Chapter 1: View from the Earth

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Chapter 1 View from the Earth
I. The Sky A. Constellations names to
groups of stars to honor heroes, gods,
and animals. 1. Each culture created their
own 2. In 1928, astronomers established 88
official constellations with defined
boundaries. a. Many of these came from
Mesopotamia 5000 years ago b.
Others came from Babylonian, Egyptian,
Greek astronomers 3. Asterism less formally
defined grouping of stars that is part
of a larger constellation. a. Ex. The Big
Dipper is part of Ursa Major
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Ursa Major (The Great Bear) The Big Dipper
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I. The Sky A. Constellations B.
Brightness of Stars 1. ancient astronomers
used a scale a. 1-6, 1 is brightest 1st
class 2. modern astronomers are more precise
and use apparent visual
magnitude a. Instead of class ? 3.34 b.
Can be brighter than 1 ? .04, even
1.47, c. Range the sun -26 to 28 3.
apparent visual magnitude how the stars look
to human eyes from earth. a. What about
distance? 4. we should measure intensity
light energy from a star to hit 1
square meter in 1 second.
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es_aao_big.jpg
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4. we should measure intensity light energy
from a star to hit 1 square meter in 1
second. a. The magnitude system is such that
if 2 stars differ by 1 magnitude, their
intensity ratio is 2.5. b. Ex. If they differ
by 2 magnitudes, their intensity ratio is
2.5 x 2.5. Etc.
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1 2x 3x

• 1/4 1/9

As the starlight spreads out the farther it
travels and the apparent brightness decreases by
a greater amount.
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I. The Sky II. The Sky Its Motion A.
Celestial Sphere the sky above and
around earth appears to be part of an imaginary
sphere. 1. Celestial equator and poles are
projected outside earths
geographical equator and poles. 2. sky
appears to rotate west around the earth
day, actually earth rotates east 3.
what we can see of the sky depends on
where we are
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• 4. Coordinates
• a. Right ascension measured in time (hrs,
  min, sec) as earth rotates 360 in 24 hours.
• b. Declination measured in degrees N or S
  of equator just like latitude.
• 5. we measure distance on the sky as
  angular distances
• a. Angular distance is represented with
  degrees
• Ex. the sun and moon are 1/2 degree in diameter

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• II. The Sky Its Motion
• A. Celestial Sphere
• B. Precession the slow change in the in the
  direction of the axis of the earths
  rotation.
• 1. one cycle takes 26,000 years
• 2. while the earth rotates, it sweeps
  around in a conical motion like a toy top.
• 3. this is due to the gravity of the sun
  and moon pulling on the bulge in the middle of
  the earth. (The earth is not a perfect sphere.)

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