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
don’t 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/
Understanding What’s 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:
1.
2.
3.
4.
Night Sky—Stars, Planets
The Four Seasons
Moon, its appearances
Lunar and Solar Eclipses
Where are the stars during the day?
A. There are no other stars around the sky near the
Sun.
B. The stars near the Sun are all too dim to be seen by
either our eyes or the telescopes.
C. The Sun is too bright compared with distant stars. If
we block Sun light, then we can see the stars.
D. The sky during the day is much brighter than the
stars.
 Answers
Why Can’t We See the Stars
During the Day?
• The daytime sky is bright because Earth’s
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
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
There are four seasons on Earth because…
A. The distance between the Earth and the Sun
changes through the year.
B. Earth’s self-rotation axis is tilted with respect
to the Earth’s rotation axis around the Sun.
C. The temperature of the Sun changes slightly
through the year.
 Answers
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 doesn’t 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?
Think about
• Motion
• Position
• Orientation
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.
The Dome of the Sky
At any given location on Earth, we can always see half of the sky...
• Where is the celestial north?
If you are at the north pole
If you are at 45º latitude
If you are on the equator
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 doesn’t 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.
Earth’s 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 Earth’s rotation and its
revolution around the Sun on the web.
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…
Trails of stars in the sky over Utah
North Celestial Pole
Mauna Kea Sky
North Celestial Pole
Photo by Wei-Ho Wang
http://www.ifa.hawaii.edu/%7Ewang/
Click on image to start animation
Precession of Earth’s 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 Earth’s
rotation axis with respect to
the axis of Earth’s 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
Where are the stars during the day?
A. There are no other stars around the sky near the
Sun.
B. The stars near the Sun are all too dim to be seen by
either our eyes or the telescopes.
C. 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.
D. The sky during the day is much brighter than
the stars.
 Back to Presentations
There are four seasons on Earth because…
A. The distance between the Earth and the Sun
changes through the year.
B. Earth’s self-rotation axis is tilted with respect
to the Earth’s rotation axis around the Sun.
C. The temperature of the Sun changes slightly
through the year.
 Back to Presentation