The Changing View from Earth

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The Changing View from Earth

• Chapter 13

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Cartoon of the Day
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What our Ancestors Saw

• For thousands of years the sky has been a source
  of information
• Time
• Date
• Weather
• Farmers took their cues from the celestial bodies
  (Sun, moon, stars) to know when to plant, and
  harvest their crops

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• Sailors used stars as a guidance system to cross
  oceans
• Astrologers were in demand
• People believed their destinies could be foretold
  by the stars.
• Ancients used the stars and motion of planets to
• Predict planetary motion, seasons, and eclipses

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Stories from Cultures

• Hindu mythology
• Seven wise men married seven sisters. Six of the
  women divorced their husbands and moved to
  another location in sky. They became the Pleiades
  (distinct star pattern)
• Asterism a distinctive star pattern
• The seven husbands became the seven stars of the
  big dipper. The wife became Alcor. A star in the
  crook of the big dipper.

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Stories from Cultures

• Algonquin, Iroquois, and Narragansett
• saw the constellation Ursa Major as a bear
  running away from hunters
• In the fall because the bear is low enough to
  brush the trees the blood from its wounds turns
  the leaves red.
• Another legend tells of 3 hunters chasing 4 elk
  as the 7 stars of the big dipper.
• One hunter is accompanied by a dog

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Celestial motion (Lunar)

• The moon traces a westward path across the sky.
• Each night it rises in the east an hour later
  than the previous night
• Its shape appears to change in phases, waxing
  from thin crescent to half and full moon. Then
  waning to a sliver again.
• http//www.astro.wisc.edu/dolan/java/MoonPhase.ht
  ml

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Solar Motion

• The Sun has no phases
• It also rises earlier and farther north each day
  from December 22 to June 22 and sets later
• Through the summer and fall it rises later and
  set earlier.
• http//www.earth.uni.edu/morgan/ajjar/SolarMotion
  /solarzenith.html

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Stellar Motion

• Stars and Planets also follow the same pattern
• They rise 4 minutes
• Earlier each night

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Planetary Motion And Retrograde

• Greeks noticed 5 objects wandering through the
  stars
• Called planets
• Mercury, Venus, Mars, Jupiter, Saturn
• Venus and Mercury seemed to stay close to the sun
• Mars, Jupiter, Saturn wandered Eastward
• Once a year they appeared to go backwards
  (westward)

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Retrograde motion

• Retrograde motion is the backwards motion that
  planets appear to trace across the sky.

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Modelling Celestial Motion

• Noticing the patterns is only half the battle
• Come up with basic ideas or theories
• Check theories using models
• Two early models
• Geocentric
• Earth centered model
• Heliocentric
• Sun centered model ( the one accepted today)

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Geocentric

• Aristotle
• Placed stars on outer circle (firmament or fixed
  stars)
• Also called the celestial sphere
• Inside the sphere he arranged more concentric
  spheres on which he placed the Sun, Moons and
  planets

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Explaining Epicycles

• Aristotles model didnt explain epicycles
• Ptolemy created a model which included an
  additional level of circles called Epicycles
• Good for predicting astronomical events

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Heliocentric

• 1500s Nicholas Copernicus
• Proposed a new model
• Fixed sun
• Planets (including Earth rotate around the sun)
• Arranged the planets orbits in a solar plane
• Imaginary disk extending out the suns Equator

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Galileo

• Italian Astronomer
• Discovered evidence supporting heliocentric
  model.
• Used a telescope
• Saw that Venus had phases like the moon did
• Spots on surface of sun
• Mountains on the moon
• Rings around saturn
• Four moons orbiting Jupiter (actually has 16)

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Galileo

• Published his ideas in Dialogue
• Galileo was required to recant his heliocentric
  ideas, which were condemned as "formally
  heretical".
• He was ordered imprisoned the sentence was later
  commuted to house arrest.
• His offending Dialogue was banned and in an
  action not announced at the trial, publication of
  any of his works was forbidden, including any he
  might write in the future.

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The solution

• The answers were discovered by a German
  mathematician Johannes Kepler
• According to calculations predictions would be
  more accurate if planetary orbits were ellipses
  (rather than circles)
• Ideas strengthened by Newtons Gravitational laws
  
• All objects in the universe are attracted to one
  another

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New Planets

• In 1781 Uranus was discovered
• Later using sun-centered model and Newtons laws
  they predicted where another planet should be
• Pointed their telescopes in its direction and
  discovered Neptune.
• Now knew of 8 planets in total (missing Pluto)

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Todays Views

• Scientist constructed geocentric model based on
  observations with unaided eye
• When Ideas were challenged a new theory was
  formulated
• This process of formulating new ideas continues
  even today.
• Thanks to technology we now have information on
  solar system, sun, 9 planets and their moons,
  meteors, asteroids, comets

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The Sun

• Made of Hydrogen gas
• Diameter 1.4 million km (110 times earth)
• Surface is constantly writhing/churning
• Solar prominance
• Streamers of hot gas that arch into space.
• Cooler regions appear darker in color (Sunspots)
• Near them violent outbursts/eruptions occur
  Solar flares
• Solar flares send high energy subatomic particles
  into space. (creates solar wind which can affect
  Earths activities)

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The Sun

• 330,000 times more massive than earth
• Hydrogen and helium
• Three layers
• Core 15,000,000 ? C
• Photosphere region of suns light
• 6,000 ? C
• Corona
• 1,000,000 ? C
• Closest star to the Earth
• Because Sun has planets orbiting it scientist
  predict other stars might have planets orbiting

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The Planets

• Inner planets
• Mercury, Venus, Mars
• Called terrestrial planets
• Because of their rocky composition
• Outer planets
• Jupiter, Saturn, Uranus, Neptune
• Have a gaseous composition
• Pluto
• Class all by itself because of strange orbit and
  tiny size

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Scale

• Earth diameter 12,750 km (1 Earth-Diameter)
• Venus 12,100 km (0.95 Earth-diameter)
• Jupiter 11.2 Earth-Diameter
• 143,200 km / 12,750 km 11.2

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Scale

• Other scales measured
• Mass
• Density
• Rotational period (time around sun)
• Orbital period

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Measuring Distance

• Distances in astronomy are so immense they are
  astonomical
• Scale used astronomical units (AU)
• 1 AU average distance from Earth to sun
  (149,599,000 km)
• Mars distance 1.5 AU
• Why is AU expressed as an average distance?

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Other Solar System Bodies

• Asteroids
• Known as minor planets (1m 100s of km)
• Ceres 1000 km
• Irregular shaped bodies of rock (silicate)
• Some cross path of earth (potential Collision)
• Comets
• Made of dust and ice
• Orbit at large distances
• Some fall towards the sun (evaporate) form tails
• Meteors, and Meteorites
• Dust and rock particles that heat up an vaporize
  in earths atmosphere (shooting stars) - Meteors
• Some remain large enough to hit the earth -
  Meteorites