1. Two-Sphere Model & Constellations

2. WebAssign Username: your Otterbein email address
Password: first initial plus last name plus IS2(IS3), no space
Uwe Trittmann  utrittmannIS2, utrittmannIS3
HW #1 due Friday 8pm
Question: Circumference of circle = pi*r^2

3. More names, now that we’ve observed the Sun’s position in the Sky
Observer Coordinates
Horizon – the plane you stand on
Zenith – the point right above you
Meridian – the line from North to Zenith to south

4. Modeling the Universe
During the night, it looks like all “specks of light” in the sky travel around us, like the are fixed to an (invisible) sphere
that turns around us
So the apparent daily motion
of the stars can be modeled
by assuming they are fixed
to a gigantic transparent ball
Call it the Celestial Sphere

5. What you see depends on where you are!
Celestial Sphere
Your local sky –
your view depends on your location on earth

6. Two-sphere Model
The Celestial Sphere is observed from the “Earth sphere”
Where an object appears in the local sky, depends on its position on the Celestial Sphere AND on the location of the observer on the Earth-sphere

7. Observation Location Matters
If we move to a different observing place on Earth, the pattern remains the same (bright light rises & sets, etc.), but:
Position of North Star changes
Maximal altitude of Sun, special stars changes
Latitude-Longitude coordinates of Earth

8. Observation Time Matters
If we observe at a different time, the relative positions of the stars stay the same, but the sky dome apprears rotated:
Each hour it rotates 15°, so 360° in 24 hr
Later: counterclockwise
Earlier: clockwise
Latitude-Longitude coordinates of Earth

9. Timezones – 15°= 360°/24 strips

10. The Sky From 40°N From 50°N From a longitude 90° further west
Plus 6 hours later  same as (a)!

11. Conclusion: Earth’s coordinates projected onto Sky
The Celestial Sphere
An imaginary sphere surrounding the earth, on which we picture the stars attached
Axis through earth’s north and south pole goes through celestial north and south pole
Earth’s equator 
Celestial equator
Relative positions of stars do not change -> attached to sphere
In ancient times was often literally thought of as a physical sphere rotating about the earth

12. Celestial Coordinates
Earth: latitude, longitude
Sky:
declination (dec) [from equator,+/-90°]
right ascension (RA) [from vernal equinox, 0-24h; 6h=90°]
Examples:
Westerville, OH °N, 88°W
Betelgeuse (α Orionis) dec = 7° 24’ RA = 5h 52m
Relative positions of stars do not change -> attached to sphere
In ancient times was often literally thought of as a physical sphere rotating about the earth

13. For Homework use Simplified Diagram
Zenith
Cel. Eq.
CNP
90⁰ A
latitude
Altitude of objects on the celestial equator is thus A = 90⁰ – latitude

14. Position II: Once you know where the Celestial Equator is, add declination δ of object
Zenith
Cel. Eq.
CNP δ
Aobj
90⁰
latitude
ACEq
Altitude of objects on the celestial equator is thus A = 90⁰ – latitude + δ

15. Examples: Position I Observing from:
Frankfurt (50°N latitude): Celestial equator is inclined 40° with respect to horizon
Equator (0°N latitude): Celestial equator is inclined 90° with respect to horizon
North Pole (90°N latitude): Celestial equator is inclined 0° with respect to horizon
Longitude of city does not matter!

16. Examples: Position II
Observing from Frankfurt (50°N latitude): Celestial equator is inclined 40° with respect to horizon. Then observe:
Star A (Declination 22.4°): Culminates with altitude angle of 62.4° in Frankfurt
Star A (Declination –12.4°): Culminates with altitude angle of 27.6° in Frankfurt
Star C (Declination – 45°): Culminates with altitude angle of – 5° ???  Is not visible in Frankfurt! Always below horizon.
RA (celestial longitude) of star does not matter!

17. Complicated! Let’s go with Patterns in the Sky!
We can group specs of light together to form triangles, squares, etc.
This allows us to find them the next night and follow their motion
Talk to other observers, and give them names: Bear, Bull, Lion, Hunter, Queen, etc.
 The Constellations

18. Constellations of Stars
About 5000 stars visible with naked eye
About 3500 of them from the northern hemisphere
Stars that appear to be close are grouped together into constellations since antiquity
Officially 88 constellations (with strict boundaries for classification of objects)
Names range from mythological (Perseus, Cassiopeia) to technical (Air Pump, Compass)

19. Constellation 1: Orion Orion as seen at night Orion as imagined by men
Also the Chinese and Egyptians; most constellations today (88) have Greek origins
Basically the constellations of the zodiac (12 in number)
Pattern not really meant to resemble Orion or whatever; likely named in honor, picture fit later
Orion as seen at night Orion as imagined by men 19

20. Orion “from the side”
Note naming scheme
Stars in a constellation are not connected in any real way; they aren’t even close together! 20

21. Constellation 1: Orion “the Hunter” Bright Stars: D) Betelgeuze
E) Rigel
Deep Sky Object:
i) Orion Nebula

22. Constellation 2: Gemini
“the Twins”
zodiacal sign
Brightest Stars:
I) Castor
J=K) Pollux

23. Constellation 3: Taurus
“the Bull”
zodiacal sign
Brightest Star:
F) Aldebaran
Deep Sky Object:
iii) Plejades

24. Constellation 4: Ursa Major
Other name:
Big Dipper
Stars:
B) Dubhe
C) Merak
Navigation: go 5 times the distance from Merak to Dubhe and you are at Polaris.

25. Constellation 5: Ursa Minor
Other name:
Little Dipper
α Ursa Minoris is
Polaris [A], the pole star

26. Constellation 6: Canis Major
“Big Dog”
Stars:
H) Sirius
(brightest fixed star)

27. Constellation 7: Cancer
“Crab”
No bright Stars

28. Constellation 8: Leo “the Lion” zodiacal sign Brightest Star:
G) Regulus

29. Constellation 9: Cassiopeia
Greek mythological figure: mother of Andromeda
the big “W” in the sky
No bright stars

30. Constellation 10: Pisces
“the Fishes”
Zodiacal sign
No bright stars

31. Constellation 11: Pegasus
Greek mythological figure: the winged horse
big rectangle in the sky
No bright stars

32. Constellation 12: Andromeda
Greek mythological figure: Daughter of Queen Cassiopeia and King Cepheus rescued from Cetus by Perseus
Deep Sky Object:
Andromeda Galaxy