1. Observing the Sky
The Birth of Astronomy

2. Pre-Historic Astronomers

3. Need for Astronomy Predicting seasons enables Survival
Migration
Store food (like squirrels)
Plant crops in spring (after last frost?)
Predict seasonal flooding
Knowing time of day enables Survival
Hide in cave at night (lions, tigers & bears!)
What else is there to do at night without a light?

4. Astronomy tells time of year
Which stars are up at night
Star patterns = Constellations
Orion high in winter
Cygnus high in summer
Altitude of sun at noon
High in summer
Low in winter
Location of sunrise/sunset
NE/NW in summer
SE/SW in winter
E/W on 1st day of spring/fall

5. Constellations and Star Maps

6. Constellations 88 Official Constellations Asterism
Examples: Ursa Major, Taurus, ...
Often drawn as “stick figures” (stick figures not official; may vary)
Official Constellations are regions of the sky (like states)
Asterism
a popular name for a group of stars that is not an official constellation
Examples: The Big Dipper, The Pleiades

9. The Celestial Sphere

10. Earth inside Celestial Sphere

11. Locations on Celestial Sphere
Zenith
straight up (overhead)
Nadir
straight down (beneath your feet)
Horizon
lowest place you see the sky
Shape: a circle (you are at the center)
Location: halfway between Zenith and Nadir
North Celestial Pole

12. Directions in the Sky Azimuth = direction (N, S, SSW, etc) you face
Measured in degrees along horizon turning eastward from N
Examples: 45 º azimuth = NE; 90º = E; 270º = W
Altitude = how high in sky
Measured in degrees above horizon

13. Motion of Objects in the Sky
What do you know about the motion of:
the Sun?
the Stars?

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

15. In-class exercise #1 - “Position” Pages 1-2 in workbook

16. 1) How much of the celestial sphere can an Earth observer see at one time?
a) less than half
b) exactly half
c) more than half
b) exactly half

17. The Spinning Celestial Sphere
Looking North
Looking South

18. Why do the stars move? The Earth Rotates (from W to E) Path of Stars
appears to us as if the sky (the Celestial Sphere) rotates (from E to W)
Path of Stars
Stars “attached” to celestial sphere
Path is a circle (like latitude circle)
Called diurnal circle (diurnal = daily)

19. Earth inside Celestial Sphere

20. Locations on Celestial Sphere
Projection of Earth’s rotation axis
North Celestial Pole / South Celestial Pole
Projection of Earth’s Equator
Celestial Equator
Shape: circle
Location: halfway between the Celestial Poles

21. Rise / Set / Transit Rise - move above horizon (appear)
Set - move below horizon (disappear)
Objects rise “in east” and set “in west”
Transit - moving past highest point in path E S W

22. Circumpolar
Some stars never rise or set
These stars are circumpolar

23. Rotation at Different Latitudes
Altitude of Pole = Latitude of Observer
Circumpolar zone depends on latitude

24. Imagine you are standing at the North Pole
Imagine you are standing at the North Pole. Of the stars that you can see, roughly how many of these stars are circumpolar?
None
less than half
more than half
all
all

25. Imagine you are standing on the Equator
Imagine you are standing on the Equator. Of the stars that you can see, roughly how many of these stars are circumpolar?
None
less than half
more than half
all
None

26. In-class exercise #2 - “Motion” Pages 3-6 in workbook

27. You are looking toward the north and see the Big Dipper to the right of Polaris. Fifteen minutes later, the Big Dipper will appear to have moved in roughly what direction?
a) east (to your right)
b) west (to your left)
c) up (away from the horizon)
d) down (closer to the horizon) ?

