1. Chapter 2

2. Naming stars:
Brightest stars were known to, and named by, the ancients (Procyon)
In 1604, stars within a constellation were ranked in order of brightness and labeled with Greek letters (Alpha Centauri)
In the early 18th century, stars were numbered from west to east in a constellation (61 Cygni)
As more and more stars were discovered, different naming schemes were developed (G51-15, Lacaille 8760, S 2398)
Now, new stars are simply labeled by their celestial coordinates

3. Patterns of Stars
People of ancient time saw the constellations as character or animals in the sky. They made up stories to explain how the object, animal, or character came into the night sky.
A pattern or group of stars in the sky is called a constellation.

4. Constellations
Earth rotates on its axis, this makes most constellations appear to rise in the east and set in the west during the night.
Most constellations appear in many different positions in the sky as the Earth revolves around the sun.
There is a group of stars that appear in the sky all night long and all year long. It seems that these stars do not rise and set, but circle the Earth’s north pole each night. These stars are called circumpolar.

5. Asterisms Less formally defined groups of stars.
Part of one or more larger constellations
Big Dipper part of Ursa Major.
The Teapot part of Sagittarius.
The Summer Triangle from the bright stars Altair in Aquila, Vega in Lyra, and Deneb in Cygnus.
Great Square of Pegasus.

6. As early as 5000 years ago, people began naming patterns of stars, called constellations, in the honor of mythological characters or great heroes.
Today, 88 constellations are recognized.
They divide the sky into disjoint units.
Every star in the sky is in one of these constellations.

7. Astronomers label stars within a constellation based on their apparent brightness,
brightest = alpha a,
second brightest = beta b, …
Some of the brightest stars have actual names, like Rigel & Betelgeuse (both in the constellation Orion)

8. The Brightness of Stars
There are approximately 5000 stars viewable with the unaided eye from the Earth’s surface
At any one position you could view approximately 300 stars
HOWEVER – Light and air pollution reduces this number significantly
A system of stellar labeling was developed by assigning numbers based on the relative brightness of stars => Magnitude system

9. Magnitude System
- original idea came during 2nd century B.C. from a Greek Scholar named Hipparchus
- He organized all the visible stars according to their apparent brightness. The brightest stars were given a magnitude of 1. The Next brightest a magnitude of 2 and so on till the faintest stars were given a magnitude of 6.
A. Apparent or Relative Brightness-
Amount of light energy striking a surface. (Observer’s eye or telescope). The brightness is a factor of two things:
1. Luminosity of Star-
The amount of Light Energy being given off by a star. IT DOES NOT DEPEND ON THE LOCATION OF THE OBSERVER
2. Distance to Star

11. A. Apparent or Relative Brightness-(cont.)
*As distance to star increases brightness decreases (Inverse Relationship)
* As luminosity of star increases brightness Increases (Direct Relationship)
B. Apparent Magnitude
A number assigned to a celestial object that is a measure of its relative brightness. (Based on distance and luminosity)
1. The more positive the number the dimmer the star
2. The more negative the number the brighter the star

12. Magnitude 4 star is 2.5 times brighter than a magnitude 5 star
B. Apparent Magnitude (cont.)
It turns out that a an average 1st magnitude star appears 100 times brighter than a 6th magnitude star so therefore a change in magnitude of 5 corresponds exactly to a factor of 100 in brightness.
*** One change in magnitude corresponds to a fifth root of 100 or 2.5 times in brightness
Ex. Magnitude 3 star is 2.5 times brighter than a magnitude 4 star
Magnitude 4 star is 2.5 times brighter than a magnitude 5 star
Question:
1. How much brighter is a magnitude 2 star than a 4 star?
2. How much dimmer is a magnitude 2 star than a –1 star?

13. Constellations

14. Celestial Sphere

15. Our Point of View

16. Angular Size and Distance

17. The Earth Rotates

18. Relevant Geography

19. Earth’s Rotation
23 hours and 56 minutes relative to background stars (sidereal period)
Spin axis of Earth defines points on the celestial sphere (using north and south poles)
North Celestial Pole
South Celestial Pole
Celestial Equator
Sky appears to rotate east to west about the celestial poles because Earth rotates west to east.

