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ASTRO 101

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Title: ASTRO 101


1
ASTRO 101
  • Principles of Astronomy

2
Instructor Jerome A. Orosz
(rhymes with boris)Contact
  • Telephone 594-7118
  • E-mail orosz_at_sciences.sdsu.edu
  • WWW http//mintaka.sdsu.edu/faculty/orosz/web/
  • Office Physics 241, hours T TH 330-500

3
Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
4
Astronomy Help Room Hours
  • Monday 1200-1300, 1700-1800
  • Tuesday 1700-1800
  • Wednesday 1200-1400, 1700-1800
  • Thursday 1400-1800, 1700-1800
  • Friday 900-1000, 1200-1400
  • Help room is located in PA 215

5
Looking Ahead
  • Tuesday, September 29 In-class review
  • Thursday, October 1 Exam 1
  • Extra review session Wednesday, September 30,
    330-500 in PA 216.
  • No class Tuesday, October 6 (furlough day).

6
Questions for Today
  • How do we measure velocities of things in the
    sky?
  • What is a telescope used for?

7
Coming Up
  • The 4 forces of Nature
  • Energy and the conservation of energy
  • The nature of light
  • Waves and bundles of energy
  • Different types of light
  • The spectrum
  • Definition
  • Emission and absorption
  • How light interacts with matter
  • Telescopes
  • Review

8
The Doppler Shift Measuring Motion
  • If a source of waves is not moving, then the
    waves are equally spaced in all directions.

9
The Doppler Shift Measuring Motion
  • If a source of waves is moving, then the spacing
    of the wave crests depends on the direction
    relative to the direction of motion.

10
The Doppler Shift Measuring Motion
  • Think of sound waves from a fast-moving car,
    train, plane, etc.
  • The sound has a higher pitch (higher frequency)
    when the car approaches.
  • The pitch is lower (lower frequency) as the car
    passes and moves further away.

11
The Doppler Shift Measuring Motion
  • If a source of light is moving away, the
    wavelengths are increased, or redshifted.

12
The Doppler Shift Measuring Motion
  • If a source of light is moving closer, the
    wavelengths are shortened, or blueshifted.

13
The Doppler Shift Measuring Motion
  • The size of the wavelength shift depends on the
    relative velocity of the source and the observer.

14
The Doppler Shift Measuring Motion
  • The size of the wavelength shift depends on the
    relative velocity of the source and the observer.
  • The motion of a star towards you or away from you
    can be measured with the Doppler shift.

15
Using a Spectrum, we can
  • Measure a stars temperature by measuring the
    overall shape of the spectrum (essentially its
    color).
  • Measure what chemical elements are in a stars
    atmosphere by measuring the lines.
  • Measure the relative velocity of a star by
    measuring the Doppler shifts of the lines.

16
With very few exceptions, the only way we have to
study objects in Astronomy is via the light they
emit.So we need to collect photons, and detect
them.
17
Telescopes
18
Telescopes
  • A telescope uses mirrors or lenses to collect and
    focus light.
  • The area of the lens or mirror can be
    considerably larger than the area of the eyes
    pupil, hence much fainter objects can be seen.

19
Telescopes
  • A refracting telescope uses a large lens to bring
    the light to a focus, as in Figure (a).
  • A reflecting telescope uses curved mirrors to
    bring the light to a focus, as in Figure (b).

20
Telescopes
  • The largest lenses that can be built have a
    diameter of about 1m, and have very long focal
    lengths.
  • A lens must be held by its edges, and large
    lenses sag under their own weight. Also lots of
    light is lost in the glass.
  • For these and more reasons, all modern telescopes
    use mirrors.

21
Telescopes
  • Using an objective mirror, plus some additional
    mirrors and lenses, light is collected and
    focused to a point.
  • This is a Newtonian telescope.

22
Telescopes
  • Using an objective mirror, plus some additional
    mirrors and lenses, light is collected and
    focused to a point.
  • This is a Cassegrain telescope.

23
Telescopes
  • A telescopes main job is collecting photons.
  • The light gathering power is proportional to the
    area of the mirror or lens. The area of a circle
    is
  • If you double the diameter of the mirror, the
    light gathering power goes up 4 times.

24
Telescopes
  • Modern mirrors can be made thin. Their shapes
    are maintained using pistons under computer
    control.
  • The Gemini telescope in Hawaii has primary mirror
    8.1m in diameter.

25
Seeing Detail
  • What does the next line say?
  • If you can read this, thank a teacher.
  • Why is so hard to read?
  • Why do binoculars help?
  • It is hard to read because the angular size is
    small. The binoculars magnify the angular size.

26
Sidebar Angular Size
  • A useful quantity in astronomy is the angular
    size of an object, which is basically how large
    something appears on the sky.
  • There are 360 degrees in a full circle, and 90
    degrees from the horizon to the point overhead.

27
The Second Use of a Telescope
  • A Telescope is used to collect photons, so you
    can see fainter objects.
  • It is also used to magnify the apparent angular
    size of an object so you can see detail.

