Class Announcements Monday, August 28

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Class AnnouncementsMonday, August 28

• Labs start tonight, 8pm -10 pm
• Homework 1 due today at end of lecture
• Homework 2 on course website
• Observing/ Clear Sky patrol
• Starts tomorrow night.
• Observing list on website late today
• East elevator, 7th floor VAN Door next to
  elevator

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Ecliptic the path of the Sun (and planets) in
the sky complicated by the fact that Earth is
tilted on its own axis by 23.5 degrees
Note Constellations of the zodiac lie on the
path of the ecliptic, since they are behind the
Suns path.
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Observer at North PoleWhere is Suns position on
Summer Solstice (June 21) ?
North Pole Sun is 23.5? above the horizon all
day on June 21
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Observer at Equator (e.g Quito) Where is Sun on
March 21 (Vernal equinox)?
Zenith at noon
Mach 21
Observer at equator on March 21 (or Sept 21) Sun
rises in E, at zenith at noon, sets in W)
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Ecliptic, Celestial Planes and the Path of the
Sun
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Path of Sun from March 21 to Sept 21
June 21
March 21
Sept 21
Sun today
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As the Earth moves in its orbit, The path of the
Sun (and planets) changes because of the tilt of
the rotation axis Note Since rotation axis is
fixed with respect to the stars, star paths do
NOT change!
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Northern Hemisphere Winter
Northern Hemisphere Summer
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What dates correspond to each figure?
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1
3
2
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Cause of seasons
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(No Transcript)
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Azimuth of sunrise changes with season
East
Northeast
Southeast
Dawn
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Stonehenge Ancient astronomical site aligned to
solar azimuth
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Seasons Summary

• Due to the Earths 23.5º tilt on its axis

N. Summer Northern Hemisphere tilted toward
Sun N. Winter Southern Hemisphere tilted toward
Sun

• 4 important dates
• 1. Summer solstice (June 21) sun most northerly
  on ecliptic
• 2. Autumnal equinox (Sep 21) sun crosses
  celestial equator
• 3. Winter solstice (Dec 21) sun most southerly
  on ecliptic
• 4. Vernal (spring) equinox (Mar 21) sun crosses
  c. equator

• Myth The Earth is closer to Sun in summer

Wrong! The suns rays reach (N. hemisphere) of
the Earth more DIRECTLY in (Northern H.) summer,
less DIRECTLY in winter (doesnt have to do with
sun-earth distance)
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Coordinates in the sky Horizon System
Zenith point directly overhead at any
time Horizon the lowest point you can see (the
ground)
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Celestial Sphere Extension of the Earths
Coordinate System

• celestial sphere
• N/S celestial poles
• celestial equator

Like a salad bowl over your head!
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Apparent westward motion of sky
Due to Earths rotation about its axis every
24h in an EASTWARD direction To us, it appears
as if objects in the sky are moving
WESTWARD Sun rises in the E, Sun sets in W
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The Night Sky at the North Pole Polaris (N.
Celestial Pole) fixed overhead
Time-lapse picture from the North Pole looking
straight up (zenith)
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The Night Sky at the Equator North South
Celestial Poles are on the horizon
North Celestial Pole (North Star)
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Most of us are in between these extremes we see
fixed Polaris (N. Celestial Pole star) over the
night, stars move WESTWARD about it
Looking North
E
W
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Transit Crossing the observers prime meridian

• All celestial bodies rise in the east, set in the
  west
• A line running from N to S crossing the zenith is
  the prime meridian
• Objects transit when crossing this imaginary line
  
• Transiting objects are always due south and are
  highest in the sky (highest elevation)

Prime meridian line
Transit
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Prime Meridian
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Earths rotation and the apparent motion of stars

• All celestial bodies (stars, planets, sun, moon)
  appear to move across the sky east to west
• This is caused by the Earth rotating (west to
  east) on its own axis, like a spinning top.
• The Earths rotational period is 23h 56m 4s. (NOT
  24h!) This is called the sidereal period.
• Hence, all stars are at the same position in the
  sky every successive sidereal period.
• This implies that all stars rise appx. 4 minutes
  earlier each night.
• Since 4 min x 365 days 24 hr, the night sky
  patterns repeat annually for a given time of
  night

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Daily Motion of Stars, Solar system objects on
the Sky

• Stars move along the same westward track every
  night (ignores precession, discussed later in
  lecture)
• Stars rise, transit, and set 4 min earlier every
  night.
• Solar system objects also move east to west each
  night, but their tracks are more complicated
  (because apparent motion is affected by Earths
  orbital motion around Sun)
• The Sun and all planets (except Pluto) move along
  the line of the ecliptic plane, described in next
  slide.
• Planets generally move eastward with respect to
  the stellar background, except during retrograde
  motion (Chap 3)
• The Moons motion is the most complicated we
  will discuss this in chapter 2.

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Night sky The Movie Iowa City looking south
West
East
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Night sky at 24 hour intervals (Solar period)
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Night sky at 23h 56m 4s intervals (Sidereal
period)
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Motion of the Sun Solar vs. Sidereal period

• Since the Earth moves in its orbit each day, the
  motion of (nearby) solar system objects is
  slightly different from stars.
• The Earth needs to rotate an additional 1 degree
  daily for the Sun to repeat its position (see
  diagram at right).
• This takes an additional 4 min
• This means that the Suns position repeats every
  24 hrs. This is called the solar period.