A Historical Perspective

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A Historical Perspective

• 2950
• Dr Bryce

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Class notices

• Labs begin this week
• Clear Sky Patrol is now operating WEATHER
  PERMITING
• First Homework Deadline is Friday at 5pm
• Remember drop/add slips are signed in the Physics
  General Office Room 203 Van Allen Hall

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Basic Observations

• Each Evening the Sun sets, the skies darken and
  on clear nights we are able to see stars/planets.
• We can also easily observe the westward motion of
  these heavenly/celestial bodies

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The paths of stars during the course of a night
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Stars appear to be grouped into Constellations

• A constellation is a region of the sky not a
  group of stars.
• 88 constellations fill the entire sky
• Although stars may be close together in a
  constellation they may actually be very distant
  from each other

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The Constellation Orion
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The Celestial Sphere
The 88 official constellations cover the
celestial sphere.
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Describing locations

• Our next problem is how do we describe or rather
  define locations in the sky?
• Consider how we describe locations on Earth
• Latitude
• Longitude
• North South East West

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Coordinates on Earth
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The Celestial Sphere

• The imaginary sphere that surrounds the Earth.
• Allows us to Map the Sky.
• North Celestial Pole is directly over the Earths
  North Pole

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More Celestial Sphere

• The South Celestial Pole is directly over the
  Earths South Pole.
• The Celestial Equator is a projector of the
  Earths equator.
• The Ecliptic is the Suns path during the Year,
  more about this later

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The Local Sky
An objects altitude (above horizon) and azimuth
(along horizon) specifies its location in your
local sky
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Special features
Zenith The point directly overhead Horizon
All points 90 away from zenith Meridian Line
passing through zenith and connecting N and S
points on horizon
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To describe Position

• Altitude will describe how far up in the sky an
  object is.
• With altitude 0o an object is on the horizon
  (setting or rising) and with altitude 90o an
  object is at the observers zenith
• Azimuth will describe how far round the sky an
  object is.
• With azimuth 0o an object is due North, 90o due
  East, 180o due South, 270o due West

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Problems

• Stars rise and set during the course of the night
• This means that their altitudes and azimuths
  are constantly changing
• A stars altitude and azimuth will change with the
  observers location
• Not the most convenient system

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Altitude of the celestial pole your latitude
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The Equatorial System

• Uses the Celestial Poles and the Celestial
  equator to define positions
• Declination (equivalent to latitude) gives the
  position angle north or south of the celestial
  equator
• Right ascension (equivalent to longitude) gives
  the position angle Eastwards around the Celestial
  equator from the Vernal Equinox
• These values remain constant as the Earth revolves

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We measure the sky using angles instead of
metres. The further away you are from an object
the smaller its angular size will be.
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Angular Measurements

• Full circle 360º
• 1º 60? (arcminutes)
• 1? 60? (arcseconds)

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Why do stars rise and set?

• Earth rotates east to west, so stars appear to
  circle from west to east.

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Our view from Earth

• The horizon (90 degrees from the zenith) splits
  the celestial sphere into two equal parts
• We can only see objects above the horizon
• An object moving below the horizon is setting
• Stars near the north celestial pole are
  circumpolar and never set.
• We cannot see stars near the south celestial
  pole.
• All other stars (and Sun, Moon, planets) rise in
  east and set in west.

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The sky varies with latitude but not longitude.
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Your location

• Polaris is located in a constellation called the
  little dipper or Ursa Minor
• The altitude of Polaris is the same angle as your
  latitude on Earth
• Remember that when you are facing Polaris you
  are facing North

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Observing at the North Pole
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Observing at the Equator
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Thought Question The North Star (Polaris) is 50
above your horizon, due north. Where are you?

• You are on the equator.
• You are at the North Pole.
• You are at latitude 50N.
• You are at longitude 50E.
• You are at latitude 50N and longitude 50E.

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Seasons

• One of our most basic observations in Astronomy.
• In Summer the Sun is above the horizon for more
  time than it is in Winter.
• The days are longer and the nights are shorter.
• In Winter the Sun is above the horizon for less
  time and so it gets darker earlier.

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The stars we can see change throughout the year

• As the Earth orbits the Sun, the Sun appears to
  move eastward along the ecliptic.
• At midnight, the stars on our meridian are
  opposite the Sun in the sky.

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The Motion of the Sun

• Throughout the year the Sun appears to move
  relative to the background stars
• The constellations through which the Sun passes
  have well know names the Zodiac
• The Sun moves at a rate of about 1 degree a day
• No coincidence that there are 360 degrees in a
  circle

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The Suns path

• The Suns path across the Celestial Sphere is
  called the Ecliptic
• Tilted in relation to the Celestial Equator
• This means that the Suns declination changes
  throughout the year
• The ecliptic is the plane of the Earths orbit
• NOTE the vernal equinox

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Seasons depend on how Earths axis affects the
directness of sunlight
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The Real Reason for Seasons

• Earths axis points in the same direction (to
  Polaris) all year round, so its orientation
  relative to the Sun changes as Earth orbits the
  Sun.
• Summer occurs in your hemisphere when sunlight
  hits it more directly winter occurs when the
  sunlight is less direct.
• AXIS TILT is the key to the seasons without it,
  we would not have seasons on Earth.

