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Note that the following lectures include
animations and PowerPoint effects such as fly-ins
and transitions that require you to be in
PowerPoint's Slide Show mode (presentation mode).
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Users Guide to the Sky
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• Chapter 2

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Guidepost
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• The previous chapter took you on a cosmic zoom
  through space and time. That quick preview sets
  the stage for the drama to come. In this chapter
  you can view the sky from Earth with your own
  eyes, and as you do, consider four important
  questions
• How do astronomers name stars and compare their
  brightness?
• How do Earths motions affect the appearance of
  the sky?
• What causes the seasons?
• How can astronomical cycles affect Earths
  climate?
• As you study the sky and its motions, you will be
  thinking of Earth as a planet rotating on its
  axis and moving in an orbit. The next chapter
  will introduce you to other impressive sky
  cycles phases of the moon and eclipses.

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Outline
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I. The Stars A. Constellations B. The Names of
the Stars C. Favorite Stars D. The Brightness
of Stars E. Magnitude and Flux II. The Sky and
Celestial Motion A. The Celestial Sphere B.
Precession III. The Cycles of the Sun A. The
Annual Motion of the Sun B. The Seasons C.
The Motion of the Planets
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Outline (continued)
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V. Astronomical Influences on Earth's Climate A.
The Hypothesis B. The Evidence
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Constellations
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In ancient times, constellations only referred to
the brightest stars that appeared to form groups.
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Constellations (2)
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They were believed to represent great heroes and
mythological figures. Their position in the sky
seemed to tell stories that were handed down from
generation to generation over thousands of years.
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Constellations (3)
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Today, constellations are well-defined regions on
the sky, irrespective of the presence or absence
of bright stars in those regions.
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Constellations (4)
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The stars of a constellation only appear to be
close to one another. Usually, this is only a
projection effect The stars of a constellation
may be located at very different distances from
us.
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Constellations (5)
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Stars are named by a Greek letter (a, b, g)
according to their relative brightness within a
given constellation the possessive form of the
name of the constellation
Orion
Betelgeuse a Orionis
Rigel b Orionis
Betelgeuse
Rigel
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Constellations (6)
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Some examples of easily recognizable
constellations and their brightest stars
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The Magnitude Scale
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• First introduced by Hipparchus (160 - 127 B.C.)
• Brightest stars 1st magnitude
• Faintest stars (unaided eye) 6th magnitude

• More quantitative
• 1st mag. stars appear 100 times brighter than
  6th mag. stars
• 1 mag. difference gives a factor of 2.512 in
  apparent brightness (larger magnitude gt fainter
  object!)

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The Magnitude Scale (Example)
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Betelgeuse
Magnitude 0.41 mag
For a magnitude difference of 0.41 0.14 0.27,
we find an intensity ratio of (2.512)0.27 1.28.
Rigel
Magnitude 0.14 mag
In other words, Rigel is 1.28 times brighter than
Betelgeuse.
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The Magnitude Scale (2)
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The magnitude scale system can be extended
towards negative numbers (very bright) and
numbers greater than 6 (faint objects)
Sirius (brightest star in the night sky) mv
-1.42Full moon mv -12.5Sun mv -26.5
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The Celestial Sphere
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Zenith Point on the celestial sphere directly
overhead
Nadir Point on the c.s. directly underneath
(not visible!)
Celestial equator projection of Earths
equator onto the c.s.
North celestial pole projection of Earths
north pole onto the c.s.
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Distances on the Celestial Sphere
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The distance between two stars on the celestial
sphere can only be given as the difference
between the directions in which we see the stars.
Therefore, distances on the celestial sphere are
measured as angles, i.e., in
degrees (o) Full circle 360o
arc minutes () 1o 60
arc seconds () 1 60
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The Celestial Sphere (2)
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• From geographic latitude l (northern hemisphere),
  you see the celestial north pole l degrees above
  the northern horizon

