AST121 Introduction to Astronomy

High Point University

Spring, 2004Dr. Aaron Titus

Getting to know you…

• Introduce yourself• The one-hand ice-breaker:

– What is something you’re good at?– Where are you headed?– What is something that makes you angry?– Who do you love or what is something that you

love?– What is something to remember you by?

Syllabus

• Course Description– An introduction to astronomy covering the motions,

distances, and physical nature of heavenly bodies. Topics include the history of astronomy, the scientific method, and current views of cosmology.

• Textbook– The Cosmos: Astronomy in the New Millenium by

Pasachoff and Filippenko– This is required for the course and will be used

extensively.– You should read the assigned chapter BEFORE class.

Syllabus

• Office: 342 HHSC• Email: titus@mailaps.org• Phone: 336-841-4668• Web: www.highpoint.edu/~atitus/• Class: M T Th 6:00 pm - 9:30 pm • Office hours

– Before or after class is generally a good time to find me, but there are occasions when I’ll be unavailable.

– Email is always a good way to reach me.

Syllabus

• Grading ScaleA+ (96), A (92), A- (88), B+ (84), B (80), B- (76), C+

(72), C (68), C- (64), D+ (60), D (56), D- (52), F (<52).

• Grade Determination– experiments and in-class activities (25%), homework

(20%), quizzes (15%), mid-term exam (20%), final exam (20%).

• Homework– WebAssign (https://www.webassign.net/v4.html)

WebAssign

• Your username is your High Point username and your initial password is the same as your username.

• Please log in tonight or tomorrow and email immediately if you have difficulty.

• After logging in, go to “options” to change your email address (to the one you check most often) and your password.

My Educational Philosophy

You learn best when you are actively engaged with the subject through activities such as reading (and answering questions about what you read), discussing, experimenting, and solving problems. Lectures are useful for motivation, but for most students listening to lectures and copying lecture notes is an ineffective method to learn.

My motivational speech

• Focus on learning, not on grades. Learning brings success, and life-long learning brings life-long success.

• If I offered to hire you for one month (30 days straight!) and offered to pay you $1,000,000 or 1 penny on the first day, but double your wages each day, which would you accept?

• If you take the penny, you make over 5 million dollars on the 30th day (over 10 million for the month)

• It’s how you grow that’s important, not how much you start with!

Chapter 1 - A Grand Tour of the Heavens

MeasurementsScientific Notation

How big is…?Scientific Method

Warm Up

Why should you learn about astronomy?

Psalm 191 The heavens declare the glory of God;the skies proclaim the work of his hands.2 Day after day they pour forth speech;night after night they display knowledge.3 There is no speech or languagewhere their voice is not heard.4 Their voice goes out into all the earth,their words to the ends of the world.

In the heavens he has pitched a tent for the sun,5 which is like a bridegroom coming forth from his pavilion,like a champion rejoicing to run his course.6 It rises at one end of the heavensand makes its circuit to the other;nothing is hidden from its heat.

Measuring things

• For us to communicate “how much” something is, we must use units.

• What if I were to offer a job to you that pays an annual salary of 60,000 per year? Would you accept the job?

• Three fundamental units used in the SI system (SI is the most common system of units used in science)– unit of mass, kilogram (kg)– unit of distance, meter (m)– unit of time, second (s)

Prefixes

prefix meaning power of 10

micro one-millionth 10-6 1/1,000,000

milli one-thousandth 10-3 1/1000

centi one-hundredth 10-2 1/100

kilo one thousand 103 1000

mega one million 106 1,000,000

giga one billion 109 1,000,000,000

tera one trillion 1012 1,000,000,000,000

Scientific Notation

• There is an easier way to express numbers so that we don’t have to count all those digits and commas.

9850 kg = 9.85 x 103 kg = 9.85 x 106 g

299,792, 458 m/s = 2.99792458 x 108 m/s

384,403 km = 3.84403 x 106 km = 3.84403 x 109 m

• The mass of the earth is 5.97 x 1024 kg. Do you really want to write that number without using scientific notation?

Mass

• a measure of inertia (how much net force is needed to accelerate an object)

• independent of location in the universe

• a measure of how much “stuff” an object is made of and what kind of “stuff” it’s made of; it depends on the atomic composition of which an object is made

Mass and Weight

• mass is NOT the same thing as weight!– near a large body such

as a planet or moon or star, w = mg where g is the local acceleration due to gravity.

