ASTRO 101

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ASTRO 101

• Principles of Astronomy

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Instructor Jerome A. Orosz
(rhymes with boris)Contact

• Telephone [phone]
• E-mail orosz_at_sciences.sdsu.edu
• WWW http//mintaka.sdsu.edu/faculty/orosz/web/
• Office Physics 241, hours T TH 330-500

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Text Perspectives on Astronomy First
Editionby Michael A. Seeds Dana Milbank.
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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

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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).

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Questions for Today

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

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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

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The Doppler Shift Measuring Motion

• If a source of waves is not moving, then the
  waves are equally spaced in all directions.

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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.

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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.

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The Doppler Shift Measuring Motion

• If a source of light is moving away, the
  wavelengths are increased, or redshifted.

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The Doppler Shift Measuring Motion

• If a source of light is moving closer, the
  wavelengths are shortened, or blueshifted.

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The Doppler Shift Measuring Motion

• The size of the wavelength shift depends on the
  relative velocity of the source and the observer.

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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.

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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.

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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.
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Telescopes
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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.

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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).

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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.

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Telescopes

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

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Telescopes

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

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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.

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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.

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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.

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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.

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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.
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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.
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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.

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Telescopes at other Wavelengths

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

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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.

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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.

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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.

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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.

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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.

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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.

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Radio Telescopes

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

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Telescopes at other Wavelengths

• For most wavelengths, you need to go into space
  to observe.

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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

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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)

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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

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Highlights

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

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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

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Highlights

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

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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 ?

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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.

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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?

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Quick Review of Chapters 4.1, 5.2 and 5.4
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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.
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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.
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What is the nature of light?Light can be
thought of as awave in an electric fieldoras
discrete particles of energy
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How light interacts with matter andthe line
spectrum.
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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.

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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.

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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?

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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.

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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.

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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.

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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.

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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)
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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.

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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)
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Emission spectraandabsorption spectra.
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Emission and Absorption
Image from Nick Strobel (http//www.astronomynotes
.com)
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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.

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

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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

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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.

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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.

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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!

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Atomic Fingerprints

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

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Atomic Fingerprints

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

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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.

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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 ?

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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.

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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.

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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

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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!

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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

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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.

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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 ?

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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.

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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).

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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.

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In Detail

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

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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.

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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.

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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.

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What Causes the Seasons?

• Here is an edge-on view, from the plane of the
  Earths orbit.

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What Causes the Seasons?

• Here is a slide from NASA and NOAA.

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What Causes the Seasons?

• A slide from Nick Strobel.

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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.

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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.

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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/).

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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.

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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.

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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.

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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 ?

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What Causes the Phases of the Moon?
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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.

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What Causes the Phases of the Moon?

• The full Moon always rises just after sunset.
• A crescent Moon sets shortly after sunset.

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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.

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• 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.