20. Rotating Celestial Sphere

21. The Celestial Sphere

22. Definitions
Celestial Sphere: An imaginary sphere where celestial objects are projected on the basis of their direction from Earth
Celestial Poles: The two points where the spin axis of the Earth’s spin axis intersects the celestial sphere
Celestial Equator: The projection of the Earth’s equator onto the celestial sphere
Great Circle: A circle on the sphere's surface whose center is the same as the sphere’s center, and divides the sphere into two equal hemispheres

23. Definitions
Zenith: The point on the sky that is directly overhead of the observer.
Horizon: The great circle on the celestial sphere that is 90 degrees from the zenith
Hour circle: The great circle through the position of a celestial body and the celestial poles
Meridian: The hour circle that passes through the zenith and both celestial poles

24. Directions on the Local Sky
Altitude: The minimum angular distance between the position of a celestial body and the horizon
Azimuth: The angular bearing of an object, measured from North (0 degrees) through East (90 degrees), South (180 degrees), West (270 degrees), and back to North (360 degrees)
Hour Angle: The angle between the meridian and an object’s hour circle (west is positive)

25. Azimuth and Altitude are observer centric.
Altitude is the angular distance of an object above the local horizon. It ranges from 0 degrees at the horizon to 90 degrees at the zenith, the spot directly overhead.
Azimuth is the angular distance of an object from the local North, measured along the horizon. An object which is due North has azimuth = 0 degrees; due East is azimuth = 90 degrees; due South is azimuth = 180 degrees; due West is azimuth = 270 degrees.

26. Celestial Coordinates
Vernal Equinox: The position of the Sun when it crosses the celestial equator from south to north
Declination: The minimum angular distance from the position of a celestial body and the celestial equator
Right Ascension: The eastward angle from the vernal equinox to the intersection of an object’s hour circle with the equator
1 hour of angle = 15 degrees

27. Right Ascension and Declination
Right Ascension (RA): Analogous to longitude, but on the celestial sphere.
It is the east-west angle between the vernal equinox and a location on the celestial sphere.
Declination (dec): Analogous to latitude, but on the celestial sphere.
It is the north-south angle between the celestial equator and a location on the celestial sphere.

28. Celestial Coordinates

30. Units of R.A.
360o = 24h
 15o/h

31. Motion Depends on Declination

32. The Sky at the North Pole
At the North Pole, the North Celestial Pole is at the zenith
Stars never rise or set
Planets, Moon, and Sun do rise and set…Why?

33. Stars Rise and Set at the Equator

34. The Sky at Our Latitude

35. Circumpolar Constellations
Circumpolar constellations never set.
Circumpolar constellations change with latitude… sky changes with latitude

36. The Sky at Southern Latitudes

37. Different sets of constellations are visible in northern and southern skies.
The Altitude of the celestial pole (Polaris) = your latitude

38. The Altitude of the celestial pole (Polaris) = your latitude
Counter-Clockwise Rotation
Clockwise Rotation
Northern Hemisphere
Southern Hemisphere
The Altitude of the celestial pole (Polaris) = your latitude

39. Tilt of Spin Axis

40. Precession of the Earth’s Axis

41. North Celestial Pole Changes

42. Solar vs. Sidereal Day

43. An Earth Day
Sidereal Day: 23 hr 56 min 4 sec Motion relative to background stars
Mean Solar Day: 24 hours The average time between meridian crossings of the Sun
Apparent Solar Day: varies The actual time between the meridian crossings

44. Movement of Stars through the sky Rising and Setting of the Sun
of the Moon
Rotation
Movement of Stars
through the sky
The Seasons
Changing
Constellations
Revolution

45. The Ecliptic
The apparent path of the sun in its yearly motion around the sky.
The projection of the Earth’s orbit on the sky.
The plane of the Earth’s orbit.
These three definitions are equivalent for one of the most important reference lines on the sky.

46. The Ecliptic

47. Ecliptic on the Celestial Sphere

48. The Equinoxes and Solstices
Tilt of the spin axis is related to the seasons
Effect of tilt on climate is not instantaneous

49. Precession of the Earth’s Axis

50. North Celestial Pole Changes

51. Why do we have seasons?

52. Earth’s rotation
The Earth rotates on its axis (imaginary vertical line around which Earth spins) every 23 hours & 56 minutes.
One day on Earth is one rotation of the Earth.
Day on Earth is when our side of the Earth faces the sun.
Night on Earth is when the side of Earth we are on faces away from the sun.

53. Earth’s revolution
It takes the Earth 365 days (or rotations) to travel or revolve around the Earth once.
This is called a year.

54. Why do we have seasons?
The Earth’s orbit around the sun is NOT a perfect circle. It is an ellipse.
Seasons are not caused by how close the Earth is to the sun.
In fact, the Earth is closest to the sun around January 3 and farthest away from the sun around July 4.
Ellipse

55. Why do we have seasons?
Seasons are the result of the tilt of the Earth's axis.
Earth’s axis is tilted 23.5°.
This tilting is why we have SEASONS like fall, winter, spring, summer.
The number of daylight hours is greater for the hemisphere, or half of Earth, that is tilted toward the Sun.

56. Why do we have seasons?
Summer is warmer than winter (in each hemisphere) because the Sun's rays hit the Earth at a more direct angle during summer than during winter

57. Why do we have seasons?
Also the days are much longer than the nights during the summer.
During the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short. These effects are due to the tilt of the Earth's axis.

