Lecture 2: Our Place in the Universe

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Lecture 2 Our Place in the Universe

• Claire Max
• April 5, 2007
• Astro 18 Planets and Planetary Systems
• UC Santa Cruz

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Practicalities

• Whats the textbook situation?
• Web site
• I will put all lectures, assignments, homeworks,
  etc on class web page http//www.ucolick.org/max/
  astro18
• BUT its not all there yet. Start-up glitches.
  Be patient.
• Signing up for this course
• Its closed - we can only fit 43 people in this
  room.
• Since theres a waiting list, Ive requested a
  bigger room. Dont know yet if well get one.
• So if you are on waiting list, keep coming to
  class and doing assignments thru end of next
  week. Hopefully Ill know about a room by then.

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Outline of this lecture

• Concepts
• Based on reading assignment - Chapter 1 of
  Bennett, Our Place in the Universe
• Five minute break
• Projects

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Concepts in Lecture 2

• Geometry of the Earth relative to the rest of the
  Solar System
• Ecliptic plane
• Tilt of the Earths axis
• Seasons
• Star trails
• Distances concept of a light-year
• The expansion of the Universe
• If all but the closest galaxies are moving away
  from us, does that mean we are at the center of
  the Universe?
• Why do galaxies that are farther and farther from
  the Milky Way zoom away from us faster and
  faster?

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Concept One Geometry of the Earth.Main Points

• Motions of Sun, moon, stars and planets in the
  sky, as we see them from Earths surface each day
  and night, are apparent, not actual
• e.g. the Sun doesnt rotate around the Earth
  daily
• Nightly patterns of star motions result from
  Earths rotation plus tilt of its axis 23.5 deg
  from plane of its orbit
• Nightly motions are different depending on where
  you are on Earth, and what season it is

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Geometry of the Earth relative to the Solar System

• The Sun and all the planets except Pluto lie in a
  plane called the Ecliptic plane

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But Earths rotation axis is not perpendicular to
this plane

• Earths rotation axis is inclined at 23.5 degrees
• North rotational pole points to the North Star,
  Polaris

Note that both rotation and motion around Sun are
counterclockwise, if you are looking from above
the N pole
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Key definitions

• Celestial sphere
• Rotation axis
• Horizon
• Zenith
• Celestial pole
• Celestial equator

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Results of the tilt of Earths axis

• Seasons
• Apparent motions of stars in sky
• and how these vary with where you are on the
  Earth
• Precession of the Earths axis
• in 15,000 AD, the North Star wont be Polaris
  any more, it will be Vega (the brightest star in
  the Summer Triangle) (see Bennett)

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Seasons Key concepts

• Earth's axis is tilted
• Tilt angle changes the angle of sunlight striking
  the Earth's surface
• At a fixed location on the Earth, the angle of
  the sunlight varies with time
• Seasons!
• Other planets have different tilts, and thus
  different types of seasons

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Seasons summer is when your hemisphere is tipped
toward Sun
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Seasons summer is when your hemisphere is tipped
toward Sun
Note Earth is closest to Sun in January,
farthest in July!
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Most extreme seasons in Solar System 42-year
summer!

• Uranus is tipped on its side
• Rotation axis lies almost in its orbital plane
• Uranus takes 84 Earth-years to go around the Sun
• So the North polar regions of Uranus have summer
  (in this case, continuous sunlight) for 42
  Earth-years!

orbital plane
rotational pole
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Apparent motion of stars
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Nightly motion of stars if youare at the equator
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Nightly motion of stars if you are at the equator
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Nightly motion of stars if youare at the North
Pole
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Nightly motion of stars if youare at the North
Pole
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Nightly motion of stars if youare at latitude 40
deg North
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Nightly motion of stars at latitude 40 deg North,
looking to East
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Nightly motion of stars at latitude 40 deg North
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Nightly motion of stars at latitude 40 deg North,
looking to North
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What about the Tropic of Cancer?
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Tropic of Cancer At summer solstice, Sun is at
Zenith at noon
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Arctic Circle At summer solstice, sun barely
stays above horizon
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First ConcepTest

• I will pose a question on next slide.
• First, each of you will have one minute to think
  about the answer (three multiple choices). This
  is not a trick question think conceptually.
• Then, break into groups of 3
• You will have two minutes to convince your
  neighbors of the best answer.
• I will then ask for a show of hands for the three
  multiple choices, and we will discuss the
  results.