28. At what time will star B appear highest in the sky?
6am
Noon
Midnight
At what time will star B appear highest in the sky?
a) early in the morning
b) around noon
c) in the afternoon
d) in the evening
e) around midnight

29. When star A is just above the eastern horizon, in what direction is star A moving?
a) up and to the north
b) west
c) up and to the south
d) south

30. a) high in the northern sky
4) You are observing the sky from your southern hemisphere location in Australia. You see a star rising directly to the 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
a) high in the northern sky

31. Motion of the Sun Diurnal (daily) motion like stars Sunrise “in east”
Transits “high” in south = Noon
Sunset “in west”
Altitude at noon depends on time of year

32. Time of Day
Meridian:
circle halfway between east and west
Stars, etc. are highest when they Transit the meridian
Time of day = solar position w.r.t transit (Noon)
am = ante meridian
pm = post meridian
1 pm
11 am
2 pm
10 am
Meridian S
(6am) E W
(6pm)

33. Motion of the Sun Annual (yearly) motion
Earth orbits Sun once per year
Sun seen in front of different constellations throughout year

34. Annual Motion of Sun

35. Motion of the Sun Annual (yearly) motion
From day to day, Sun “slips” a little bit on Celestial Sphere
Appears to shift all the way around the Celestial Sphere once per year
Appears to move “from W to E” relative to the background of stars
So from day to day, any given star rises earlier

36. SOHO Observes Solar MotionW E

37. Annual Path of Sun Ecliptic Zodiac path of sun around celestial sphere
shape: circle
Zodiac
Set of 12 Constellations containing Ecliptic
Sun in each constellation for about one month
(solar) signs of the zodiac

38. For today, you should have done:
Homework: “Seasonal Stars” Exercise in workbook (p. 7-10)
Let’s go over the workbook exercise…

39. One evening at midnight, you observe Leo high in the southern sky at midnight. Virgo is to the east of Leo and Cancer is to the west. One month earlier, which of these constellations was high in the southern sky in at midnight?
a) Leo
b) Virgo
c) Cancer

40. You go out tonight and see the brightest star in the constellation Orion just rising above your eastern horizon at 10 PM. One week later at 10 PM this same star will be
a) slightly higher in the sky.
b) at the same height as before.
c) below your horizon.
d) setting on your western horizon.

41. One night, you see the star Sirius rise at exactly 7:36pm
One night, you see the star Sirius rise at exactly 7:36pm. The following night it will rise
a) slightly earlier.
b) at the same time.
c) slightly later.

42. The Day
1 day = time for object to return to same point on sky (e.g. transit to transit)
Solar (Sun) day
Time from noon until next noon
Sidereal (star) day
Time for a star to return to same point.
Solar day is ~4 min longer than Sidereal day

43. In-class exercise“Solar vs. Sidereal Day” Pages 11-12 in workbook

44. What component of Earth’s motion causes the stars to rise earlier on successive nights?
a) its rotation about its axis
b) its orbit around the Sun
c) the tilt of its rotation axis

45. Which takes longer to complete? a) one solar day b) one sidereal day
c) Both take the same amount of time.
Solar Day = 24 hours Sidereal Day = 23 hrs 56 min

46. Ecliptic on Celestial Sphere
Earth’s axis tilted 23º with respect to orbit

47. Locations on Ecliptic Solstice: sun stops (moving N or S)
Summer Solstice
Jun 21=1st day of summer
Sun farthest N (from celestial equator)
Longest day of year
Winter Solstice
Dec 21=1st day of winter
Sun farthest S (from celestial equator)
Shortest day of year

48. Locations on Ecliptic Equinox: equal night and day Vernal Equinox
~March 21 = 1st day of spring
Sun on equator (crossing from S to N)
Autumnal Equinox
~Sept 21 = 1st day of fall (autumn)
Sun on equator (crossing from N to S)
Equinoxes are intersection points of Ecliptic and Celestial Equator

49. Diurnal Path of Sun Revisited
Summer: Sun above Celestial Equator
Sun high in south at noon
Days are long
Winter: Sun below Celestial Equator
Sun low in south at noon
Days are short
Zenith W S N E

50. For today, you should have done:
Homework: “Path of the Sun” Exercise in workbook (p )
Let’s go over the workbook exercise…