Chapter I:

0. Roots of Astronomy and our View of the World

Why does Man study Astronomy?

organize time:	clock, calendar, seasons
understand how the world is built:	model of the universe

• We face the discussions between Science and Religion:
  • Where do we come from?
• We are confronted with the limitations of Science:
  • Will we ultimately understand "everything"?

Mathematics:	sizes, "language for modeling"
Physics:	understand processes
Chemistry:	composition of planets and comets
Geology:	appearance and evolution of planets
Meteorology:	planetary atmospheres

Contents of the Universe:

Planets,	Moons,	comets
Sun and stars,	gas nebulae,
Clusters of stars,	galaxies,	clusters of galaxies

Learning outcomes:

Astronomy spans all sciences, in particular all branches of physics

Physics is the art of measuring objects and processes

Science is a never-ending enterprise

Contents of the universe

I. Use of Scientific Methods

(Chapter I. is to be used as a Compendium throughout the Course. We browse through it and fill some blanks with details in following Chapters.)

1) To Measure Means to Compare

We measure an unknown quantity by comparison with a known quantity.

A. Powers of ten

Large and small numbers are presented easier as powers of ten:

• E.g.: 2,300 -> 2.3 x 10³; 0.000,000,2 -> 2 x 10^-7

B. Geometry

a) Length:

Basic Unit: m (cm, km)

Compared with a meter stick (original meter, Paris)

Reality			Model
	Size in km	Distance in km	Size in cm	Distance (m)	Location
Sun	1.4 *106		13.9		Front of Auditorium
Mercury	4,880	5.8 * 107	0.05	5.8	2 steps up
Venus	12,100	1.0 * 108	0.12	10.8	7 steps up
Earth	12,760	1.5 * 108	0.13	15.0	Rear of Auditorium
Mars	6,800	2.3 * 108	0.07	22.8	Tree in front of Auditorium
Jupiter	143,800	7.8 * 108	1.44	77.8	UNH Bookstore
Saturn	120,000	1.4 * 109	1.2	143.0	Morse Hall
Uranus	50,500	2.8 * 109	0.51	287.0	Hood House
Neptune	49,500	4.5 * 109	0.50	450.0	Tin Palace
Pluto	2,300	5.9 * 109	0.02	590.0	Young's Restaurant
Closest Star:	Light Years
a Centauri	4.34			4,108,799	San Francisco

Remember, in a Geometric Model you are shrinking all Distances and Sizes! That means:
- All Ratios remain unchanged
- All Angles remain unchanged

Derived from Length:

Area

<= Length * Length

m * m = m2

Used for example for Surface of objects

• E.g.: circle 2x in radius = area of circle: 2*2 = 4 x as large!!

Volume

= Length * Length * Length

= m * m * m = m3

Used for example for Volume of objects

• E.g.: sphere 2x in radius = volume of sphere: 2*2*2 = 8 x as large!!

c) Length determination in Astronomy:  

To get distances and sizes in astronomy we use angle measurements and a baseline.

Skinny triangle: (in Astronomy always a "skinny triangle")

i) Distance r known

ii) Obs. separation d known

-> determine

by measuring q, plugging into: Q = 360 * d/2'r = 57.3 * d/r

and solving for:

"Parallax" is the basis of all distance measurements in Astronomy.

Other useful length units:

• Longest Baseline for Parallax: 1 AU [Astronomical Unit = Earth - Sun distance]= 1.5 10⁸ km
• 1 Parsec = distance giving a Parallax of 1 arcsec = 3.1 10¹³ km = 3.26 LY
• 1 LY [Light-year = distance light travels in 1 year] = 9.5 10¹² km

  C. Action:

1) Time

Basic unit:

sec (second), hour, day

Compared with regular motion

• sun -> sun clock (position of shadow with time) -> hours, days
• sand flowing through an orifice -> sand clock -> minutes
• mechanical pendulum -> mechanical clock -> seconds
• oscillation of crystal -> quartz clock -> fractions of second

2) Velocity:

• Derived Unit: m/sec, km/h
• Distance traveled per unit time: Velocity = Distance/Time

Change of velocity (acceleration, deceleration)

• Derived Unit: m/sec/sec = m/sec2
• Velocity change/unit time: Acceleration = Velocity/Time

D. Substance: In Chapter IV

Basic Unit: kg (kilogram)

• Compared with original kg (Paris)

Inertial mass: resists the attempt to move it

Gravitational mass: ability of objects to mutually attract one another

m_i = m_g= Einstein's General Theory of Relativity

Derived Unit:kg/m3

• Idea of planet's interior:
• denser than rocks = rock + iron core
• or of stars' interior =compressed gas
• or neutron stars = denser than atomic nuclei

But do not mix Mass with Weight!!

