Chapter 1,2 Our Place in the Universe, Night sky

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Chapter 1,2 Our Place in the Universe, Night
sky
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To understand the universe, astronomers use the
laws of physics and construct testable theories
and models.

• Scientific method
• Based on observation, logic, and skepticism.
• Hypothesis
• A collection of well thought-out ideas to explain
  a phenomena
• Model.
• Hypotheses that have withstood observational and
  experimental tests.
• Theory
• A well-founded body of related hypotheses and
  models that form a self-consistent description of
  nature.

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Science, religion, and pseudo-science

• Scientific theories are experimentally verifiable
  (or falsifiable) and predictive. They address how
  questions (e.g. How do stars form? When will the
  next lunar eclipse be?)
• Religious and ethical statements are (generally)
  not verifiable. They address why questions (e.g.
  Why does the Universe exist? What kind of life is
  worth living?). These are not intrinsically less
  worthwhile than scientific inquiries, they are
  simply addressing different questions.
• Pseudo-science theories pretend to be scientific
  but are either not falsifiable (e.g. seances,
  intelligent design) or supporters or use
  anecdotal evidence to support claims (e.g.
  astrology, magnetic healing, ).

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Scepticism and Truth

• Role of scepticism. An essential part of the
  scientific method. Scientists always question the
  basis for an scientific assertion. This is often
  considered impolite behavior in social
  settings, but is not impolite in scientific
  discussion.
• What is truth? - Scientific theories are not
  statements of truth. They are the best
  explanation for observed facts, but are subject
  to revision or falsification. Example
  Gravitation (Newton vs. Einstein).

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Which are valid scientific hypotheses?

• Pluto is made of green cheese.
• The soul lives on after the body dies.
• Jon Stewart is the funniest person alive.
• Osama Bin Laden is dead.
• All asteroids contain fossil material.
• God made all things.

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Astronomers use angles to denote the positions
and apparent sizes of objects in the sky.
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Angular Measure

• Basic unit of angular measure is the degree º
• Full circle measures 360º
• Right angle measures 90º
• Angular distance is the number of degrees across
  the sky between two points.
• Angular diameter or angular size is the number of
  degrees from one side of an object to the other
  side.
• The angular sizes of the Moon and the Sun are
  both 0.5º

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Try this Estimate approximate angular size of
this disk? (use your hand)
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Angular Measure for Small Angles

• 1º 60 arcminutes 60'
• 1' 60 arcseconds 60?
• e.g., On January 1, the planet Saturn had an
  angular diameter of 19.7? as viewed from Earth.

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Moon Angular sizes
Sea of Tranquility (Apollo 11 site)
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Example On November 28, 2000, the planet Jupiter
was 609 million kilometers from Earth and had an
angular diameter of 48.6?. Using the small-angle
formula, determine Jupiters actual diameter.

• D 48.6? x 609,000,000 km / 206206 143,000 km

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Powers-of-ten notation is a useful shorthand
system of writing really large numbers.
100 1 101 10 102 100 103 1,000 104
10,000 106 1,000,000 109 1,000,000,000
One Ten (deca-) Hundred (centa-) Thousand
(kilo-) Ten thousand Million (mega-) Billion
(giga-)
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Powers-of-ten notation is a useful shorthand
system of writing really small numbers.
100 1 10-1 0.10 10-2 0.01 10-3 0.001 10-4
0.0001 10-6 0.000001 10-9 0.000000001
One One-tenth (deci-) One-hundredth
(centi-) One-thousandth (milli-) One-ten-thousandt
h One-millionth (micro-) One-billionth (nano-)
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Scientific Notation example Population of Earth
6.5 billion 6.5109
(6,500,000,000)
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Astronomical distances
Astronomical Unit (AU) One AU is the average
distance between Earth and the Sun (150 million
km or 93 million miles). Light Year (ly) One
light year (ly) is the distance light can travel
in one year at a speed of about 3 x 105 km/s or
186,000 miles/s (9.46 X 1012 km or 63,240
AU). Parsec (pc) One parsec (pc) is the
distance from which Earth would appear to be one
arcsecond from the Sun. One parsec 3.27 light
years.
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1.2 The Scale of the Universe
Guiding questions

• How big is Earth compared to our solar system?
• How far away are the stars?
• How big is the Milky Way Galaxy?
• How big is the universe?
• How do our lifetimes compare to the age of the
  universe?

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How big is Earth compared to our solar
system?Lets reduce the size of the solar
system by a factor of 10 billion the Sun is now
the size of a large grapefruit (14 cm
diameter).How big is Earth on this scale?

• an atom
• a ball point
• a marble
• a golf ball

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Lets reduce the size of the solar system by a
factor of 10 billion the Sun is now the size of
a large grapefruit (14 cm diameter).How big is
Earth on this scale?

• an atom
• a ball point
• a marble
• a golf ball

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The scale of the solar system

• On a 1-to-10 billion scale
• Sun is the size of a large grapefruit (14 cm)
• Earth is the size of a ball point, 15 meters away.