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Variation in the Azimuthal angle of Sunrise
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Suns altitude also changes with seasons
Suns position at noon in summer higher
altitude means more direct sunlight.
Suns position at noon in winter lower altitude
means less direct sunlight.
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Fig.02.15
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We can recognize solstices and equinoxes by Suns
path across sky
Summer solstice Highest path, rise and set at
most extreme north of due east. Winter solstice
Lowest path, rise and set at most extreme south
of due east. Equinoxes Sun rises precisely due
east and sets precisely due west.
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Dates to remember

• Sept 22, Autumnal Equinox, Sun will rise in the
  East and set in the West, the day and night will
  be of equal length.
• Dec 21, Winter Solstice, Sun rises in SE sets in
  SW, least amount of day
• March 21, Vernal Equinox
• June 21, Summer Solstice, Sun rises in NE, sets
  in NW, longest day

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Extreme days

• Path of the Sun on the summer solstice at the
  Arctic Circle

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What do we mean by day?

• A Solar day is the amount of time between
  successive Meridian crossings by the Sun
• A Sidereal day is the amount of time for a star
  to return
• Sidereal means relative to the Stars
• The Sun is moving Eastwards by about a degree a
  day relative to the stars
• Solar day is longer!

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The Sun passes through the meridian for different
observers at different times
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The Moon

• Earths Satellite
• Takes about 27 days to
  orbit the Earth
• On average 380,000,000 m
• Or 3.8108 m from the Earth

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In another example

• Remember that Moonlight is reflected Sunlight.
• Please attempt the online tutorial for the phases
  of the Moon (good idea to do this before
  attempting the homework)

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Phases of the Moon 29.5-day cycle
new crescent first quarter gibbous full gibbous la
st quarter crescent

• waxing
• Moon visible in afternoon/evening.
• Gets fuller and rises later each day.

• waning
• Moon visible in late night/morning.
• Gets less and sets later each day.

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There is no dark side!
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Remember the horns point away from the Sun
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Time and the Moon
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Eclipses

• This photo shows an annular Solar Eclipse.
• The Moons angular diameter is too small to cover
  the Sun
• Just the Earth the Moons orbit is an ellipse so
  the distance to the Moon varies
• If the Moon is further away its angular diameter
  is smaller.

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Time lapse photography of a Total Solar Eclipse
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Some definitions

• Solar Eclipse the Moon is between the Earth and
  the Sun.
• Lunar Eclipse the Earth is between the Moon and
  the Sun
• As the Earth is much larger it has a bigger
  shadow so Lunar Eclipses are more common and
  visible from more of the Earth

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More definitions

• Totality The time period in which the Moon
  completely covers the Sun, normally just a couple
  of minutes. The sky becomes dark and you can see
  the stars.
• Partial The Moon doesnt completely cover the
  Sun and you see a crescent or a bite taken out of
  the Sun.
• Health warning Do not look at the Sun with your
  naked eye or with binoculars, even during an
  partial eclipse you can blinded.

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The Earths shadow

• The Earth and Moon cast shadows.
• When either passes through the others shadow, we
  have an eclipse.

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The Moons shadow

• Solar eclipses can occur only at new moon.
• Solar eclipses can be partial, total, or annular.
  
• Your view of the eclipse depends on your
  location, you have to be in the Moons shadow
• Annular eclipses occur when the Moons angular
  size is smaller than the Suns angular size

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• The Earth-Moon distance varies
• The Moon is not orbiting the Earth in a perfect
  circle

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The Moon isnt in the same plane as the Earth/Sun
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So we dont get eclipses every month
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Path of Totality
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Summary Two conditions must be met to have an
eclipse

• It must be full moon (for a lunar eclipse) or new
  moon (for a solar eclipse).
• AND
• The Moon must be at or near one of the two
  points in its orbit where it crosses the ecliptic
  plane (its nodes).

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Planets

• A view of the Solar System
• Notice Plutos orbit
• Notice that our part of
  the Solar System is
  close to the
  Sun.

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Our view of the planets
Mercury difficult to see always close to Sun in
sky Venus very bright when visible morning or
evening star Mars noticeably red Jupiter very
bright Saturn moderately bright
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The Planets and Our Culture
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Westwards motion?

• We see the Sun, Moon and the Superior Planets
  moving Eastwards relative to the Stars
• But occasionally we see Westwards motion

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The explanation
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Next Stop

• A Historical overview of Astronomy!
• What did ancient people believe about the Heavens
• Daily timekeeping
• Tracking the seasons and calendar
• Monitoring lunar cycles
• Monitoring planets and stars
• Predicting eclipses
• And more

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The Mesopotamians

• Region around the Euphrates and Tigris rivers
• Made astronomical observations which have
  somewhat survived to the modern day
• Noted the Zodiac
• Used a sexagesimal numeral system (base 60)
• Many of our star names come from Mesopotamian
  astronomers, Betelgeuse
• Predicted planetary orbits and eclipses, the
  origination of astrology (Magi)

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Egyptian Astronomy

• We know that the Ancient Egyptians made
  astronomical observations from for example the
  alignments of the pyramids
• No written record.
• Most likely for practical (ie time keeping)
  reasons

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Modern Science and the Greeks

• Greeks were the first people known to make
  models of nature.
• They tried to explain patterns in nature without
  resorting to myth or the supernatural.

