Title: The Ever-Changing Sky
1The 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/
2Understanding 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
3- 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
4Why 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
5Stars 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
6- 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
7The 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?
8- Think about
- Motion
- Position
- Orientation
9The 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.
10The Dome of the Sky
At any given location on Earth, we can always see
half of the sky...
- Where is the celestial north?
11The 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.
12Earths 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.
13The 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
14Trails of stars in the sky over Utah
15Mauna Kea Sky
North Celestial Pole
Click on image to start animation
Photo by Wei-Ho Wang http//www.ifa.hawaii.edu/7E
wang/
16Precession 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
17- 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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18- 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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