Relatively good angular resolution.
28
The Second Use of a Telescope
  • A Telescope is used to collect photons, so you
    can see fainter objects.
  • It is also used to magnify the apparent angular
    size of an object so you can see detail.

Relatively poor angular resolution.
29
What a Telescope Does
  • A Telescope is used to collect photons, so you
    can see fainter objects.
  • A telescope is also used to magnify the apparent
    angular size of an object, thereby allowing one
    to see more detail.

30
Telescopes at other Wavelengths
  • Recall that there other forms of light,
    including radio waves, X-rays, UV light, etc.

31
Telescopes at other Wavelengths
  • Recall that there other forms of light,
    including radio waves, X-rays, UV light, etc.
  • The goal of collect and detect is still the
    same.

32
Telescopes at other Wavelengths
  • Recall that there other forms of light,
    including radio waves, X-rays, UV light, etc.
  • The goal of collect and detect is still the
    same.
  • However, the technologies used to collect and
    detect are different at different wavelengths.

33
X-ray Telescopes
  • For example, X-ray light cannot be reflected like
    visible light can. X-ray telescopes use grazing
    incidence mirrors to collect X-rays.

34
Radio Telescopes
  • Radio telescopes use mirrors made from steel
    plates.
  • Radio receivers collect the focused radio waves.
  • The radio telescopes are huge because of the long
    wavelengths of the radio waves.

35
Radio Telescopes
  • Radio telescopes use mirrors made from steel
    plates.
  • Radio receivers collect the focused radio waves.
  • The radio telescopes are huge because of the long
    wavelengths of the radio waves.

36
Radio Telescopes
  • The GBT is the largest steerable radio telescope
    in the world, with a diameter of 100 meters. It
    is perhaps the largest movable land-based object
    in the world.

37
Radio Telescopes
  • With modern computers and electronics, one can
    combine the signals from several radio telescopes
    to synthesize a much larger telescope.

38
Telescopes at other Wavelengths
  • For most wavelengths, you need to go into space
    to observe.

39
Review
  • Thursday Exam 1 Chapters 1-4, plus 5.2 and 5.4
    (e.g. the parts of Chapter 5 dealing with light)
  • Bring the Scantron No. F-288-PAR-L

40
Breakdown
  • There will be three types of questions
  • multiple choice questions (2 pts each)
  • long answer (5 pts each)
  • fill in the blank (1 pt each)

41
Highlights
  • Astronomy without a telescope
  • Celestial sphere
  • Stellar coordinates
  • Stellar brightnesses
  • The clockwork of the Universe
  • The day/night cycle
  • The reason for the seasons
  • The phases of the moon

42
Highlights
  • A brief history of Astronomy
  • The geocentric model Aristotle, Ptolomy
  • The heliocentric model Copernicus, Galileo,
    Kepler
  • Isaac Newton
  • Gravitation
  • Physical model

43
Highlights
  • Energy
  • Definition
  • Forms of energy
  • Conservation of energy
  • Light as a form of energy
  • Light
  • Light as particles
  • Light as a wave
  • The electromagnetic spectrum
  • Emission lines and absorption lines
  • The uses of a spectrum

44
Highlights
  • Observational astronomy collecting and
    detecting photons.
  • Telescopes
  • Refracting (ones with lenses)
  • Reflecting (ones with mirrors)
  • Detectors

45
Good Review Questions, Chapter 2
  • 5. In what ways is the celestial sphere a
    scientific model?
  • 6. Where would you go on Earth if you wanted to
    be able to see both the north celestial pole and
    the south celestial pole at the same time?
  • 8. Explain how to make a simple astronomical
    observation to determine your latitude.
  • 13. Why are the seasons reversed in the southern
    hemisphere relative to the northern hemisphere?
  • 15. Do the phases of the Moon look the same from
    everywhere on Earth ?

46
Good Review Questions, Chapter 3
  • 2. How did the Ptolemaic model explain
    retrograde motion of the planets?
  • 3. In what ways were the models of Ptolomy and
    Copernicus similar?
  • 7. Explain how Keplers laws contradict uniform
    circular motion.
  • 9. Review Galileos telescope discoveries and
    explain why they supported the Copernican model
    and contradicted the Ptolemaic model?
  • 13. Explain why you might describe the orbital
    motion of the moon with the statement The moon
    is falling.

47
Review Questions Chapter 4
  • Why do nocturnal animals usually have large
    pupils in their eyes? How is that related to the
    design of astronomical telescopes?
  • 5. Optical and radio astronomers both try to
    build large telescopes. How are their goals
    similar, and how are they different?

48
Quick Review of Chapters 4.1, 5.2 and 5.4
49
RECAPEnergy is the ability to do work, i.e. the
ability to move or change the state of
matter.The conservation of energy Energy is
neither created nor destroyed, but may be changed
in form.
50
Light is a form of energy.Why is this
important?With very few exceptions, the only
way we have to study objects in Astronomy is via
the light they emit.
51
What is the nature of light?Light can be
thought of as awave in an electric fieldoras
discrete particles of energy
52
How light interacts with matter andthe line
spectrum.
53
What are Things Made of?
  • Among other things, chemistry is the study of
    matter and its composition.
  • Most substances around us can be divided
    chemically into simpler things
  • Water --gt hydrogen and oxygen
  • Table salt --gt sodium and chlorine
  • At some point, certain things dont chemically
    break down into different parts. These are
    called elements.

