Download introtoastronomy part 2

Origin of Modern
Astronomy
Early Ideas of the Heavens
• Ancient Greek Astronomers
– Through the use of models and observations, they
were the first to use a careful and systematic
manner to explain the workings of the heavens
– Limited to naked-eye observations, their idea of
using logic and mathematics as tools for
investigating nature is still with us today
– Their investigative methodology is in many ways
as important as the discoveries themselves
History of Astronomy
2
Early Ideas of the Heavens
• The Shape of the Earth
– Pythagoras taught as early as 500 B.C. that the
Earth was round, based on the belief that the
sphere is the perfect shape used by the gods
– By 300 B.C., Aristotle presented naked-eye
observations for the Earth’s spherical shape:
• Shape of Earth’s shadow on the Moon during an
eclipse
• A traveler moving south will see stars previously
hidden by the southern horizon
History of Astronomy
3
Early Ideas of the Heavens
• The Size of the Earth
– Eratosthenes (276-195 B.C.) made the first measurement
of the Earth’s size
– He obtained a value of 25,000 miles for the
circumference, a value very close to today’s value
– His method entailed measuring the shadow length of a
stick set vertically in the ground in the town of Alexandria
on the summer solstice at noon, converting the shadow
length to an angle of solar light incidence, and using the
distance to Syene, a town where no shadow is cast at noon
on the summer solstice
History of Astronomy
4
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.
Johannes Kepler (1571 – 1630)
• Used the precise observational tables of Tycho
Brahe (1546 – 1601) to study planetary motion
mathematically.
• Found a consistent description by
abandoning both
1. Circular motion and
2. Uniform motion.
• Planets move around the sun on elliptical
paths, with non-uniform velocities.
Kepler’s Laws of Planetary Motion
1. The orbits of the planets are ellipses with the
sun at one focus.
c
Eccentricity e = c/a
Eccentricities of Ellipses
1)
2)
e = 0.02
3)
e = 0.1
e = 0.2
5)
4)
e = 0.4
e = 0.6
Eccentricities of Planetary Orbits
Orbits of planets are virtually
indistinguishable from circles:
Most extreme example:
Earth: e = 0.0167
Pluto: e = 0.248
Planetary Orbits (2)
• A line from a planet to the sun sweeps
over equal areas in equal intervals of time.
• A planet’s orbital period (P) squared is
proportional to its average distance from the
sun (a) cubed:
Py2
= aAU
3
(Py = period in years; aAU
= distance in AU)
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.
Galileo Galilei (1564-1642)
• Galileo was one of the first to use a telescope
to study astronomical objects, starting in about
1609.
• His observations of the moons of Jupiter and
the phases of Venus provided strong support for
the heliocentric model.
Jupiter’s Moons
• The 4 objects circled Jupiter, and not the Earth!
Jupiter’s Moons
• You can watch Jupiter’s moons move from one
side of Jupiter to the other in a few days.
Jupiter’s Moons
• Not all bodies go around the Earth!
Venus
• Venus, the brightest planet, is never far from
the Sun: it sets at most a few hours after
sunset, or rises at most a few hours before
sunrise.
Venus
• Venus, the brightest planet, is never far from
the Sun: it sets at most a few hours after
sunset, or rises at most a few hours before
sunrise.
• It is never out in the middle of the night.
Venus
• Galileo discovered that Venus had phases, just like the
Moon.
Venus
• Galileo discovered that Venus had phases, just like the
Moon.
• Furthermore, the crescent Venus was always larger than
the full Venus.
Venus
• Galileo discovered that Venus had phases, just like the
Moon.
• Furthermore, the crescent Venus was always larger than
the full Venus.
• Conclusion: Venus shines by reflected sunlight, and it
is closer to Earth when it is a crescent.
Venus in the Geocentric View
• Venus is always
close to the Sun on
the sky, so its
epicycle restricts its
position.
• In this view, Venus
always appears as a
crescent.
Venus in the Heliocentric View
• In the heliocentric
view, Venus orbits
the Sun closer than
the Earth does.
• We on Earth can see
a fully lit Venus
when it is on the far
side of its orbit.
Venus in the Heliocentric View
• The correlation between
the phases and the size
is accounted for in the
heliocentric view.
• Galileo’s observations of Jupiter and Venus
strongly favored the heliocentric view of the
Universe.
