1. Starry Monday at Otterbein
Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the month-
March 6, 2006
Dr. Uwe Trittmann

2. Today’s Topics
Basic Observations in Astronomy
The Night Sky in March

3. On the Web
To learn more about astronomy and physics at Otterbein, please visit
(Observatory)
(Physics Dept.)

4. Basic Observations in Astronomy
Positions of objects (sun, moon, planets, stars …)
Motion of objects
– with respect to you, the observer
- with respect to other objects in the sky
Changes (day/night, seasons, etc.)
Appearance of objects (phases of the moon, etc.)
Special events (eclipses, transitions, etc.)
 All “in the sky”, i.e. on the Celestial Sphere

5. What’s up in the night 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

6. 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, 83°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

7. What’s up for you? Observer Coordinates
Horizon – the plane you stand on
Zenith – the point right above you
Meridian – the line from North to Zenith to south

8. …depends where you are! Your local sky –
your view depends on your location on earth

9. Look North in Westerville

10. Look North on Hawai’i

11. Daily Rising and Setting
Due to the rotation of the Earth around its axis
Period of rotation: siderial day= 23h56m4.1s
1 solar day (Noon to Noon) =24h
Stars rotate around the North Star – Polaris
Circumpolar – never rise or set

12. Solar vs Siderial Day Earth rotates in 23h56m also rotates around sun
 needs 4 min. to “catch up”
Consequence: stars rise 4 minutes earlier each night
after 1/2 year completely different sky at night!

13. What time is it? Depends on where you are on the Earth!
Time zones ensure that the noon is really noon, i.e. sun is at highest point
To avoid confusion, use universal time (UT), the time at the meridian in Greenwich
UT = EST + 5 hrs
Daylight savings adds one hour in spring, so UT = EDT+ 4 hrs

14. The Time Zones
Established to insure that sun is at highest point approximately at noon in the middle of the time zone

15. Another Complication: Axis Tilt!
The Earth’s rotation axis is tilted 23½ degrees with respect to the plane of its orbit around the sun (the ecliptic)
It is fixed in space  sometimes we look “down” onto the ecliptic, sometimes “up” to it
Rotation axis
Path around sun

16. Position of Ecliptic on the Celestial Sphere
Earth axis is tilted w.r.t. ecliptic by 23 ½ degrees
Equivalent: ecliptic is tilted by 23 ½ degrees w.r.t. equator!
 Sun appears to be sometime above (e.g. summer solstice), sometimes below, and sometimes on the celestial equator
Skyglobe demo
7 visible “planets” incl. the sun and moon
Planet = “wanderer”
Days of the week named after the planets

17. The Seasons
Change of seasons is a result of the tilt of the Earth’s rotation axis with respect to the plane of the ecliptic
Sun, moon, planets run along the ecliptic
Equinoxes: day and night of equal length
Accounts for the N-S motion of the sun on the celestial sphere

18. The Zodiac throughout the Year
Slow drift across background of fixed stars caused by rotation of earth about sun
Period = 1 year (365 ¼ days)
Example: In Winter sun in Sagittarius, Gemini at night sky; in summer sun in Gemini, Sagittarius at night sky

19. 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)

20. Constellations of Stars (cont’d)
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

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

22. Zodiacal signs vs. Constellations
“Constellation” is a modern, well-defined term
- Some constellations are big, some are small on the celestial sphere
“Zodiacal sign” is the old way of dividing the year and the Sun’s path into 12 equal parts
Slow drift across background of fixed stars caused by rotation of earth about sun
Period = 1 year (365 ¼ days)
360/12=30, so each zodiacal sign is exactly 30 degrees “long”
0 degrees: Aries, 30 degrees: Taurus, 60 degrees: Gemini, 90 degrees: Cancer, etc.

23. Example
The vernal equinox happens when the sun enters the zodiacal sign of Aries, but is actually located in the constellation of Pisces.

24. Reason: Precession of the Equinoxes
Precession period
about 26,000 years
“The dawning of the age of
Aquarius”
A result of the combined grav pull of the sun and moon on the earth
Vernal equinox now in Pisces years ago in Aries; around AD 2600 will move into Aquarius

25. Motion of Sun, Moon and other Planets
All major bodies in the Solar System move around ecliptic
Slow drift (from W to E) against the background of stars
Skyglobe demo
7 visible “planets” incl. the sun and moon
Planet = “wanderer”
Days of the week named after the planets

26. Reason: All planets move in same plane!
all planets orbit in same direction (ccw as seen from above the north pole)
all orbits lie nearly in a single plane (Mercury (7deg) and Pluto (17deg) being most notable exceptions)
inner planets are small, dense, rocky (Terrestrial); outer planets are large, gaseous, low density (Jovian)
density = mass/volume
inner planets close together, outer planets further apart