54
What are Things Made of?
  • At some point, certain things dont chemically
    break down into different parts. These are
    called elements.
  • Examples of elements hydrogen, helium, carbon,
    oxygen, gold, silver, mercury, uranium,
  • There are 92 stable and common elements.

55
What are Things Made of?
  • At some point, certain things dont chemically
    break down into different parts. These are
    called elements.
  • Suppose you took a sample of an element and
    physically divided the sample into two, and took
    one of the halves and divided it into two, and so
    on. Can you go on forever dividing by two?

56
What are Things Made of?
  • At some point, certain things dont chemically
    break down into different parts. These are
    called elements.
  • Suppose you took a sample of an element and
    physically divided the sample into two, and took
    one of the halves and divided it into two, and so
    on. Can you go on forever dividing by two?
  • No, at some point you reach individual atoms. An
    atom cannot be split into parts without changing
    it.

57
How Light Interacts with Matter.
  • Atoms are the basic blocks of matter.
  • They consist of heavy particles (called protons
    and neutrons) in the nucleus, surrounded by
    lighter particles called electrons.
  • The number of protons determines which element
    the atom is.

58
How Light Interacts with Matter.
  • An electron will interact with a photon.
  • An electron that absorbs a photon will gain
    energy.
  • An electron that loses energy must emit a photon.
  • The total energy (electron plus photon) remains
    constant during this process.

59
How Light Interacts with Matter.
  • Electrons bound to atoms have discrete energies
    (i.e. not all energies are allowed).
  • Thus, only photons of certain energy can interact
    with the electrons in a given atom.

60
How Light Interacts with Matter.
  • Electrons bound to atoms have discrete energies
    (i.e. not all energies are allowed).
  • Thus, only photons of certain energy can interact
    with the electrons in a given atom.

Image from Nick Strobel (http//www.astronomynotes
.com)
61
How Light Interacts with Matter.
  • Electrons bound to atoms have discrete energies
    (i.e. not all energies are allowed).
  • Each element has its own unique pattern of
    energies.

62
How Light Interacts with Matter.
  • Electrons bound to atoms have discrete energies
    (i.e. not all energies are allowed).
  • Each element has its own unique pattern of
    energies, hence its own distinct line spectrum.

Image from Nick Strobel (http//www.astronomynotes
.com)
63
Emission spectraandabsorption spectra.
64
Emission and Absorption
Image from Nick Strobel (http//www.astronomynotes
.com)
65
Tying things together
  • The spectrum of a star is approximately a black
    body spectrum.
  • Hotter stars are bluer, cooler stars are redder.
  • For a given temperature, larger stars give off
    more energy than smaller stars.

66
  • In the constellation of Orion, the reddish star
    Betelgeuse is a relatively cool star. The blue
    star Rigel is relatively hot.

67
Tying things together
  • The spectrum of a star is approximately a black
    body spectrum.
  • Hotter stars are bluer, cooler stars are redder.
  • For a given temperature, larger stars give off
    more energy than smaller stars.
  • However, a closer look reveals details in the
    spectra

68
The Line Spectrum
  • Upon closer examination, the spectra of real
    stars show fine detail.
  • Dark regions where there is relatively little
    light are called lines.

69
The Line Spectrum
  • Today, we rarely photograph spectra, but rather
    plot the intensity vs the wavelength.
  • The lines where there is relatively little
    light show up as dips in the curves.

70
The Line Spectrum
  • Today, we rarely photograph spectra, but rather
    plot the intensity vs the wavelength.
  • The lines where there is relatively little
    light show up as dips in the curves.
  • These dips tell us about what elements are
    present in the star!

71
Atomic Fingerprints
  • Hydrogen has a specific line spectrum.
  • Each atom has its own specific line spectrum.

72
Atomic Fingerprints
  • These stars have absorption lines with the
    wavelengths corresponding to hydrogen!

73
Atomic Fingerprints.
  • One can also look at the spectra of other objects
    besides stars, for example clouds of hot gas.
  • This cloud of gas looks red since its spectrum is
    a line spectrum from hydrogen gas.

74
Good Review Questions, Chapter 2
  • 5. In what ways is the celestial sphere a
    scientific model?
  • 6. Where would you go on Earth if you wanted to
    be able to see both the north celestial pole and
    the south celestial pole at the same time?
  • 8. Explain how to make a simple astronomical
    observation to determine your latitude.
  • 13. Why are the seasons reversed in the southern
    hemisphere relative to the northern hemisphere?
  • 15. Do the phases of the Moon look the same from
    everywhere on Earth ?

75
The Celestial Sphere
  • Imagine the sky as a hollow sphere with the stars
    attached to it. This sphere rotates once every
    24 hours. This imaginary sphere is called the
    celestial sphere.
  • Even though we know it is not the case, it is
    useful to imagine the Earth as being stationary
    while the celestial sphere rotates around it.

