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Lecture
2: The Night Sky - I
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| Astronomy
101/103 |
Terry
Herter, Cornell University
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Lecture
Topics
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Understand
why the sky changes
- During
the night
- From
night to night
Some
definitions
- Celestial
sphere, ecliptic, zodiac, zenith, meridian
SkyGazer
demo
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Sights
in the
Night
Sky
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- The
Moon & Planets
- Stars
- Constellations
- Asterisms
(groupings of stars, for example The Pleiades)
- Binaries
- Variable
Stars
Some
sample objects are given in the table below. Click the link
to get more information and see a picture.
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The
Need
to
Know
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But
Where to Look?
We
need to know:
- Where
an object is on the sky (coordinates or it location relative
to some reference stars or constellations)
- What
time that part of the sky is viewable, e.g. the sun is
not up.
- Time
of night and
- Time
of year
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Where
and
When
to Look
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Consider
the major motions that affect the night sky:
- Rotation
of the earth
- Causes
the stars to move through the sky at night.
- Revolution
of the earth around the sun
- Causes
different stars to be available at different times of
the year.
The
figure below demonstrates how different constellations are
viewable at various times of the year. For instance, in
winter Auriga and Orion are visible but Lyra and Hercules
are hidden "behind" the sun. This situation reverses
itself in the summer months.
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Class
Demo
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| Cycles
of the Sky
A
class demo, wherein
- the
part of the sun is play by a volleyball
- while
the earth is played by a tennis ball and then a volleyball,
- and
the students are, of course, stars.
Real
Proportions
This
table below give the relative sizes of the earth-sun system
by relating them to everyday objects/distances.
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OBJECT
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EXAMPLE
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If
the sun were a volley ball
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~
8 in = 20 cm in diameter
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The
earth is small than a pea
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~
1.8 mm in diameter
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sun-earth
distance
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22
m (about 71 feet)
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The
Celestial
Sphere
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- The
celestial sphere is the vast hollow sphere on which the
stars appear fixed.
- The
celestial equator is defined by extending the earth's
equator outward.
- The
N & S poles of the celestial sphere correspond to
the earth's poles.
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The
Ecliptic
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- The
ecliptic is the apparent path of the sun through the sky.
- It
is also the plane of the earth's orbit about the sun on
the celestial sphere.
- Note:
The ecliptic is tilted w.r.t. the earth's equatorial plane
by 23.5 deg.
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The
Zodiac
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- The
zodiac is a band of celestial sphere that represents the
path of the planets, the moon and the sun.
- Extends
~8 deg to either side of the ecliptic.
- In
astrology the zodiac is divided into 12 equal parts called
signs, each bearing the name of a constellation.
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Astrology
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Astrology
is NOT a science!
- Propagates
the claim that a person's life is determined by the
position of the sun, moon, and planets at birth.
- This
notion is patently false, and potentially harmful.
- Astrology
is neither a science nor a religion. It is probably
better characterized as entertainment.
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Zenith
and
Meridian
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The
zenith is the point on the celestial sphere that
is directly above the observer.
The
meridian is the great circle passing through the
two poles of the celestial sphere and the observer's zenith.
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