Greek geocentric model (c. 400 B.C.)
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A distant Massive Sun

• If the Sun is more massive than the Earth it is
  natural to think of it as stationary
• If the Earth is revolving why dont we feel a
  Westwards wind?
• And why arent the stars brighter when we are
  close to them?

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Eratosthenes measures the Earth (c. 240 BC)
Measurements Syene to Alexandria distance
5000 stadia angle 7
Calculate circumference of Earth 7/360 ?
(circum. Earth) 5000 stadia ? circum. Earth
5000 ? 360/7 stadia 250,000 stadia
Compare to modern value ( 40,100 km) Greek
stadium 1/6 km ? 250,000 stadia 42,000 km
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• The most sophisticated geocentric model was that
  of Ptolemy (A.D. 100-170) the Ptolemaic model
• Sufficiently accurate to remain in use for 1,500
  years.
• Arabic translation of Ptolemys work named
  Almagest (the greatest compilation)

Ptolemy
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So how does the Ptolemaic model explain
retrograde motion? Planets really do go backward
in this model..
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Copernicus (1473-1543)

• Proposed Sun centre-ed Heliocentric Solar System
  model in 1543
• Planets still moved on perfect circles
• No contemporary observations could differentiate
  between the Geo and Helio centric models

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Tycho Brahe (1546-1601)

• Compiled the most accurate (one arcminute) naked
  eye measurements ever made of planetary
  positions.
• He could not detect stellar parallax, and thus
  still thought Earth must be at center of solar
  system (but recognized that other planets go
  around Sun)
• Hired Kepler, who used Tychos observations to
  discover the truth about planetary motion.

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Stellar Parallax
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Johannes Kepler

• Used Brahes planetary observations.
• Found that the orbits did not match perfect
  circles.
• Compiled 3 LAWS

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Keplers First Law The orbit of each planet
around the Sun is an ellipse with the Sun at one
focus.
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What is an ellipse?
An ellipse looks like an elongated circle
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Keplers Second Law As a planet moves around its
orbit, it sweeps out equal areas in equal times.

• means that a planet travels faster when it is
  nearer to the Sun and slower when it is farther
  from the Sun.

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Keplers Third Law

• More distant planets orbit the Sun at slower
  average speeds, obeying the relationship
• p2 a3
• p orbital period in years
• a avg. distance from Sun in AU

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Thought Question An asteroid orbits the Sun at
an average distance a 4 AU. How long does it
take to orbit the Sun?

• 4 years
• 8 years
• 16 years
• 64 years
• Hint Remember that p2 a3

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Galileo Galilei (1564-1642)
Galileo overcame major objections to Copernican
view. Three key objections rooted in Aristotelian
view were

• Earth could not be moving because objects in air
  would be left behind.
• Non-circular orbits are not perfect as heavens
  should be.
• If Earth were really orbiting Sun,wed detect
  stellar parallax.

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In Pisa

• According to legend he dropped balls of different
  masses from the tower to show that they fell at
  the same rate.
• In reality his experiment used rolling balls to
  show that a moving object remains in motion
  unless a force acts to stop it.

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The telescope

• Galileo did not invent the telescope.
• He did build his own
• And use it in a logical and systematic way
• He made the first recorded observations of many
  celestial objects.

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One of his first targets

• Was the Sun
• Not recommended, Galileo lost his own sight
• Sunspots, the Sun is rotating and not a perfect
  sphere!

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Galileo and the Moon

• Again not a perfect sphere
• Moon

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Galileo and Stars

• The ancient world had a heaven that did not
  change but observations such as a supernova and
  comet during Tychos lifetime were contradictory
  to that.
• Galileo observed that the Milky Way was made up
  of many stars rather than being a diffuse cloud
  (or creamy nougat and caramel).
• Stars were further away hence no stellar
  parallax measurements

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Galileo and Jupiter

• Observed stars close to Jupiter.
• Overtime the stars positions changed.
• Are in fact Moons.
• How can Earth have the Universe orbit around it
  when there are objects orbiting Jupiter

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Galilean Moons

• Io
• Europa
• Ganymede
• Callisto
• And the Great Spot was first recorded by Galileo

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Saturn and Galileo

• 1610 large Moons on both sides
• 1612 the objects have disappeared, Earth was
  crossing the same plane as the rings
• 1616 Two half ellipses
• 1655 Huygens proposes that the objects are rings
• 1883 First photograph of Saturns rings

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Venus

• Venus goes through phases
• Which only makes sense if it is orbiting the Sun
  and not the Earth
• Was the observation that the Copernican
  revolution needed!
• Landed Galileo in hot water.

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