76
The Celestial Sphere
  • The north celestial pole is directly above the
    north pole on the Earth.
  • The south celestial pole is directly above the
    south pole on the Earth.
  • The celestial equator is an extension of the
    Earths equator on the sky.
  • The zenith is the point directly over your head.
    The horizon is the circle 90 degrees from the
    zenith.

77
The Celestial Sphere
  • The celestial poles and the celestial equator are
    the same for everyone.
  • The zenith and the horizon depend on where you
    stand.
  • http//www.astronomynotes.com/nakedeye/s4.htm

78
Stellar Coordinates and Precession
  • There are a few ways to specify the location of a
    star (or planet) on the sky
  • Altitude/Azimuth
  • The altitude describes how many degrees the star
    is above the horizon, the azimuth describes how
    far the star is in the east-west direction from
    north.
  • The altitude and azimuth of a star is constantly
    changing owing to the motion of the star on the
    sky!

79
Stellar Coordinates and Precession
  • There are a few ways to specify the location of a
    star (or planet) on the sky
  • Equatorial system
  • Lines of longitude on the earth become right
    ascension, measured in units of time. The RA
    increases in the easterly direction.
  • Lines on latitude on the earth become
    declination, measured in units of degrees.
    DEC90o at the north celestial pole, 0o at the
    equator, and -90o at the south celestial pole.
  • http//www.astronomynotes.com/nakedeye/s6.htm

80
Stellar Coordinates and Precession
  • The north celestial pole moves with respect to
    the stars very slowly with time, taking 26,000
    years to complete one full circle.

81
Good Review Questions, Chapter 2
  • 5. In what ways is the celestial sphere a
    scientific model?
  • 6. Where would you go on Earth if you wanted to
    be able to see both the north celestial pole and
    the south celestial pole at the same time?
  • 8. Explain how to make a simple astronomical
    observation to determine your latitude.
  • 13. Why are the seasons reversed in the southern
    hemisphere relative to the northern hemisphere?
  • 15. Do the phases of the Moon look the same from
    everywhere on Earth ?

82
In Detail
  • If we do some careful observations, we find
  • The length of the daylight hours at a given spot
    varies throughout the year the Sun is out a
    longer time when it is warmer (i.e. summer), and
    out a shorter time when it is colder.
  • On a given day, the length of the daylight hours
    depends on where you are on Earth, in particular
    it depends on your latitude e.g. in the summer,
    the Sun is out longer and longer the further
    north you go.

83
In Detail
  • Near the North Pole, the Sun never sets in the
    middle of the summer (late June).
  • Likewise, the Sun never rises in the middle of
    the winter (late December).

84
In Detail
  • In most places on Earth, the weather patterns go
    through distinct cycles
  • Cold weather winter, shorter daytime
  • Getting warmer spring, equal daytime/nighttime
  • Warm weather summer, longer daytime
  • Cooling off fall, equal daytime/nighttime
  • These seasons are associated with the changing
    day/night lengths.

85
In Detail
  • When it is summer in the northern hemisphere, it
    is winter in the southern hemisphere, and the
    other way around.

86
What Causes the Seasons?
  • Is the Earth closer to the Sun during summer,
    and further away during winter? (This was the
    most commonly given answer during a poll taken at
    a recent Harvard graduation).
  • No! Otherwise the seasons would not be opposite
    in the northern and southern hemispheres.

87
What Causes the Seasons?
  • The Earth moves around the Sun. A year is
    defined as the time it takes to do this, about
    365.25 solar days.
  • This motion takes place in a plane in space,
    called the ecliptic.
  • The axis of the Earths rotation is inclined from
    this plane by about 23.5 degrees from the normal.

88
What Causes the Seasons?
  • The axis of the Earths rotation points to the
    same point in space (roughly the location of the
    North Star).
  • The result is the illumination pattern of the Sun
    changes throughout the year.

89
What Causes the Seasons?
  • Here is an edge-on view, from the plane of the
    Earths orbit.

90
What Causes the Seasons?
  • Here is a slide from NASA and NOAA.

91
What Causes the Seasons?
  • A slide from Nick Strobel.

92
What Causes the Seasons?
  • Because of the tilt of the Earths axis, the
    altitude the Sun reaches changes during the year
    It gets higher above the horizon during the
    summer than it does during the winter.

93
What Causes the Seasons?
  • Because of the tilt of the Earths axis, the
    altitude the Sun reaches changes during the year
    It gets higher above the horizon during the
    summer than it does during the winter.
  • Also, the length of the daytime hours changes
    during the year the daylight hours are longer
    in the summer and shorter in winter.

94
What Causes the Seasons?
  • The altitude of the Sun matters when the Sun is
    near the horizon, it does not heat as efficiently
    as it does when it is high above the horizon.
  • Image from Nick Strobels Astronomy Notes
    (http//www.astronomynotes.com/).

95
What Causes the Seasons?
  • Winter The combination of a short daytime and a
    Sun that is relatively low above the horizon
    leads to much less heating in the day, plus a
    longer period of cooling at night. Overall, it
    is colder.

