1. Astronomy
Elementary Astronomy

2. Astronomy Study of the Heavens Science Physics (some math)
verifiable predictions
Physics (some math)

3. Finding One’s Way in the Sky
Celestial Sphere
imaginary sphere
Horizon
line where sky meets ground
Zenith
point on celestial sphere directly overhead
Meridian
imaginary line running N-S that passes through zenith

4. Finding One’s Way in the Sky
Celestial Poles
projection of Earth’s rotation axis on celestial sphere
Celestial Equator
imaginary circle lying halfway between north and south poles
projection of Earth’s equator

5. Finding One’s Way in the Sky
Ecliptic
annual path of the Sun projected on celestial sphere
Zodiac
narrow band, around the ecliptic
many of the constellations have animal names
zoo zodiac

6. Finding One’s Way in the Sky

7. Finding One’s Way in the Sky
Locating Objects
celestial sphere has 2-d surface
two numbers to locate an object
Origin
reference point
Units
measuring system

8. Finding One’s Way in the Sky
Linear vs. Angular Measurement
linear distances meaningless on celestial sphere
angular measurements
360o in a circle
60 minutes (`) in one degree
60 seconds (``) in one minute
43o52`34``

9. Finding One’s Way in the Sky
Earth
Latitude
natural reference points, poles and equator
90o N=north pole, 90o S=south pole, 0o =equator
Longitude
arbitrary reference point
prime meridian through Greenwich, England = 0o

10. Latitude and Longitude

11. Latitude and Longitude

12. Finding One’s Way in the Sky
Celestial Sphere
need way of measuring angles (compass, sextant, protractor)
Two systems for determining position
Altitude-Azimuth
Equatorial

13. Altitude - Azimuth Coordinates
angular distance above horizon
Azimuth
angular distance measured in CW direction from N*
Advantages
very easy to determine
Disadvantages
depends on location

14. Equatorial Coordinates
Declination
angular distance from celestial equator
north (+), south (-) celestial poles  90o
Right Ascension
measured in an easterly direction from the location of the vernal equinox
Advantages
same for all observers
Disadvantages
more difficult to determine

15. Motion of Heavenly Objects
Observations and Explanations

16. Sun Observation Explanations rises in East, sets in West
daily (diurnal)
Explanations
Sun moves through the sky (old)
Earth rotates on axis once per day (accepted)

17. Sun
Observation
moves west to east wrt to stars
yearly (annual)

18. June 20

19. July 20

20. Sun Observation Explanation Ecliptic moves west to east wrt to stars
yearly (annual)
Explanation
Earth orbits Sun
Ecliptic
apparent path of the Sun through sky

21. Sun Observation Explanation sunrise and sunset move north and south
hours of daylight increase and decrease
annual
Explanation
Earth’s rotation axis is tilted with respect to plane of orbit

22. Northern Summer
Northern Winter
Southern Summer
Southern Winter

23. Seasons Northern Hemisphere summer solstice vernal equinox (Spring)
rotational axis points toward Sun
most hours of daylight
Sun rises northernmost along eastern horizon
vernal equinox (Spring)
equal hours of day and night
Sun rises directly East
moving north

24. Seasons Northern Hemisphere winter solstice fall equinox
axis points away from Sun
fewest hours of daylight
Sun rises southernmost along eastern horizon
fall equinox
equal hours of day and night
Sun rises directly East
moving south

25. Seasons and the Celestial Sphere

26. Keeping Time Solar Day Sidereal Day
time between two successive crossings of the meridian by the Sun
24 hours
Sidereal Day
time between two successive crossings of the meridian by a star
23 hrs 56 min

27. Day 2
Day 1
difference between sidereal day and solar day:
sidereal day is shorter

28. Keeping Time
Tropical Year
time for sun to complete one trip around ecliptic
solar days
Sidereal Year
time for constellations to complete one trip around the sky
solar days
Difference due to Precession
Difference due to precession time between two successive crossings of the meridian by a star
23 hrs 56 min

29. North Star Observation Explanation
different stars play the role of north star
difficult observation to make
time scale of 26,000 years
Explanation
Earth’s rotation axis wobbles
precession

30. North Star and Precession

31. North Star

32. Planets (“wanderers”)
Observation
five visible to naked eye
Mercury, Venus, Mars, Jupiter, Saturn
east to west
daily
west to east
move in zodiac (narrow band surrounding ecliptic)
retrograde (east to west)

