Chapter 2: The Sky

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Chapter 2The Sky
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If you look up into the sky, towards the south,
then

1. east is to your right, north is behind you, west
   is to your left.
2. east is to your left, north is behind you, west
   is to your right.
3. east is to your left, west is behind you, north
   is to your right.
4. east is to your right, west is behind you, and
   north is to your right.
5. east is behind you, west is to your left, north
   is to your right.

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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 the Earths
  equator onto the c. s.
• North celestial pole projection of the Earths
  north pole onto the c.s.

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The Celestial Sphere (II)
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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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Example
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New York City l 40.70
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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Athens, OH, is located at l 39o. Where in the
sky would you see the highest point of the
celestial equator?

1. North, 39o above the horizon.
2. South, 39o above the horizon.
3. North, 51o above the horizon.
4. South, 51o above the horizon.
5. South, 45o above the horizon.

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Athens, OH l 390
Celestial North Pole
Celestial Equator
510
390
Horizon
Horizon
North
South
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The Celestial Sphere (III)
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Apparent Motion of the Celestial Sphere
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Apparent Motion of the Celestial Sphere II
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Where will the sun be in the evening before
sunset?
A View of the Sugarloaf Mountain (Rio de Janeiro,
Brazil)
Suns position in the morning

1. Far off to the right
2. Far off to the left
3. Near the Zenith
4. Near the Nadir
5. Close to where it is now.

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The Magnitude Scale
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• First introduced by Hipparchus (160 - 127 B.C.)
• Brightest stars 1st magnitude (mv 1)
• Faintest stars (unaided eye) 6th magnitude (mv
  6)
• More quantitative
• 1st mag. stars apear 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 system can be extended
towards negative numbers (very bright) and
numbers gt 6 (faint objects) Sirius (brightest
star in the sky) mv -1.42 Full moon mv
-12.5 Sun mv -26.5
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The Sun and its Motions (I)
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Earths rotation is causing the day/night cycle.
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The Sun and its Motions (II)
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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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What is causing the seasons?

1. Brightness variations of the sun.
2. The Earth being closer to the sun in the summer
   and further away in the winter.
3. A steeper angle of incidence of the suns rays in
   the summer than in the winter.
4. A denser cloud cover in the winter than in the
   summer.
5. Stronger heat output from within the Earth in the
   summer than in the winter.

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The Seasons (I)
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The Earths axis of rotation is inclined vs. the
normal to its orbital plane by 23.50, which is
causing the seasons.
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The Seasons (II)
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• The Seasons are only caused by a varying angle of
  incidence of the suns rays.
• They are not related to the Earths distance from
  the sun.
• In fact, the Earth is slightly closer to the sun
  in (northern-hemisphere) winter than in summer.

Steep incidence ? Summer
Light from the sun
Shallow incidence ? Winter
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The Seasons (III)
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Earths orbit (eccentricity greatly exaggerated)
Earth in January
Earth in July
Sun
The Earths distance from the sun has only a very
minor influence on seasonal temperature
variations.
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When its summer in the U.S., it is in Argentina

1. spring
2. summer
3. fall
4. winter
5. midnight

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The Seasons (IV)
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Northern summer southern winter
Northern winter southern summer
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What happens to a spinning top that you set on a
table top slanted?

1. It will immediately fall over
2. It will quickly adjust itself to a perfectly
   upright position.
3. It will wobble around in a circular motion around
   the vertical.
4. It will wobble around back and forth through the
   vertical position.
5. It will take off and become airborne.

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Precession (I)
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Gravity is pulling on a slanted top. gt Wobbling
around the vertical. The Suns gravity is doing
the same to the Earth. The resulting wobbling
of the 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 (II)
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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. 12,000 years from now, it will be
close to Vega in the constellation Lyra.
There is nothing peculiar about Polaris at all
(neither particularly bright nor nearby etc.)
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What is the radius of the circle that the
celestial north pole traces out on the celestial
sphere in the course of a precession cycle?

1. 90o
2. 45o
3. 39o
4. 23.5o
5. 66.5o

Radius
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The Motion of the Planets (I)
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• The planets are orbiting the sun almost exactly
  in the plane of the Ecliptic.

Jupiter
Venus
Mars
Earth
Mercury
The Moon is orbiting Earth in almost the same
plane (Ecliptic).
Saturn
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The Motion of the Planets (II)
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• All outer planets (Mars, Jupiter, Saturn, Uranus,
  Neptune and Pluto) generally appear to move
  eastward along the Ecliptic.
• The inner planets Mercury and Venus can never be
  seen at large angular distance from the sun and
  appear only as morning or evening stars.

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Which planet is the most difficult one to observe?

1. Mercury
2. Venus
3. Mars
4. Jupiter
5. Saturn

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Mercury appears at most 280 from the sun. It can
occasionally be seen shortly after sunset in the
west or before sunrise in the east. Venus appears
at most 460 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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