– g depends on the mass of the large body

Bodyg

(m/s2)

weight

(N) of

Dr. Titus

weight

(lbs) of

Dr. Titus

Earth 9.8 800 180

Moon 1.6 130 29

Jupiter 25 2050 460

Mars 3.7 300 67

Pluto 0.1 8 1.8

Masses of a few common objects

Object mass (kg)

1 liter of water 1

Earth 5.97 x 1024

Moon 7.36 x 1022

Sun 1.99 x 1030

me 82

universe ~1050

hydrogen atom 1.67 x 10-27

Distance

• “how far” one point is from another point.• need to define the two points

– distance of the center of the moon from the center of the earth

– distance of the center of the moon from the center of the sun

• need a standard– a meter, as defined by the length of a meterstick, is too

arbitrary– a meter should be based on another unchanging and

unarbitrary value.

What about light?

• Albert Einstein made a presupposition that the speed of light is the same in all reference frames (i.e. no matter how fast you are traveling and no matter what direction you’re traveling in, you will measure the same value for the speed of light).

• Einstein also proposed that nothing travels faster than the speed of light in a vacuum.

• The speed of light is a universal speed limit!• The speed of light is a universal constant!

c

• c = 299,792,458 m/s and is DEFINED to be this value.

• Therefore, in one second, light travels 299,792,458 meters.

• 1 m = the distance light travels in 1/299,792,458 second

light year

• 1 light year is the distance light travels in one year.

• 1 ly = 9.46 x 1015 m

• It takes about 8.3 minutes for light from the Sun to reach Earth.

distance to the Sun / speed of light = 500 seconds = 8.3 minutes

second

• Throughout history, time has been measured in various ways.

• Astronomy has long been important in defining time.

• A month was originally measured by lunar phases, the day is measured by the earth’s rotation about its axis, and the year is measured by the earth’s motion about the sun.

• A day is broken into hours, hours into minutes, and minutes into seconds.

• But what are some problems with using this method to define a second?

second

• The time duration for Earth to rotate once about its axis (a day) is not necessarily constant. Earth’s rate of rotation is slowing down.

• A day cannot be measured as precisely as we would like.• We now define a second in terms of the frequency of the

light emitted by excited electrons in cesium atoms. The electrons emit light at a frequency of 9,192,631,770 Hz. Thus, one second is the time required for this light to make 9,192,631,770 oscillations. The official clock of the United States is the cesium fountain clock, NIST-F1, at NIST.

• The uncertainty of NIST-F1 is so low that it will neither gain nor lose a second in 20 million years!

The nature of science

• Science is the systematic enterprise of gathering knowledge about the world and organizing and condensing that knowledge into testable theories.

• You will notice four important things in this definition– observing (i.e. experimenting)

– developing a model for understanding and predicting

– testing the model

– doing this in a systematic way

“Doing” science

• There’s a definite methodology to “knowing” - we call this the scientific method.

• The scientific method is the process of formulating a model to explain and predict observed phenomena.

• It’s not a single method, but rather a process that has certain characteristics.

• There’s no correct definition of the scientific method.

Experiment

• “The test of all knowledge is experiment. Experiment is the sole judge of scientific ‘truth.’” -- Richard Feynman, The Feynman Lectures on Physics.

• Scientific theories and models are tested by experiment. If they don’t hold up, there are two possibilities: (1) our theory or model is incomplete or invalid; (2) our experiment is flawed and we’ve made invalid conclusions from our experiment.

Characteristics of the Scientific Method

• observation or research question• hypothesis

– Sometimes this is a creative idea based on what you already know

– Sometimes this seems completely “off the wall”– This is often based on a “model” which is a well-organized

concept used to explain and understand.

• experiment– Are the results explainable by the hypothesis?– Was the experiment well-designed?

• prediction and further testing

a few more comments...

• A model is an organized way of thinking about something.

• A model is often approximate and has known limitations (i.e. it’s simplistic).

• A model is our way of understanding a complex phenomenon in a simple way.

• A model is used to formulate specific hypotheses.

examples of models

• We might model the earth as a sphere. It’s actually squashed at the poles.

• We might model the earth and moon as a two-body system even though they also orbit the sun, and the entire solar system is also in orbit within the Milky Way, etc.

• We model solid matter as if it was made up of atoms connected by tiny springs.

For practice

• A jar is full of candies. In a group of two, determine how you can estimate the number of candies in the jar without actually counting ALL of the candies (counting a few is acceptable)?

• Determine at least two methods.

• Estimate the number of candies in the jar.

The Universe in 11 Steps

Nightwatch

Terence Dickinson

Step 1 (20,000 km)

Step 2 (2 x 106 km)

Step 3 (1.2 AU, ~200 x 106 km)

Step 4 (120 AU)

Step 5 (12,000 AU = 0.19 light-years)

Step 6 (20 light-years)

Step 7 (2,000 light-years)

Step 8 (200,000 light-years)

Step 9 (20 million light-years)

Step 10 (2 billion light years)

Step 11 (greater than known universe)