Motions in the sky: The celestial sphere

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Motions in the sky The celestial sphere
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What is the celestial sphere?

• An imaginary sphere surrounding Earth that
  contains all the objects we see when we look at
  the sky.
• Imagine that there is a giant, spherical
  projector screen around the earth and all the
  light from objects in space is shining onto it to
  make the patterns we see (like constellations).

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Star trails Why do stars appear to move across
the sky?
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Diurnal motion
The entire sky appears to rotate once in 24
hours. This is termed the daily or diurnal motion
of the celestial sphere, and results from the
daily rotation of the earth on its axis. The
diurnal motion affects all objects in the sky and
does not change their positions relative to each
other. Some objects also appear to move with
respect to the other objects on the celestial
sphere. These are the "wanderers" of the ancient
astronomers the planets, the Sun, and the Moon.
This is due to the Earths orbit around the sun.
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The Celestial Sphere
We can use the celestial sphere idea to help
locate objects in the sky.
The stars rotate around the North and South
Celestial Poles. These are the points in the sky
directly above the Earths north and south
poles. Fortunately, for those in the northern
hemisphere, there is a fairly bright star very
close to the North Celestial Pole called Polaris
(or the North star). Another important reference
marker is the celestial equator an imaginary
circle around the sky directly above the Earth's
equator. All the stars rotate in a path that is
parallel to the celestial equator.
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Stars that are close to the celestial poles never
appear to rise or set. These are called
circumpolar stars.
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The "Road of the Sun" on the Celestial Sphere

1. The sun has a diurnal (daily) motion from east to
   west due to the earths spinning around its axis,
   24 h
2. The sun also changes its position in the sky 1
   degree per day, 365.25 days

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What is the ecliptic?

• The Earth and sun orbit each other in the same
  plane, called the ecliptic.
• That means the path of the sun across the sky
  will be on the ecliptic.

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• Constellations of the Zodiac
• Your zodiac sign is the constellation the sun
  was in on the day you were born.
• Since the Earth orbits the sun, when standing on
  Earth and looking up at the sun, there will be a
  different constellation in the background
  depending on the time of year.

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Ecliptic and Zodiac
Sun travels 360o/365.25 days 1o/day
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axis
Celestial equator
As a result, planes of the ecliptic and celestial
equator make an angle 23.5o
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The Solstices

• The solstices are the two spots farthest away
  from the celestial equator.
• Summer solstice longest day of the year
• Winter solstice shortest day of the year

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Longer day
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The Equinoxes

• The ecliptic and the celestial equator cross in
  two places.
• In these two spots, there is an equal amount of
  day and night (12 hours each). These are the
  equinoxes.

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Shorter day
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The tilt of the Earths axis, NOT distance from
the sun!!!
What causes the seasons?
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Tilt of the axis

• When the northern hemisphere is tilted towards
  the sun, we receive more direct sunlight.
• When we are tilted away from the sun, its rays
  are spread out over a larger area so they dont
  give us as much warmth.

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Seasons - summary

• Seasons are NOT caused by varying distances from
  the Earth to the Sun
• The primary cause of seasons is the 23.5 degree
  tilt of the
• Earth's rotation axis with respect to the plane
  of the ecliptic.

The Seasons in the Northern Hemisphere
Note the Earth is actually closest to the Sun in
January 4!
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• Thus, we experience Summer in the Northern
  Hemisphere when the Earth is on that part of its
  orbit where the N. Hemisphere is oriented more
  toward the Sun and therefore
• the Sun rises higher in the sky and is above the
  horizon longer,
• The rays of the Sun strike the ground more
  directly.
• Likewise, in the N. Hemisphere Winter the
  hemisphere is oriented away from the Sun, the Sun
  only rises low in the sky, is above the horizon
  for a shorter period, and the rays of the Sun
  strike the ground more obliquely.