• From geographic latitude l (southern
  hemisphere), you see the celestial south pole l
  degrees above the southern horizon

90o - l
l

• Celestial equator culminates 90º l above the
  horizon

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The Celestial Sphere (Example)
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New York City l 40.7º
Celestial North Pole
Celestial Equator
49.30
40.70
Horizon
Horizon
North
South
The Celestial South Pole is not visible from the
northern hemisphere.
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The Celestial Sphere (3)
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Apparent Motion of The Celestial Sphere
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Looking north, you will see stars apparently
circling counterclockwise around the Celestial
North Pole.
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Apparent Motion of The Celestial Sphere (2)
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Some constellations around the Celestial North
Pole never set. These are called circumpolar.
The circle on the celestial sphere containing the
circumpolar constellations is called the
circumpolar circle.
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Apparent Motion of The Celestial Sphere (3)
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Looking east, you see stars rising and moving to
the upper right (south)
Looking south, you see stars moving to the right
(west)
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Precession (1)
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At left, gravity is pulling on a slanted top. gt
Wobbling around the vertical.
The Suns gravity is doing the same to Earth. The
resulting wobbling of Earths axis of rotation
around the vertical w.r.t. the Ecliptic takes
about 26,000 years and is called precession.
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Precession (2)
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As a result of precession, the celestial north
pole follows a circular pattern on the sky, once
every 26,000 years.
It will be closest to Polaris A.D. 2100. There
is nothing peculiar about Polaris at all (neither
particularly bright nor nearby etc.) 12,000
years from now, the celestial north pole will be
close to Vega in the constellation Lyra.
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The Sun and Its Motions
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Earths rotation is causing the day/night cycle
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The Sun and Its Motions (2)
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Due to Earths revolution around the sun, the sun
appears to move through the zodiacal
constellations.
The Suns apparent path on the sky is called the
Ecliptic. Equivalent The Ecliptic is the
projection of Earths orbit onto the celestial
sphere.
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The Seasons
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Earths axis of rotation is inclined vs. the
normal to its orbital plane by 23.5, which
causes the seasons.
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The Seasons (2)
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The Seasons are caused only by a varying angle of
incidence of the suns rays.
We receive more energy from the sun when it is
shining onto the Earths surface under a steeper
angle of incidence.
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The Seasons (3)
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Steep incidence ? Summer
Light from the sun
Shallow incidence ? Winter
The seasons are not related to Earths distance
from the sun. In fact, Earth is slightly closer
to the sun in (northern-hemisphere) winter than
in summer.
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The Seasons (4)
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Northern summer southern winter
Northern winter southern summer
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The Seasons (5)
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Earths distance from the sun has only a very
minor influence on seasonal temperature
variations.
Earths orbit (eccentricity greatly exaggerated)
Earth in January
Earth in July
Sun
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The Motion of the Planets
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The planets are orbiting the sun almost exactly
in the plane of the Ecliptic.
Jupiter
Venus
Mars
Earth
Mercury
Saturn
The Moon is orbiting Earth in almost the same
plane (Ecliptic).
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The Motion of the Planets (3)
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Mercury appears at most 28 from the sun. It can
occasionally be seen shortly after sunset in the
west or before sunrise in the east.
Venus appears at most 46 from the sun. It can
occasionally be seen for at most a few hours
after sunset in the west or before sunrise in the
east.
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Astronomical Influences on Earths Climate
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• Factors affecting Earths climate
• Eccentricity of Earths orbit around the Sun
  (varies over period of 100,000 years)
• Precession (Period of 26,000 years)
• Inclination of Earths axis versus orbital plane

Milankovitch Hypothesis Changes in all three of
these aspects are responsible for long-term
global climate changes (ice ages).
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Astronomical Influences on Earths Climate (2)
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Polar regions receive more than average energy
from the sun
Last glaciation
Polar regions receive less than average energy
from the sun
End of last glaciation