Lecture 2 11603 Ch1 pages 1621, 2527, Ch3 5863

1
Lecture 2 - 1/16/03Ch1 pages 16-21, 25-27, Ch3
58-63

• Old business
• Text reading assignments, listed on link on
  website given at top of lecture notes
• Scale of Universe
• Distance from sun to nearest star 4.2 light yrs

2
Location and Motions of Objects in Nighttime Sky

• Motivation
• 1) How why do stars appear to travel across the
  sky during the night?
• 2) How do planets travel across the sky?
• 3) Future research on positions of stars in the
  sky (somebody is interested in this stuff?)

3
Why do celestial objects move across the sky?

• 1) Daily rotation of Earth (diurnal motion)
• 2) Annual motion of Earth around Sun
• 3) Wobble of Earth's rotation axis
• Long term effect
• 4) Relative motions of stars
• Long term effect (ignore for purposes of our
  discussion)

4
Diurnal Motions

• Most stars appear to migrate east to west
  across the sky each night
• Cause rotation of Earth (NOT the physical
  movement of the stars)

5
Diurnal Motions/Celestial Sphere

• 'Easy' way to represent stars we see on Earth is
  to project the sky onto a CELESTIAL SPHERE
• Positions of stars on sphere given by 'right
  ascension' and 'declination'
• Analogous to familiar longitude (EW) latitude
  (NS)
• Important terms
• North Celestial Pole, South Celestial Pole,
  Celestial Equator
• Intersections of celes sphere w/ ...

6
Celestial Sphere (cont.)

• More terms
• Horizon
• Point where 'earth meets celestial sphere' - what
  you can see - 180 deg - when stars emerge from
  your horizon they are 'rising', when they
  disappear from your horizon the are 'setting'
• Zenith
• Single point directly overhead
• Meridian
• Line where stars appear highest in sky (midpoint
  at which stars are neither rising or setting
  (except for circumpolar stars))

7
http//www.sal.wisc.edu/HPOL/manual.pdf
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All Stars Seen From Toledo Rise Set, Right??

• www.math.nus.edu.sg/aslaksen/teaching/heavenly.sht
  ml

9
Circumpolar Stars

• Stars which never go below the horizon (never
  set) are called circumpolar stars
• When is a star circumpolar?
• Stars above (90 - declination)
• Toledo (90 -41 49) --gt all stars w/ dec higher
  than 49
• N Pole?
• Somewhere on equator?
• S hemisphere?
• Follow 1 cycle, circumpolar star moves both E-W
  and W-E!! (it must to make a circle)

10
Why do stars rise 4 min earlier each night?

• Ex observation Assume that Betelguese rises
  above the horizon tonight at 9pm ... if you look
  at sky tomorrow night, Betelguese will rise at
  856pm (etc)
• Solar Day
• From one day to another, time it takes sun to
  reach meridian ... 'normal time' ... sun crosses
  meridian at noon each day
• Sidreal Day
• Time for a specific star to return to its exact
  location in sky on a subsequent day
• Sidreal day is 4 min shorter than solar day

11
Solar vs. Sidreal Days

• Sidreal Day
• Cause Earth moves a finite distance along its
  orbit around the sun each day --gt Earth must
  rotate a little more than 360 deg to complete one
  solar day
• Because we all set our clocks to correspond to a
  'Solar Day', stars rise 4 min earlier each day

12
Annual Motions

• Earth's rotation axis is inclined 23.5 degrees
• Effects?
• Apparent motion of sun in sky throughout the year
• Path it follows eccliptic
• Causes sun to appear high in the sky in our
  summers(days longer), low in sky in our
  winters(days shorter)
• Summer Solstice - N.Hem. faces sun most
  directly(sun is highest in sky, it is directly
  overhead for people at dec of 23.5)
• Vernal(spr) Autumnal Equinox - day when sun's
  path passes equator - length of day/night equal
  in both hemispheres - sun directly overhead for
  people living on equator

13
www.astro.columbia.edu/.../labs/fall2001/lec02_fal
l01.html
14
Precession

• Sun Moon exert gravitational forces on the
  Earth which causes the Earth's rotation axis to
  precess (like the motion of a top)
• Pole remains tilted at 23.5 degrees, gradually
  traces out a circle (different stars become the
  'north star')
• 1 cycle takes 26,000 yrs
• astro.wsu.edu/worthey/astro/html/lec-precession.ht
  ml
• http//www.opencourse.info/astronomy/introduction/
  03.motion_earth/precession_north_pole.gifl

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Precession (cont.)

• Historical evidence for precession?
• Air shafts in pyramids (Great Pyramid at Giza
  2600 BC)
• Effects
• 1) 'North star' will change with time
• 2) Stars that are circumpolar today may not be in
  thousands of years
• 3) Some stars not visible today in Toledo (too
  far south) will be visible in thousands of years

16
Planetary Motions

• Mercury Venus orbiting closer to Sun compared
  to Earth - called 'inferior planets'
• Never appear far from the Sun as seen on Earth
• 'Superior planets' (everything past Earth) each
  have a successively slower orbital speed around
  the sun (to be discussed in a few lectures)
• Since it takes the Earth a shorter time period to
  complete 1 revolution about the sun, the Earth
  will periodically 'pass up/overtake' the superior
  planets
• RETROGRADE MOTION

17
Retrograde Motion
astrosun.tn.cornell.edu/courses/astro201/retrograd
e.htm
18
Future Research?

• Astrometry
• Study of the positions of celestial objects -
  'positional astronomy'
• Useful (discuss later in semester) in determining
  distances
• Hipparcos (ESA, 1989-93)
• FAME (Full-sky Astrometric Mapping Explorer)
• NASA, launch 2004
• GAIA (ESA, 2010)

19
'Pre-Test'

• You observe a star rising due east. When this
  star reaches its highest position above the
  horizon, where will it be?
• a) high in the northern sky
• b) high in the eastern sky
• c) high in the southern sky
• d) high in the western sky
• e) directly overhead

20
'Post-Test'

• You are in the southern hemisphere, and observe a
  star rising directly to the east. When the star
  reaches its highest position above the horizon,
  where will it be?
• a) high in the northern sky
• b) high in the eastern sky
• c) high in the southern sky
• d) high in the western sky
• e) directly overhead

21
'Pre-Test'

• You are in Toledo. Looking directly north, you
  see a star just above the northern horizon. A
  little while later you notice that it has shifted
  positions slightly. Which way did it move?
• a) to the right (east)
• b) to the left (west)
• c) up (rising)
• d) down (setting)