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ConcepTest One

• You are having an argument with a friend about
  what causes Earths seasons. Your friend insists
  the difference between summer and winter is that
  the Earth is closer to the Sun in summer than it
  is in winter. Which of the following is the best
  fact you can use to convince your friend that
  his/her explanation must be wrong?
• a) days are shorter in winter than in summer
• b) if you are above the Arctic Circle in winter,
  there is a long period of time when the sun never
  rises
• c) when it is winter in the Northern Hemisphere,
  it is summer in the Southern Hemisphere

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Concept Two What is a light-year?First discuss
speed of light.

• Light doesnt travel infinitely fast.
• If light propagates in a vacuum (as in outer
  space), its speed is a very specific number
• c 300,000 km/sec 3 x 1010 cm/sec
• At this speed, light would circle the Earth eight
  times in 1 second

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Since speed of light is constant, can use it to
measure distance

• distance speed x time
• Use dimensional analysis
• Write down units of each quantity in an equation
• Then cross out places where the same unit is in a
  numerator and denominator
• Example

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Since speed of light is constant, can use it to
measure distance

• distance speed x time
• Use dimensional analysis
• Write down units of each quantity in an equation
• Then cross out places where the same unit is in a
  numerator and denominator
• Example

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Define a light-year

• A light-year is the distance that light travels
  in one Earth-year
• How big is it?

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Some examples of light travel-time

• The Moon
• It takes light 1 sec to travel from the moon to
  the Earth, so the Moon 1 light-sec away
• The Sun
• It takes light 8 minutes to travel from the Sun
  to the Earth, so the Sun is 8 light-minutes away
• The nearest star, Proxima Centauri
• It takes light about 4 years to travel from
  Proxima Centauri to the Earth, so this star is 4
  light-years away

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Implications of the finite speed of light

• Because it takes light a finite amount of time to
  reach us,
• the farther away we look in distance, the further
  back we look in time
• In 1987 when we saw a supernova explosion in the
  Large Magellanic Cloud (a neighboring
  galaxy150,000 light-years away), the supernova
  had actually exploded 150,000 years ago
• When we look at galaxies that are more and more
  distant from us, we are seeing them at younger
  and younger stages of their evolution

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ConcepTest Two

• If the speed of light were half what it is now,
  then a light-year would
• a) take half as long to traverse at light speed
• b) take the same amount of time to traverse at
  light speed
• c) last twice as many months
• d) last half as many months

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Concept Three The Expanding Universe

• Observation
• Virtually every galaxy outside our Local Group is
  moving away from us
• The farther away a galaxy is, the faster it is
  moving away from us
• How is the observation made? From Doppler shift
  of spectral lines (will discuss in later lecture)
• Recession velocities are large
• tens of thousands to 100s of thousands of km/sec

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Whats going on?

• Entire universe is expanding
• (Its not that everybody hates us....)
• Furthermore, at every place in the universe, it
  looks like the rest of the galaxies are all
  receding, and more distant galaxies are receding
  faster
• Analogies to help understand this
• A jungle gym that whose bars are all getting
  longer
• A sponge cake that is expanding as it bakes

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Local Sponge Cake Example

• Every raisin sees all the other raisins moving
  away from it
• More distant raisins move away faster

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The Big Bang

• This is as far back as we can hope to measure
• Every place in the universe was (almost)
  infinitely dense and infinitely hot
• Since the Big Bang the universe has been
  expanding, becoming less dense (on the average),
  and cooling off

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ConcepTest Three

• There must be some very large distance such that
  light from a galaxy at that distance hasnt
  reached us during the age of the universe. The
  expansion velocity of galaxies at that distance,
  relative to us, must be
• a) zero
• b) infinite
• c) less than the speed of light
• d) the speed of light or greater

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Projects

• Topic selection
• First meeting of your group next week (it will
  meet weekly thereafter)
• Weekly e-mails to me
• Other milestones
• Meetings with Prof. and TA
• Presentation dates
• Report due date

