Announcements Wed, Jan 25

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AnnouncementsWed, Jan 25

• Clear Sky Patrol (observing test)
• Started this week
• Every clear night Mon-Thurs
• Observing test REQUIRED
• See course website for list, CSP status
• Smart idea get started on this early
• Special bonus variable star U Orionis (next
  page)
• Homework assignment 2
• Due Sunday at midnight
• Trouble registering? See me after lecture

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U Orionis

• Variable star in Orion (long period variable)
• Distance 850 ly
• Period 374 days
• Varies in brightness by a factor of 150x

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Light curve of U Orionis
Binoculars needed
Telescope needed
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Finding Chart
Observing bonus Observe twice (once at maximum
next 2 weeks, and once in April (telescope!) ,
receive 10 pts on observing test!
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Lecture learning Goals

• Celestial sphere (definition and usage)
• Daily motion of stars and solar system bodies in
  sky
• Definitions of zenith, horizon, ecliptic, zodiac
• Constellations
• Cause of the seasons
• Vernal, autumnal equinox
• Summer, winter solstice
• Precession Motion of North celestial pole
• Calendars (Julian, Gregorian, others)

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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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So why is a day 24 hours ?(not Earth true
rotation period of 23h 56m 4s?)

• Answer Because in a day the Earth moves a
  little in its orbit, so the position of the Sun
  changes
• It takes an extra 4 minutes for the Sun to
  appear in the same place in the sky

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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.

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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 next lecture.

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Ecliptic Plane(plane of the Solar System)
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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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Analemma Solar Position at Noon throughout year
Sun does not transit prime meridian at 1200 noon!
Expected position at along prime meridian at noon
(mean Sun)
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Why the Analemma? Because the Earth orbit around
the Sun is not perfectly circular Equation of
Time

• This effect is caused by
• Obliquity ( tilt of Earths axis)
• Ellipticity of Earths orbit (Earth moves more
  slowly when farther away from Sun)

Maximum difference from uniform motion (Sun
transits at noon daily) is 16 min
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Check your knowledge--
An observer at the equator (Quito) sees a star at
the zenith at midnight on June 21. Which of the
following is true? (hint more than one might be
true)
False! stars have same path all year
(a) The next night, the same star is north of the
zenith.
(b) The star sets in the west near dawn.
True!
(c) Exactly 24 hours later, the star is east of
the zenith.
False! star will be west of zenith
(d) The star is on the celestial equator.
True!
(e) At this time, the star is on the horizon as
viewed from the South Pole
True!
(f) On Sept 21 at midnight, the same star is on
the eastern horizon.
True!