synthesis of Helium
•The current CMB temperature tells us precisely how
hot the universe was when it appeared.
•It tells us how much helium was initially produced.
•A helium nucleus contains 2 protons and 2 neutrons.
•At T > 1011 K, nuclear reactions converted protons into
neutrons and back, keeping their numbers nearly equal.
•Between 1010 and 1011 K, neutron – proton reactions
favor protons, because neutrons are heavier than
protons.
Synthesis of Helium
•Energy is required to convert protons to neutrons.
•At T < 1010 K, only neutrons can be changed into
protons.
•However, fusion continued to operate
•and protons and neutrons combined into deuterium.
•Then deuterium fused into helium.
•During the early era of nucleosynthesis, helium nuclei
were being destroyed by gamma-rays.
•At ~1 minute, gamma-rays were gone and the proton –
neutron ratio was set to 7:1.
Synthesis of Helium
Big Bang
theory
prediction:
75% H, 25%
He (by mass)
Matches
observations
of nearly
primordial
gases
Synthesis of Helium
Abundances
of other light
elements
agree with
Big Bang
model having
4.4% normal
matter – more
evidence for
WIMPS!
Nebular Hypothesis of Solar System Formation.