The Ever-Changing Sky

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The Ever-Changing Sky

• The sky is constantly changing. We experience
  the day-night cycle every day. Night after night,
  the pattern of the stars seems identical, yet it
  changes with the seasons. The motions of some of
  the celestial objects dont seem to follow that
  of the others
• Why?
• How do we explain the motion of the celestial
  objects?

Check out the all-sky images at
http//nightskylive.net/
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Understanding Whats Going On Around Us

• In Chapter I., we have discussed in very broad
  terms what our universe looks like, what is in
  it, and how things are moving around, as if we
  can elevate ourselves to a vintage point outside
  Earth. In this chapter, we will move ourselves
  back to Earth. Knowing how Earth is moving in the
  universe, with respect to the distant stars, and
  with respect to the Sun, allows us to explain
  what we see on Earth in our sky, and how the
  environment (e.g., the four seasons) on Earth
  changes through time.
• In the next couple of classes, we will be
  talking about
• Night SkyStars, Planets
• The Four Seasons
• Moon, its appearances
• Lunar and Solar Eclipses

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• Where are the stars during the day?
• There are no other stars around the sky near the
  Sun.
• The stars near the Sun are all too dim to be seen
  by either our eyes or the telescopes.
• The Sun is too bright compared with distant
  stars. If we block Sun light, then we can see the
  stars.
• The sky during the day is much brighter than the
  stars.

? Answers
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Why Cant We See the Stars During the Day?

• The daytime sky is bright because Earths
  atmosphere is illuminated by direct sunlight. The
  brightness of the stars are much less than the
  daytime sky.
• If we go into space where there is no atmosphere,
  then we can see stars around the Sun, if the Sun
  is blocked out.

Your horizon
Your Location
Sun
Earth
Day Side
Night Side
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Stars During the Day

• The reason that we cannot see stars during the
  day is because of the existence of the
  atmosphere, which brightens up when illuminated
  by direct sunlight.
• If we go into space, then we can see stars right
  next to the Sun (as long as you block sunlight),
  because there is no atmosphere in space.

Movie from SoHO LASCO Instrument. Light from the
Sun is blocked by the occulter. The bright radial
structures are the
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• There are four seasons on Earth because
• The distance between the Earth and the Sun
  changes through the year.
• Earths self-rotation axis is tilted with respect
  to the Earths rotation axis around the Sun.
• The temperature of the Sun changes slightly
  through the year.

? Answers
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The Night Sky

• What we observe in the sky during the night
• Stars move across the sky during the night
• Some move faster
• Some move very little
• One doesnt seem to move at all!
• The positions of stars with respect to each
  others seem to be fixed
• Star patterns repeat itself about every 24 hours
• Star patterns do not change much from night to
  night.
• Star patterns in the winter are different from
  that in the summer.
• Positions of stars (with respect to the horizon
  and the zenith) in the sky are different at
  different locations (latitudes) on Earth.
• Some stars seem to wander around with respect
  to other stars (you have to be real patient and
  careful to see this).
• The motion of the Moon is quite different from
  the other celestial objects.
• Can we explain these phenomena?

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• Think about
• Motion
• Position
• Orientation

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The Celestial Sphere

• The celestial sphere is an imaginary sphere on
  which all the celestial objects are located.
  However, the location of a celestial object on
  the celestial sphere only tells us about the
  orientation of the objects, but not the distant.
  Two stars located very close together in the sky
  may in fact very far apart because their distance
  to Earth are very different.

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The Dome of the Sky
At any given location on Earth, we can always see
half of the sky...

• Where is the celestial north?

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The Night Sky

• What we observe in the sky during the night
• Stars move across the sky during the night
• Some move faster
• Some move very little
• One doesnt seem to move at all!
• The positions of stars with respect to each
  others seem to be fixed
• Star patterns repeat itself about every 24 hours
• Star patterns do not change much from night to
  night.
• Star patterns in the winter are different from
  that in the summer.
• Positions of stars (with respect to the horizon
  and the zenith) in the sky are different at
  different locations (latitudes) on Earth.
• Some stars seem to wander around with respect
  to other stars (you have to be real patient and
  careful to see this).
• The motion of the Moon is quite different from
  the other celestial objects. The appearance and
  motion of the moon is a subject by itself.

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Earths Rotation and Revolution Around the Sun

• The day-night cycle, and the seasonal change of
  the night sky pattern can be explained by
• the rotation of the Earth, and
• the revolution of the Earth around the Sun.
• Here is a good animation of the Earths rotation
  and its revolution around the Sun on the web.

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The 24-hour Cycle

• Sidereal Day The Earth is rotating with respect
  to the distant stars with a 23 hours and 56
  minutes period.
• Solar Day The average time (24 hours) it takes
  the Sun to make one circle in the sky.
• Solar day is longer than sidereal day because of
  the revolution of the Earth around the Sun.. It
  takes a little extra rotation by Earth to catch
  up

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Trails of stars in the sky over Utah
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Mauna Kea Sky
North Celestial Pole
Click on image to start animation
Photo by Wei-Ho Wang http//www.ifa.hawaii.edu/7E
wang/
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Precession of Earths Rotation Axis

• Precession The rotation of the rotation axis of
  Earth (with respect to distant stars) with
  respect to the rotation axis of Earth around the
  Sun.
• The tilt of the Earths rotation axis with
  respect to the axis of Earths rotation around
  the Sun does not change because of precession.
• The period of precession is 26,000 years
• About 13,000 year from now, Vega would be our
  north star.

Click on the image to start animation
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• Where are the stars during the day?
• There are no other stars around the sky near the
  Sun.
• The stars near the Sun are all too dim to be seen
  by either our eyes or the telescopes.
• The Sun is too bright compared with distant
  stars. If we block Sun light, then we can see the
  stars---this is true only in space.
• The sky during the day is much brighter than the
  stars.

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• There are four seasons on Earth because
• The distance between the Earth and the Sun
  changes through the year.
• Earths self-rotation axis is tilted with respect
  to the Earths rotation axis around the Sun.
• The temperature of the Sun changes slightly
  through the year.

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