33. Planetary Configurations
Conjunction
lies in same direction of the sky as the Sun
Superior
Sun between planet and us
Inferior
planet between Sun and us
only occurs for Mercury and Venus

34. Planetary Configurations
Opposition
planet lies directly opposite the Sun in the sky
only occurs for Mars, Jupiter & Saturn
Opposition
Superior Conjunction
Earth
Inferior Conjunction

35. Planetary Configurations
Transit
object moves across the Sun’s disk
occurs only for inferior planets
Greatest Elongation
greatest angular separation of a planet from Sun
Mercury - 28o
Venus - 47o
superior planets - 180o

46. Historical Perspective
Explanations
Historical Perspective

47. Shape of the Earth (Sphere)
Pythagoras
sphere is perfect shape
Aristotle
change in stars with change in latitude
appearance of ships’ sails
shadow of Earth during lunar eclipse

48. Size of Earth Erastothenes (276 - 195 B.C.) geometry
Parallel Light Rays
(40,000 km at equator)

49. Sun, Earth and Moon Aristarchus Moon about 1/3 size of Earth
Sun about 20 times farther away than Moon
Sun bigger than Earth
heliocentric theory

50. Geocentric Theory Earth is center of motion.
Sun, Moon, planets, stars all move around Earth
Fast moving objects are near Earth
Epicycle (Ptolemy )
circles on circles
could explain retrograde motion

51. Heliocentric Theory Copernicus (1473 - 1543) Sun is center of motion
Earth, planets, stars move around Sun in circles
explains retrograde motion
allows calculation of relative distances
Problems:
no parallax observed
predictions were not much better

52. Tycho Brahe (1546 -1601) superior observations Heavens are changeable
supernova
beyond planets
comet
outside of Earth’s atmosphere
compromise
all planets orbit Sun
Sun orbits Earth

53. Johannes Kepler (1571 - 1630) analysis of Tycho’s data
Three Laws of Planetary Motion
Planets move in elliptical orbits with the Sun at one focus.
Equal areas in equal times.
P2 =k a3

54. Law 1 - Ellipse squashed circle eccentricity most orbits have small e
measure of flattening
ratio of distance between foci to length of major axis
0 = perfect circle
most orbits have small e
semimajor axis

55. Law 2 - Equal Areas
Planets move faster when closer to the Sun

56. Law 3 - P2 = ka3 P is the period of the planet
how long it takes the planet to complete one orbit
a is the length of the semimajor axis

57. Galileo Galilei (1564 - 1642) observed moons of Jupiter
orbiting something other than the Earth
phases of Venus
Venus in orbit around Sun
Sunspots
Sun is not unchanging
Craters on the Moon
ordinary rock

58. Isaac Newton ( )
Laws of Motion
Theory of Gravity

59. Newton’s Laws of Motion

60. Describing the Motion position velocity acceleration location
speed and direction
acceleration
change in velocity
changing speed
changing direction
both changing

61. Changes in Motion Galileo rolling on inclines inertia
tendency of an object at rest to remain at rest and an object in motion to keep moving

62. Changes in Motion Newton=s First Law of Motion
An object at rest remains at rest.
An object in motion continues to move in a straight line at a constant speed unless a net force acts on it.
Such motion is called "uniform"
Non-uniform motion is accelerated motion.

63. Orbital Motion non-uniform First Law º a force must act on it
object follows a curved path
even if speed is constant
First Law º a force must act on it
astronomical objects
force is Gravity

64. Orbital Motion Law of Gravity described by Newton F = GMm/r2
M = mass of one object
m = mass of the other object
r = distance between the two objects
G = 6.7 x N-m2/kg2

65. Newton's Second Law of Motion
F = ma
change in motion depends on force
change in motion depends on mass
mass measures amount of material
more technically, its inertia
allows shape of orbit and details of motion to be worked out

66. Newton's Third Law action = reaction
the force that the Earth exerts on you is equal in size to the force you exert on the Earth

67. Measuring a Body's Mass
Newton's laws of gravity and motion allow mass of object to be deduced from orbital motion of object moving around it
Example using planets
example using Galilean satellites

68. Gravity Surface Gravity gravitational force of planet on object
this is the weight of object

69. Gravity Escape Velocity
speed needed to move away from an object and not fall back
depends on gravitational force
higher speed
mass will go higher
escape velocity for Earth = 11 km/s

70. Gravity Escape Velocity atmospheres black holes low escape velocity
high temperature
little atmosphere
black holes
escape velocity exceeds speed of light