27. All planets? - Pluto’s strange Orbit
Very far out there: 40 A.U.
Pluto’s year = 248 Earth years
Orbit inclined 17° w.r.t. ecliptic
Very eccentric orbit:
Perihelion: 30 A.U. (inside the Neptune orbit!)
Aphelion: 50 A.U.
A bit smaller than our moon
Icy, basically like Triton

28. Is Pluto a planet? Orbit too weird Too small
 Today Pluto would probably not be classified as a planet!

29. Why do all planets move in the same plane?
Reason: Formation process of the Solar System
Condenses from a rotating cloud of gas and dust
Conservation of angular momentum flattens it
Dust helps cool the nebula and acts as seeds for the clumping of matter

30. Formation of Planets Orbiting dust – planitesimals
Planitesimals collide
Different elements form in different regions due to temperature
Asteroids
Remaining gas

31. Motion of the Moon
Moon shines not by its own light but by reflected light of Sun
 Origin of the phases of the moon
Moon revolves around the Earth
period of revolution = 1 month
Planets also shine by reflected light!

32. Phases of the Moon

33. Phases of the Moon (cont’d)
Moon rotates around earth in one month
Moon rotates around itself in the same time
 always shows us the same side!
 “dark side of the moon” (not dark at all!)

34. Motion of the Planets Along the ecliptic as Sun and Moon
But: exhibit weird, “retrograde” motion at times

35. “Strange” motion of the Planets
Note: outer planets are slower
Planets usually move from W to E relative to the stars, but sometimes strangely turn around in a loop, the so called retrograde motion.

36. The heliocentric Explanation of retrograde planetary motion
See also: SkyGazer

37. SkyGazer
A computer program that simulates the vision of the sky during day and night
Things to observe:
Set your position on Earth: observe how view of sky changes as you move E,W, N,S
Note the distribution of sunlight on Earth!
Rotation is around Polaris which is not in zenith

38. SkyGazer Things to observe (cont’d):
Sun, moon, planets, stars rise (E) and set (W)
In the southern hemisphere the sun is highest in the north
Planets sometimes move backward
Moon phases
Planets have phases, too!

39. Eclipses
One celestial object hidden by other or in the shadow of another
Solar eclipse: sun hidden by the moon
Lunar eclipse: moon in earth’s shadow (sun hidden from moon by earth)
Also: eclipses of Jupiter’s moons, etc.
Most spectacular because moon and sun appear to be the same size from earth

40. Solar Eclipses Umbra – region of total shadow
Penumbra – region of partial shadow
Totality lasts only a few minutes!
Why isn’t there a solar eclipse every month?

41. Solar Eclipse

42. Solar Corona
Thin outer atmosphere, usually swamped by other light and hence invisible

43. Question Why isn’t there an eclipse every month ?
Answer: because the Moon’s orbit is inclined w.r.t. the ecliptic

44. Lunar Eclipses Moon moves into earth’s shadow… …and out of it
Moon appears reddish due to light bent by Earth’s atmosphere
Aristotle argues earth is round based on shape of its shadow on the moon
…and out of it
(takes hours!)

46. Partial Eclipse

47. Not an Eclipse !

48. Towards Totality

49. Almost total…

50. Totality
Totality

51. The Night Sky in March Long nights, early observing!
Winter constellations are up early: Orion, Taurus, Gemini, Auriga, Canis Major & Minor, the spring constellations come up: Cancer, Leo, Big Dipper
Saturn dominates the evening, Jupiter early morning.

52. Moon Phases Today (First quarter Moon) 3/ 14 (Full Moon)
3 / 22 (Last Quarter Moon)
3 / 29 (New Moon)

53. Today at Noon
Sun at meridian, i.e. exactly south

54. 10 PM
Typical observing hour, early March
Saturn
Mars
Moon

55. South-West
Plejades
Mars in Aries / Taurus

56. Zenith
Big Dipper points to the north pole

57. with Plejades and the Double Cluster
West
High in the sky:
Perseus and
Auriga
with Plejades and the Double Cluster

58. South-West The Winter Constellations Orion Taurus Canis Major Gemini
Canis Minor

59. The Winter Hexagon
Sirius
Procyon
Pollux
Capella
Aldebaran
Rigel

60. South
Saturn near Praesepe, an open star cluster

61. East
Spring constellations:
Leo
Hydra

62. Mark your Calendars! Next Starry Monday: April 3, 2005, 8 (!!!) pm
(this is a Monday )
Observing at Prairie Oaks Metro Park:
Friday, May 5, 9:00 pm
Web pages:
(Obs.)
(Physics Dept.)

63. Mark your Calendars II Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 256
Free coffee, cookies, etc.