58. Seasons…in a nut shell

59. Solstices
Solstices occur twice a year, when the tilt of the Earth's axis is oriented directly towards or away from the Sun, causing the Sun to appear to reach its northernmost and southernmost extremes.
Winter solstice is the shortest day of the year. In the Northern Hemisphere. It occurs on December 21 and marks the beginning of winter.
The Summer Solstice is the longest day of the year. It occurs on June 21 and marks the beginning of summer.
Tyrrhenian Sea and Solstice Sky Credit & Copyright: Danilo Pivato

60. SOLSTICE
During the winter the Northern Hemisphere day lasts fewer than 12 hours and the Southern Hemisphere day lasts more than 12 hours.
During the winter solstice, the North Pole has a 24-hour night and the South Pole has a 24-hour day.
Sunlight strikes the earth most directly at the Tropic of Capricorn.

62. Equinoxes
A day lasts 12 hours and a night lasts 12 hours at all latitudes.
Equinox literally means "equal night".
Sunlight strikes the earth most directly at the equator.
This occurs twice a year.

63. Equinox The vernal (spring) equinox occurs March 21.
The autumnal (fall) equinox occurs September 21.

64. The Earth's seasons are not caused by the differences in the distance from the Sun throughout the year.

65. The seasons are the result of the tilt of the Earth's axis.
I know this is a repeat, but it is important that you understand this idea. Many Americans, including Harvard graduates, do not know what causes seasons!

67. How the Sun’s location affects the seasons:
The angle of the sun’s rays:
In the summer it passes closer to overhead and therefore shines more directly on the summer hemisphere
The time the Sun is up
In the summer it spends more than 12 hours above the horizon.
The seasons are NOT due to the slightly elliptical shape of the Earth’s orbit and the fact that it is slightly closer to the Sun during part of the year.
Test of that hypothesis: If the distance were the cause, then when it was summer in the northern hemisphere, what season would it be in the southern hemisphere?
SUN
(From our Text: Horizons, by Seeds)

68. Special Locations on the Earth
How close to the North Pole do we need to go before the Summer Solstice sun becomes a “circumpolar star” and is above the horizon all day?
Within 23.5o of the pole: THE ARCTIC CIRCLE
How close to the equator do we need to get before the Summer Solstice sun passes directly overhead rather than somewhat to the south:
Within 23.5o of the equator: The TROPICS
(From our Text: Horizons, by Seeds)

69. Why are the planets found near the ecliptic?
The ecliptic is defined by the plane of the Earth’s orbit around the Sun.
If the other planets are always found near the ecliptic, they must always be located near the plane of the Earth’s orbit – at most slightly above or below it.
The planes of their orbits around the sun must almost match the Earth’s.
Their slight motions above and below the ecliptic means the match isn’t exact. (Their orbits are slightly tilted relative to ours.)
From our text: Horizons, by Seeds

70. Superior vs. Inferior Planets
Superior planets (Mars, Jupiter, Saturn, Uranus, Neptune, Pluto) have orbits larger than the earth and can appear opposite the sun in the sky. They can be up at midnight. Never show phases.
Inferior planets (Mercury, Venus) have orbits smaller than earth and can never appear far from the Sun. They form “morning stars” or “evening stars” visible a little before sunrise or after sunset. Show phases.
From our text: Horizons, by Seeds

71. Apparent Motion of Inferior Planets
Inferior planets (Mercury, Venus) have orbits smaller than earth and can never appear far from the Sun. They form “morning stars” or “evening stars” visible a little before sunrise or after sunset.
If the inferior planet sets before the Sun it won’t be visible in the evening sky. Look for it instead in the morning sky and vice versa.
From our text: Horizons, by Seeds

72. Ecliptic Constellations & Zodiac Signs
A 16º - 18º band of 12 constellations around the sky entered on the ecliptic (apparent path of the sun on the earth as the earth revolves around it).
Aries, Leo, Sagittarius, Taurus, Virgo, Capricorn, Gemini, Libra, Aquarius, Cancer, Scorpio, and Pisces.

73. Ecliptic Constellations

74. Astronomical Factors that Influence Climate
Eccentricity
Obliquity
Precession

75. Eccentricity
Eccentricity = (distance from focus to center) / (length of semimajor axis)
Eccentricity of Earth’s orbit varies from 0 to 0.05, with 100-kyr, 400-kyr and 2 Myr periodicities.
Fall 2007

76. Obliquity
Obliquity (i.e., tilt) of Earth’s axis varies from 22° to 24.5°, with a 41-kyr periodicity.
Fall 2007

77. Precession
The Earth’s axis precesses, or wobbles, with periodicities of 19 kyr and 23 kyr.
Fall 2007