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Topics suggested (see web or handout at last
lecture)

• The Hazard from Outer Space Killer asteroids
  and comets
• The Earths Climate How and why does it change?
• Our Nearest Neighbor Planets Were Mars and Venus
  more hospitable in the past?
• Solar Systems How do they form, and why do many
  look so different from our own?
• Life in the Universe What are the conditions for
  life to form and prosper?
• Voyaging to Mars What can be learned by going
  there? What are the pluses and minuses of
  sending people or robots to Mars?
• Volcanoes and Underground Oceans on the Moons of
  the Outer Planets
• Vast Clouds of Comets and Asteroids

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Topics for Projects

• Please sign up for your first and second choice
  topics
• List your name, email address, and whether the
  topic is your 1st or 2nd choice
• Stefan and I will attempt to put together groups
  of four students per topic. Will announce groups
  on Tuesday at lecture.
• Topics not on list please see me after class

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First milestone for your project

• By end of next week (Week 2) Brainstorming
• Hold at least one in-person meeting of your
  project group
• Purpose to brainstorm a list of all the
  possible questions you can ask about your chosen
  topic
• Brainstorming
• One person serves as scribe (at blackboard, for
  example)
• Everyone in group suggests ideas
• Scribe writes each one down
• No criticisms allowed! Just put all the ideas
  down.

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Brainstorming, continued

• Main point of brainstorming is to build on each
  others ideas
• Keeping the discussion positive (no criticisms
  allowed) encourages creativity.
• Nobody feels turned off or discouraged
• A generally useful method in brainstorming, the
  group can actually be more creative than its
  component individuals

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Example of brainstorming list for Pluto - an
imaginary project

• Why is Pluto so small?
• What is Pluto made of? How do we know?
• How come Plutos orbit is so elliptical?
• Did Pluto used to be an asteroid? How do we
  know?
• Does Pluto look like the Moon, with lots of
  craters?
• Does Pluto have an atmosphere?
• How cold is it on Pluto and Charon? Was it
  always that cold?
• What could we learn from sending a spacecraft to
  Pluto and Charon?
• How long would it take to get there? Could it go
  into orbit there?
• Does Pluto have seasons? What are they like?
• Could we change Plutos orbit into a circle if we
  wanted to?
• What sort of science fiction story could we write
  about Pluto?

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Due at of day next Monday Jan 20

• From each group
• A list of all the questions that youve
  brainstormed about your topic.
• Either handed in to me in class on Thursday, or
  emailed to me by end of the day Friday.
• From each individual (all of you)
• A very short email to me, giving me feedback on
  how your group is going did everyone participate
  in your brainstorming session, did you feel
  included or left out, did you enjoy it
• Ill ask you to do this each week, for a while at
  least

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Milestones for Projects

• Jan 20 Brainstorming list
• Jan 26
• Narrow down brainstorming list to the most
  important four questions
• Each person sign up to research one question
• (Group) Hand in a one page summary of what your
  project is - what are the questions, why are they
  important, what methods do you plan to use to
  address them, what help can we give you, etc.
• Each Project should contain at least one
  calculation you can do
• Email to me

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Milestones, continued

• Feb 4 Do preliminary exploration of your topics
• Find one or two web sites, one or two books about
  your topics
• Skim them to see if they are going to be useful
• Use them to put together list of other
  potentially useful books and web sites
• Get together and compare notes, tell each other
  what youve found out so far
• Discuss whether new questions have arisen that
  seem interesting enough to pursue
• Hand in one page list of at least 4 books and 4
  websites that you plan to use, who will read
  which, list new questions that have arisen,
  describe whether if and how you plan to pursue
  these new questions

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Milestones, continued

• Feb 4 - 23rd
• Work on your projects
• Week of Feb 24-28th
• Meet with Stefan and me a second time
• Week of March 1 - 7
• Finish up research and rehearse presentations

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Assignments for next week

• Tuesday Jan 14 before class
• Chapter 5 of Bennett
• Thursday Jan 16 before class
• Handout Celestial Mechanics
• Do Homework 2 (will be on web by tomorrow)
• Hold project group meeting