96
What Causes the Seasons?
  • Summer The combination of a long daytime and a
    Sun that is relatively high above the horizon
    leads to much more heating in the day, plus a
    shorter period of cooling at night. Overall, it
    is warmer.

97
What Causes the Seasons?
  • Spring and Fall The number of hour of daylight
    is about equal to the number of nighttime hours,
    leading to roughly equal times of heating and
    cooling.

98
Good Review Questions, Chapter 2
  • 5. In what ways is the celestial sphere a
    scientific model?
  • 6. Where would you go on Earth if you wanted to
    be able to see both the north celestial pole and
    the south celestial pole at the same time?
  • 8. Explain how to make a simple astronomical
    observation to determine your latitude.
  • 13. Why are the seasons reversed in the southern
    hemisphere relative to the northern hemisphere?
  • 15. Do the phases of the Moon look the same from
    everywhere on Earth ?

99
What Causes the Phases of the Moon?
100
What Causes the Phases of the Moon?
  • The full Moon always rises just after sunset.
  • The crescent Moon always points towards the Sun.
  • A crescent Moon sets shortly after sunset, or
    rises just before sunrise.
  • The Moon is illuminated by reflected sunlight.

101
What Causes the Phases of the Moon?
  • The full Moon always rises just after sunset.
  • A crescent Moon sets shortly after sunset.

102
What Causes the Phases of the Moon?
  • The full Moon always rises just after sunset.
  • A crescent Moon sets shortly after sunset.

103
What Causes the Phases of the Moon?
  • The lit side of the Moon always faces the Sun.
  • Because of the motion of the Moon relative to the
    Sun, we see different amounts of lit and dark
    sides over the course of a month.

104
What Causes the Phases of the Moon?
  • The lit side of the Moon always faces the Sun.
  • Because of the motion of the Moon relative to the
    Sun, we see different amounts of lit and dark
    sides over the course of a month.

Image from Nick Strobel (http//www.astronomynotes
.com/)
105
Good Review Questions, Chapter 2
  • 5. In what ways is the celestial sphere a
    scientific model?
  • 6. Where would you go on Earth if you wanted to
    be able to see both the north celestial pole and
    the south celestial pole at the same time?
  • 8. Explain how to make a simple astronomical
    observation to determine your latitude.
  • 13. Why are the seasons reversed in the southern
    hemisphere relative to the northern hemisphere?
  • 15. Do the phases of the Moon look the same from
    everywhere on Earth ?

106
Good Review Questions, Chapter 3
  • 2. How did the Ptolemaic model explain
    retrograde motion of the planets?
  • 3. In what ways were the models of Ptolomy and
    Copernicus similar?
  • 7. Explain how Keplers laws contradict uniform
    circular motion.
  • 9. Review Galileos telescope discoveries and
    explain why they supported the Copernican model
    and contradicted the Ptolemaic model?
  • 13. Explain why you might describe the orbital
    motion of the moon with the statement The moon
    is falling.

107
Aristotle (385-322 B.C.)
  • Aristotle was perhaps the most influential Greek
    philosopher. He favored a geocentric model for
    the Universe
  • The Earth is at the center of the Universe.
  • The heavens are ordered, harmonious, and perfect.
    The perfect shape is a sphere, and the natural
    motion was rotation.

108
Geocentric Model
  • The motion of the Sun around the Earth accounts
    for the rising and setting of the Sun.
  • The motion of the Moon around the Earth accounts
    for the rising and setting of the Moon.
  • You have to fiddle a bit to get the Moon phases.

109
Geocentric Model
  • The fixed stars were on the Celestial Sphere
    whose rotation caused the rising and setting of
    the stars.

110
  • This is the constellation of Orion

111
  • The constellations rise and set each night, and
    individual stars make a curved path across the
    sky.
  • The curvature of the tracks depend on where you
    look.

112
Geocentric Model
  • The fixed stars were on the Celestial Sphere
    whose rotation caused the rising and setting of
    the stars.
  • However, the detailed motions of the planets were
    much harder to explain

113
Planetary Motion
  • The motion of a planet with respect to the
    background stars is not a simple curve. This
    shows the motion of Mars.
  • Sometimes a planet will go backwards, which is
    called retrograde motion.

114
Planetary Motion
  • Here is a plot of the path of Mars.
  • Other planets show similar behavior.

Image from Nick Strobel Astronomy Notes
(http//www.astronomynotes.com/)
115
Aristotles Model
  • Aristotles model had 55 nested spheres.
  • Although it did not work well in detail, this
    model was widely adopted for nearly 1800 years.

116
Better Predictions
  • Although Aristotles ideas were commonly
    accepted, there was a need for a more accurate
    way to predict planetary motions.

117
Better Predictions
  • Although Aristotles ideas were commonly
    accepted, there was a need for a more accurate
    way to predict planetary motions.
  • Claudius Ptolomy (85-165) presented a detailed
    model of the Universe that explained retrograde
    motion by using complicated placement of circles.