• Weight: means a force in a given gravitation (e.g., on Earth)
• Mass of the same body remains the same, but its Weight may vary from location to location (on Earth, Moon or in free space)!!!

Unit: see Force (or 1 kilopond (kp) = 1 kg on Earth)

c) Force ability to pull or push an object (to change its motion)

• Derived Unit: kg * m/sec2 = Newton (N) in Chapter IV

d) Energy Ability to change motion or generate heat

• Derived Unit: kg*m2/sec2 = Joule in Chapter IV

For example: Falling object vaporizes rock --> explosion --> round crater

Gravitation --> Motion --> Heat

e) Pressure = Force per area in Chapter VIII

• Derived Unit: N/m2 = Pascal Å 1/50 Pounds/foot2
• Holds things up against gravity:
• Atmospheres: pressure = weight of overlying atmosphere
• Stars: internal oven keeps material above in balance
• Black holes: no pressure high enough to keep it from collapse?

f) Temperature Average energy per molecule in Chapter VI, VIII

• Basic Unit: Kelvin = Centigrade + 273 (always starts with 0 degree!!)
• 0 Kelvin = -273 Celsius = - 404 Fahrenheit
• 273 Kelvin = 0 Celsius = 32 Fahrenheit
• Ability of gas to resist gravity Þ

Determines properties of atmospheres

• Hot sun has an extended atmosphere
• Warm earth's atmosphere is only tens of miles deep
• Hot and small Mercury with its weak gravity lost its atmosphere

Determines life of stars

• Stars form from collapse of cool interstellar clouds ("stellar nurseries")
• Nuclear Fusion possible in hot interior of tars ("ignition temperature")

2) What we measure

To get information over large distances in astronomy: in Chapter V

A) Electromagnetic radiation:

• Radio, microwave, IR, light, UV, X, gamma rays

B) Particles: Cosmic rays (sample of Milky Way Galaxy)

• Meteors (sample of early solar system)
• Comets (sample of early solar system)
• Solar wind (sample of sun)

C) Forces: Gravity (the key to deducing masses)

3) Deduction

We use Deduction, i.e. we measure something to deduce something else:

Measure	Deduce:
A) angle --> skinny triangle	size or distance
B) orbits --> gravity	masses in Chapter IV
C) color --> blackbody	temperature in Chapter VII (red:cool; yellow:hot; blue:very hot)
D) change in --> frequency	velocities in Chapter VII Doppler shift (frequency increases = moves toward; frequency decreases = away).

4) Scientific Reasoning

We use Scientific Reasoning to test our view of the world and to develop new measurement methods.

Scientific Reasoning consists of:

A) Earth goes around sun.

(Analogy: raindrops seen from moving car)

B) i. Earth rotates on its axis.

• (Coriolis also gives circulation of weather systems.)
• (Jupiter's red spot and Neptune's dark spot have circulation of high-pressure systems.)

5) Scientific Models

To understand the universe and its contents we build models of what is going on.

Geometry is maintained, length scales reduced to fit

• Globe -> Earth
• Map -> orientation in surrounding area

B) Physical models

Main functions behave like the real thing (details may be different)

• Constant improvement of models is what we do in Science

In the beginning: 1. A) Through D) b), 2), 3) A), and 4) A)

Other items will be included as course proceeds!

• Use of Powers of Ten
• Define length, area, volume, angle, mass, mass density and weight
• Use of these quantities in astronomy
• How to determine distances and sizes in astronomy (Parallax)
• Use of radiation, particles and forces to measure astronomical quantities

What is deduction? Example: parallax to determine distances

What is scientific reasoning? Examples: Earth moves -> parallax and aberration of stars