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How far away are the stars?On our 1-to-10
billion scale, its just a few minutes walk to
Pluto.How far would you have to walk to reach
Alpha Centauri?

• 1 mile
• 10 miles
• 100 miles
• the distance across the U.S. (2500 miles)

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Answer D, the distance across the U.S.
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How big is the Milky Way Galaxy?
The Milky Way has about 100 billion stars. On the
same ten billion-to-one scale.
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Thought QuestionSuppose you tried to count the
more than 100 billion stars in our galaxy, at a
rate of one per second How long would it take
you?

• a few weeks
• a few months
• a few years
• a few thousand years

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Suppose you tried to count the more than 100
billion stars in our galaxy, at a rate of one per
second How long would it take you?

• a few weeks
• a few months
• a few years
• a few thousand years

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How big is the Universe?

• The Milky Way is one of about 100 billion
  galaxies.
• 1011 stars/galaxy x 1011 galaxies 1022 stars

As many stars as grains of (dry) sand on all
Earths beaches
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• Now lets step through the Universe in powers of
  10

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How do our lifetimes compare to the age of the
Universe?

• The Cosmic Calendar a scale on which we compress
  the history of the universe into 1 year.

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How do our lifetimes compare to the age of the
Universe?

• The Cosmic Calendar a scale on which we compress
  the history of the universe into 1 year.

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Review

• How big is Earth compared to our solar system?
• The distances between planets are huge compared
  to their sizeson a scale of 1-to-10 billion,
  Earth is the size of a ball point and the Sun is
  15 meters away
• How far away are the stars?
• On the same scale, the stars are thousands of km
  away
• How big is the Milky Way galaxy?
• It would take more than 3,000 years to count the
  stars in the Milky Way Galaxy at a rate of one
  per second, and they are spread across 100,000
  light-years

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• How big is the universe?
• The observable universe is 14 billion light-years
  in radius and contains over 100 billion galaxies
  with a total number of stars comparable to the
  number of grains of sand on all of Earths
  beaches
• How do our lifetimes compare to the age of the
  universe?
• On a cosmic calendar that compresses the history
  of the Universe into one year, human civilization
  is just a few seconds old, and a human lifetime
  is a fraction of a second

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1.3 Spaceship Earth
Key questions

• How is Earth moving in our solar system?
• How is our solar system moving in the Galaxy?
• How do galaxies move within the Universe?
• Are we ever sitting still?

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How is Earth moving in our solar system?

• Contrary to our perception, we are not sitting
  still.
• We are moving with the Earth in several ways, and
  at surprisingly fast speeds

The Earth rotates around its axis once every day.
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Earth orbits the Sun (revolves) once every year

• at an average distance of 1 AU 150 million km.
• with Earths axis tilted by 23.5º (pointing to
  Polaris)
• and rotating in the same direction it orbits,
  counter-clockwise as viewed from above the North
  Pole.

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Our Sun moves randomly relative to the other
stars in the local Solar neighborhood

• typical relative speeds of more than 70,000
  km/hr
• but stars are so far away that we cannot easily
  notice their motion

And orbits the galaxy every 230 million years.

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More detailed study of the Milky Ways rotation
reveals one of the greatest mysteries in
astronomy
Most of Milky Ways light comes from disk and
bulge
. but most of the mass is in its halo
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How do galaxies move within the universe?
Galaxies are carried along with the expansion of
the Universe. But how did Hubble figure out that
the universe is expanding?
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Hubble discovered that

• All galaxies outside our Local Group are moving
  away from us.
• The more distant the galaxy, the faster it is
  racing away.

Conclusion We live in an expanding universe.
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Are we ever sitting still?
Earth rotates on axis gt 1,000 km/hr
Earth orbits Sun gt 100,000 km/hr
Solar system moves among stars 70,000 km/hr
Milky Way rotates 800,000 km/hr
Milky Way moves in Local Group
Universe expands
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Review

• How is Earth moving in our solar system?
• It rotates on its axis once a day and orbit the
  Sun at a distance of 1 A.U. 150 million km
• How is our solar system moving in the Milky Way
  galaxy?
• Stars in the Local Neighborhood move randomly
  relative to one another and orbit the center of
  the Milky Way in about 230 million years

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• How do galaxies move within the universe?
• All galaxies beyond the Local Group are moving
  away from us with expansion of the Universe the
  more distant they are, the faster theyre moving
• Are we ever sitting still?
• No!

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1.4 The Human Adventure of Astronomy
Key question

• How has the study of astronomy affected human
  history?

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How has the study of astronomy affected human
history?

• Copernican Revolution showed that Earth was not
  the center of the universe (Chapter 3)
• Study of planetary motion led to Newtons Laws of
  motion and gravity (Chapter 4)
• Newtons laws laid the foundation of the
  industrial revolution
• Modern discoveries are continuing to expand our
  cosmic perspective

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Review

• How has the study of astronomy affected human
  history?
• Throughout history, astronomy has provided an
  expanded perspective on Earth that has grown hand
  in hand with social and technological developments