The Shape of the Sky

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The Shape of the Sky
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Measuring the Sky
Angles Why are they convenient?
How do we measure them?
If we see two objects very far away - how can we
describe the distance between them without
knowing how far away they are? How can we
describe the distance between them?
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Cardinal Points
N
S
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The Star and Planet Locator (A two dimensional
view of the sky)
zenith
Overhead point,
(on the celestial meridian
South
midway between N and S)
A
Our horizon
Z
East
West
Brass tack, the North Star.
North
Date
9 pm mid-September
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Dubhe
Merak
North
Looking north about 900 p.m. EDT September 9
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Dubhe
Merak
North
Looking north about 300 a.m. EDT September 10
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Merak
Dubhe
North
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N
North
E
W
S
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Earths Rotation
The most obvious motion. How do we measure it?
by watching when a celestial object crosses the
celestial meridian Celestial Meridian - a line
passing from the point due south on our
horizon, through our zenith, down to the
point due north on our horizon
But - When the Sun crosses the celestial meridian
- Solar Day 24 hours When a star crosses the
celestial meridian - Sidereal Day 23 hours 56m
4.01s (and the Moon crosses later each day) So
the daily or diurnal motion of the sky is due to
more than the rotation of the Earth

just rotation A celestial object is
at its highest point for that day when it
crosses the celestial meridian - it is
transiting
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zenith
Overhead point,
(on the celestial meridian
South
midway between N and S)
A
Our horizon
Z
East
West
Brass tack, the North Star.
North
Date
9 pm mid-September
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Ecliptic
Annual path of the Sun
zenith
Celestial Meridian
Overhead point,
(on the celestial meridian
South
midway between N and S)
Celestial Equator
A
Our horizon
Z
East
West
Brass tack, the North Star.
North
Date
9 pm mid-September
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Solar and Sidereal Day
Revolution of the the Earth around the Sun

Solar Day and Sidereal Day Differ
The sidereal day the period of rotation of the
Earth with respect to
the stars. Sidereal refers to the stars, from
the Latin, sidereus - star
(Vernal Equinox
Celestial Meridian
actually is used)
Looking South
Looking South
23 Hours 56m 4.01s
Solar Day 24 Hours
Sidereal Day
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Since the earth revolves around the Sun and
thereby moves about 1 along its orbit each day,
the earth must rotate 361, or an additional 1,
so that the Sun appears on the celestial meridian
at noon. And so, it appears as though the Sun is
moving 1 eastward through the fixed stars (or
the fixed stars are moving westward 1) per
day. Since the earth rotates 360 in 24 hours
(actually 23 hours, 56 minutes, and 4.01 secs),
or 1 every 4 minutes, the stars rise and set 4
minutes earlier every day.
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Due to the Earths revolution around the Sun
- It appears as though the Sun is moving 1
eastward through the fixed stars per day or the
fixed stars are moving westward 1 per day.
Since the Earth takes 1 hour to rotate 15 or 4
minutes to rotate 1, The stars rise, transit and
set 4 minutes earlier each day. And each month
the stars rise
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Planet Positions - September 5, 2006
Sunrise 627 AM EDT Sunset 722 PM EDT (1922)
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Planet Positions - May 15, 2006
Sunrise 539 AM EDT Sunset 806 PM EDT 2006
EDT Sunrise 439 AM EST Sunset 706 PM EST 1906
EST
Note Times in table are EST
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Planet Positions - August 31, 2005
Sunrise 624 AM EDT Sunset 731 PM EDT 1931
EDT Sunrise 524 AM EST Sunset 631 PM EST 1831
EST
Note Times in table are EST
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Starfinder Lab Checklist
Answer each question, neatly Indicate star
motion with a sketch Note where constellations
of the ecliptic are found Use Planet Table to
determine planet positions Indicate Dipper
positions for each season Use the starfinder to
identify and label star patterns.
Next Week - Math Lab!