118
Ptolomys Epicycles
  • By adding epicycles, very complicated motion
    could be explained.

119
Ptolomys Epicycles
Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com/).
120
Ptolomys Epicycles
121
Ptolomys Epicycles
  • Ptolomys model was considered a computational
    tool only.
  • Aristotles ideas were true. They eventually
    became a part of Church dogma in the Middle Ages.

122
The Sun-Centered Model
  • Nicolaus Copernicus (1473-1543) proposed a
    heliocentric model of the Universe.
  • The Sun was at the center, and the planets moved
    around it in perfect circles.

123
The Sun-Centered Model
  • Nicolaus Copernicus (1473-1543) proposed a
    heliocentric model of the Universe.
  • These stamps mark the 500th anniversary of his
    birth.

124
The Sun-Centered Model
  • The Sun was at the center. Each planet moved on
    a circle, and the speed of the planets motion
    decreased with increasing distance from the Sun.

125
The Sun-Centered Model
  • Retrograde motion of the planets could be
    explained as a projection effect.

126
The Sun-Centered Model
  • Retrograde motion of the planets could be
    explained as a projection effect.

Image from Nick Strobels Astronomy Notes
(http//www.astronomynotes.com/)
127
Copernican Model
  • The model of Copernicus did not any better than
    Ptolomys model in explaining the planetary
    motions in detail.
  • He did work out the relative distances of the
    planets from the Sun.
  • The philosophical shift was important (i.e. the
    Earth is not at the center of the Universe).

128
Good Review Questions, Chapter 3
  • 2. How did the Ptolemaic model explain
    retrograde motion of the planets?
  • 3. In what ways were the models of Ptolomy and
    Copernicus similar?
  • 7. Explain how Keplers laws contradict uniform
    circular motion.
  • 9. Review Galileos telescope discoveries and
    explain why they supported the Copernican model
    and contradicted the Ptolemaic model?
  • 13. Explain why you might describe the orbital
    motion of the moon with the statement The moon
    is falling.

129
Johannes Kepler (1571-1630)
  • Kepler was a mathematician by training.
  • He believed in the Copernican view with the Sun
    at the center and the motions of the planets on
    perfect circles.
  • Tycho hired Kepler to analyize his observational
    data.

130
Johannes Kepler (1571-1630)
  • Kepler was a mathematician by training.
  • He believed in the Copernican view with the Sun
    at the center and the motions of the planets on
    perfect circles.
  • Tycho hired Kepler to analyize his observational
    data.
  • After years of failure, Kepler dropped the notion
    of motion on perfect circles.

131
Keplers Three Laws of Planetary Motion
  • Starting in 1609, Kepler published three laws
    of planetary motion

132
Keplers Three Laws of Planetary Motion
  • Starting in 1609, Kepler published three laws
    of planetary motion
  • Planets orbit the Sun in ellipses, with the Sun
    at one focus.

133
Ellipses
  • An ellipse is a flattened circle described by a
    particular mathematical equation.
  • The eccentricity tells you how flat the ellipse
    is e0 for circular, and e1 for infinitely flat.

134
Ellipses
  • You can draw an ellipsed with a loop of string
    and two tacks.

135
Keplers Three Laws of Planetary Motion
  • Starting in 1609, Kepler published three laws
    of planetary motion
  • Planets orbit the Sun in ellipses, with the Sun
    at one focus.

136
Keplers Three Laws of Planetary Motion
  • Starting in 1609, Kepler published three laws
    of planetary motion
  • Planets orbit the Sun in ellipses, with the Sun
    at one focus.
  • The planets sweep out equal areas in equal times.
    That is, a planet moves faster when it is closer
    to the Sun, and slower when it is further away.

137
Keplers Second Law
  • The time it takes for the planet to move through
    the green sector is the same as it is to move
    through the blue sector.
  • Both sectors have the same area.

138
Keplers Three Laws of Planetary Motion
  • Starting in 1609, Kepler published three laws
    of planetary motion
  • Planets orbit the Sun in ellipses, with the Sun
    at one focus.
  • The planets sweep out equal areas in equal times.
    That is, a planet moves faster when it is closer
    to the Sun, and slower when it is further away.

139
Keplers Three Laws of Planetary Motion
  • Starting in 1609, Kepler published three laws
    of planetary motion
  • Planets orbit the Sun in ellipses, with the Sun
    at one focus.
  • The planets sweep out equal areas in equal times.
    That is, a planet moves faster when it is closer
    to the Sun, and slower when it is further away.
  • (Period)2 (semimajor axis)3

140
Keplers Third Law
141
Good Review Questions, Chapter 3
  • 2. How did the Ptolemaic model explain
    retrograde motion of the planets?
  • 3. In what ways were the models of Ptolomy and
    Copernicus similar?
  • 7. Explain how Keplers laws contradict uniform
    circular motion.
  • 9. Review Galileos telescope discoveries and
    explain why they supported the Copernican model
    and contradicted the Ptolemaic model?
  • 13. Explain why you might describe the orbital
    motion of the moon with the statement The moon
    is falling.