• Galileo’s observations of Jupiter and Venus
strongly favored the heliocentric view of the
Universe.
• Galileo was put before the Inquisition and
forced to recant his views.
• Galileo’s observations of Jupiter and Venus
strongly favored the heliocentric view of the
Universe.
• Galileo was put before the Inquisition and
forced to recant his views.
• Pope John Paul II admitted in 1992 that the
Church was wrong to denounce Galileo.
Isaac Newton (1642-1727)
http://www-history.mcs.st-andrews.ac.uk/history/PictDisplay/Newton.html
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”
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
• He was professor of mathematics at
Cambridge University in England. (Steven
Hawking currently hold’s Newton’s Chair at
Cambridge).
Isaac Newton (1642-1727)
• Isaac Newton was born the year Galileo
died.
• He was professor of mathematics at
Cambridge University in England. (Steven
Hawking currently hold’s Newton’s Chair at
Cambridge).
• He was later the Master of the Mint in
London, where first proposed the use of
grooved edges on coins to prevent shaving.
Isaac Newton (1642-1727)
• Newton was perhaps the greatest scientist of
all time, making substantial contributions to
physics, mathematics (he invented calculus
as a college student), optics, and chemistry.
Isaac Newton (1642-1727)
• Newton was perhaps the greatest scientist of
all time, making substantial contributions to
physics, mathematics (he invented calculus
as a college student), optics, and chemistry.
• His laws of motion and of gravity could
explain Kepler’s Laws of planetary motion.
Newton’s Laws of Motion
Newton’s Laws of Motion
1.
2.
3.
A body in motion tends to stay in motion in a straight
line unless acted upon by an external force.
The force on an object is the mass times the acceleration
(F=ma).
For every action, there is an equal and opposite reaction.
(For example, a rocket is propelled by expelling hot gas
from its thrusters).
What is Gravity?
What is Gravity?
• Gravity is a force between all matter in the
Universe.
What is Gravity?
• Gravity is a force between all matter in the
Universe.
• It is difficult to say what gravity is.
However, we can describe how it works.
What is Gravity?
• Gravity is a force between all matter in the
Universe.
• It is difficult to say what gravity is.
However, we can describe how it works.
What is Gravity?
• The gravitational force between larger
bodies is greater than it is between smaller
bodies, for a fixed distance.
What is Gravity?
• As two bodies move further apart, the
gravitational force decreases. The range of the
force is infinite, although it is very small at
very large distances.
Newton’s Laws
• Using Newton’s Laws, we can…
Newton’s Laws
• Using Newton’s Laws, we can…
 Derive Kepler’s Three Laws.
Newton’s Laws
• Using Newton’s Laws, we can…
 Derive Kepler’s Three Laws.
 Measure the mass of the Sun, the Moon, and
the Planets.
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!
Isaac Newton & Birth of Astrophysics
• Isaac Newton (1642-1727) was born the
year Galileo died
• He made major advances in mathematics,
physics, and astronomy
• He pioneered the modern studies of motion,
optics, and gravity and discovered the
mathematical methods of calculus
• It was not until the 20th century that
Newton’s laws of motion and gravity were
modified
by
the
theories
of
relativity
History of Astronomy
63
The Growth of Astrophysics
• New Discoveries
– In 1781, Sir William Herschel discovered
Uranus; he also discovered that stars can have
companions
– Irregularities in Uranus’s orbit together with law
of gravity leads to discovery of Neptune
• New Technologies
– Improved optics lead to bigger telescopes and
the discovery of nebulas and galaxies
– Photography allowed the detection of very faint
objects
History of Astronomy
64
The Growth of Astrophysics
• The Nature of Matter and Heat
– The ancient Greeks introduced the idea of the atom
(Greek for “uncuttable”), which today has been modified
to include a nucleus and a surrounding cloud of electrons
– Heating (transfer of energy) and the motion of atoms was
an important topic in the 1700s and 1800s
• The Kelvin Temperature Scale
– An object’s temperature is directly related to its energy
content and to the speed of molecular motion
– As a body is cooled to zero Kelvin, molecular motion
within it slows to a virtual halt and its energy approaches
zero  no negative temperatures
– Fahrenheit and Celsius are two other temperature scales
that are easily converted to Kelvin
History of Astronomy
65
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