142
Heliocentric or Geocentric?
  • The year is around 1610. The old school is
    Aristotle and a geocentric view. The new
    school is the heliocentric view (Copernicus and
    Kepler).
  • Which one is correct?
  • Observational support for the heliocentric model
    would come from Galileo.
  • Theoretical support for the heliocentric model
    would come from Isaac Newton.

143
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.

144
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.

145
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.

146
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.

147
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.

148
Venus in the Heliocentric View
  • The correlation between the phases and the size
    is accounted for in the heliocentric view.

149
Good Review Questions, Chapter 3
  • 2. How did the Ptolemaic model explain
    retrograde motion of the planets?
  • 3. In what ways were the models of Ptolomy and
    Copernicus similar?
  • 7. Explain how Keplers laws contradict uniform
    circular motion.
  • 9. Review Galileos telescope discoveries and
    explain why they supported the Copernican model
    and contradicted the Ptolemaic model?
  • 13. Explain why you might describe the orbital
    motion of the moon with the statement The moon
    is falling.

150
Newtons Laws of Motion
  • 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 (Fma).
  • For every action, there is an equal and opposite
    reaction. (For example, a rocket is propelled by
    expelling hot gas from its thrusters).

151
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.

152
What is Gravity?
  • The gravitational force between larger bodies is
    greater than it is between smaller bodies, for a
    fixed distance.

153
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.

154
Newtons Laws
  • Using Newtons Laws, we can
  • Derive Keplers Three Laws.
  • Measure the mass of the Sun, the Moon, and the
    Planets.
  • Measure the masses of distant stars in binary
    systems.

155
Laws of Physics
  • The models of Aristotle and Ptolomy were based
    mainly on beliefs (i.e. that motion should be on
    perfect circles, etc.).
  • Starting with Newton, we had a physical model of
    how the planets moved the laws of motion and
    gravity as observed on Earth give a model for how
    the planets move.
  • All modern models in Astronomy are based on the
    laws of Physics.

156
Newtons Laws and Orbits
  • Newton realized that since the Moons path is
    curved (i.e. it is accelerating), there must be a
    force acting on it.

157
Newtons Laws and Orbits
  • If you shoot a cannonball horizontally, it
    follows a curved path to the ground. The faster
    you launch it, the further it goes.

158
Newtons Laws and Orbits
  • If you shoot a cannonball horizontally, it
    follows a curved path to the ground. The faster
    you launch it, the further it goes.
  • If it goes really far, the Earth curves from
    under it

159
Newtons Laws and Orbits
  • Newton showed mathematically that the expected
    shape for a closed orbit is an ellipse (i.e. he
    explained the origin of Keplers first law).

160
Newtons Laws and Orbits
  • A geosynchronous satellite has an orbital period
    around the Earth of 24 hours (23 hours and 56
    minutes actually), which is the rotation period
    of the Sun.
  • The net effect is that the satellite is always
    above the same spot.

161
Good Review Questions, Chapter 3
  • 2. How did the Ptolemaic model explain
    retrograde motion of the planets?
  • 3. In what ways were the models of Ptolomy and
    Copernicus similar?
  • 7. Explain how Keplers laws contradict uniform
    circular motion.
  • 9. Review Galileos telescope discoveries and
    explain why they supported the Copernican model
    and contradicted the Ptolemaic model?
  • 13. Explain why you might describe the orbital
    motion of the moon with the statement The moon
    is falling.

162
Review Questions Chapter 4
  • Why do nocturnal animals usually have large
    pupils in their eyes? How is that related to the
    design of astronomical telescopes?
  • 5. Optical and radio astronomers both try to
    build large telescopes. How are their goals
    similar, and how are they different?

163
Nature of Light
  • Recall that light can be thought of as a wave.
    We will make use of the wave properties when
    collecting light.
  • Also, light can be thought of as a collection of
    discrete particles called photons. This
    viewpoint is important when considering the
    detection of light.

164
All of us have a photon collection and detection
system
165
The Eye
  • The eye is a photon collection and detection
    system
  • Light passes through the cornea,
  • passes through the pupil (a few mm in diameter),
  • and is focused by the lens onto the retina.
  • The optic nerve carries a signal to the brain,
    and you can see.

166
The Eye
  • The imaged focused on the retina is upside down!
  • The image is collected and erased from the retina
    20 to 30 times per second, allowing us to sense
    seamless motion.

167
At some point, objects are too faint to see
there are not enough photons per unit time to
make a significant signal at the retina.What to
do?
168
  • (1) Increase the photon rate
  • and/or
  • (b) Collect photons for a longer period of time

169
Telescopes and Light Detection
170
Collecting more Photons
  • Light rays from distant objects are essentially
    parallel. This makes the design of telescope
    optics much simpler.

171
Collecting More Photons
  • A lens or a curved mirror can focus parallel
    light onto a point.
  • The focal length depends on the curvature of the
    surface.

172
Telescopes
  • A telescope uses mirrors or lenses to collect and
    focus light.
  • The area of the lens or mirror can be
    considerably larger than the area of the eyes
    pupil, hence much fainter objects can be seen.

173
Telescopes
  • A refracting telescope uses a large lens to bring
    the light to a focus, as in Figure (a).
  • A reflecting telescope uses curved mirrors to
    bring the light to a focus, as in Figure (b).

174
Telescopes
  • The largest lenses that can be built have a
    diameter of about 1m, and have very long focal
    lengths.
  • A lens must be held by its edges, and large
    lenses sag under their own weight. Also lots of
    light is lost in the glass.
  • For these and more reasons, all modern telescopes
    use mirrors.

175
Telescopes
  • Using an objective mirror, plus some additional
    mirrors and lenses, light is collected and
    focused to a point.
  • This is a Newtonian telescope.

176
Telescopes
  • Using an objective mirror, plus some additional
    mirrors and lenses, light is collected and
    focused to a point.
  • This is a Cassegrain telescope.

177
Telescopes
  • A telescopes main job is collecting photons.
  • The light gathering power is proportional to the
    area of the mirror or lens.

178
Telescopes
  • Mirrors can be made to be much larger (up to 8
    meters or 25 feet in diameter) since they can be
    supported at their backs.
  • This mirror is 8 feet across.

179
Telescopes
  • Modern mirrors can be made thin. Their shapes
    are maintained using pistons under computer
    control.
  • The Gemini telescope in Hawaii has primary mirror
    8.1m in diameter.

180
Telescopes
  • Mirrors can also be made out of smaller segments.
  • The Keck telescopes in Hawaii have primary
    mirrors 10m in diameter.

181
Seeing Detail
  • What does the next line say?
  • If you can read this, thank a teacher.
  • Why is so hard to read?
  • Why do binoculars help?
  • It is hard to read because the angular size is
    small. The binoculars magnify the angular size.

182
Sidebar Angular Size
  • The amount of detail you can see depends on the
    angular size.
  • For a given detector (the eye, a telescope, etc),
    there is an angular size below which you cannot
    see detail. The larger the telescope, the smaller
    this limit.

183
The Second Use of a Telescope
  • A Telescope is used to collect photons, so you
    can see fainter objects.
  • It is also used to magnify the apparent angular
    size of an object so you can see detail.

Relatively good angular resolution.
184
The Second Use of a Telescope
  • A Telescope is used to collect photons, so you
    can see fainter objects.
  • It is also used to magnify the apparent angular
    size of an object so you can see detail.

Relatively poor angular resolution.
185
What a Telescope Does
  • A Telescope is used to collect photons, so you
    can see fainter objects.
  • A telescope is also used to magnify the apparent
    angular size of an object, thereby allowing one
    to see more detail.

186
Telescopes at other Wavelengths
  • Recall that there other forms of light,
    including radio waves, X-rays, UV light, etc.

187
Telescopes at other Wavelengths
  • Recall that there other forms of light,
    including radio waves, X-rays, UV light, etc.
  • The goal of collect and detect is still the
    same.

188
Telescopes at other Wavelengths
  • Recall that there other forms of light,
    including radio waves, X-rays, UV light, etc.
  • The goal of collect and detect is still the
    same.
  • However, the technologies used to collect and
    detect are different at different wavelengths.

189
Radio Telescopes
  • Radio telescopes use mirrors made from steel
    plates.
  • Radio receivers collect the focused radio waves.
  • The radio telescopes are huge because of the long
    wavelengths of the radio waves.

190
Radio Telescopes
  • Radio telescopes use mirrors made from steel
    plates.
  • Radio receivers collect the focused radio waves.
  • The radio telescopes are huge because of the long
    wavelengths of the radio waves.

191
Radio Telescopes
  • The GBT is the largest steerable radio telescope
    in the world, with a diameter of 100 meters. It
    is perhaps the largest movable land-based object
    in the world.

192
Radio Telescopes
  • With modern computers and electronics, one can
    combine the signals from several radio telescopes
    to synthesize a much larger telescope.

193
Review Questions Chapter 4
  • Why do nocturnal animals usually have large
    pupils in their eyes? How is that related to the
    design of astronomical telescopes?
  • 5. Optical and radio astronomers both try to
    build large telescopes. How are their goals
    similar, and how are they different?

194
The Solar Cycle
  • In the mid 1800s, a Swiss astronomer made
    detailed observations of sunspots in order to
    search for transits of a possible planet interior
    to Mercury.

195
The Solar Cycle
  • No planets were found, but it was discovered that
    the number of sunspots varies with an 11 year
    cycle.
  • This is not fully understood.

196
Sunspots
  • Galileo used sunspots to track the rotation of
    the Suns surface

197
Sunspots
  • Galileo was the first to sunspots to track the
    rotation of the Suns surface.

198
Sunspots
  • Galileo was the first to sunspots to track the
    rotation of the Suns surface.
  • The Sun does not rotate as a solid body. The
    equator rotates once every 25 days. At 45o
    latitude, it takes 27.8 days.

199
The Sun and Space Weather
  • Violent activity can occur in regions near
    sunspots.
  • A solar flare is a giant eruption of particles
    and radiation.
  • The radiation and particles can interact with the
    Earths upper atmosphere, disrupting satellite
    communications and power grids.
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