Astronomy 101 – Spring 2003
Essential information:
Lecturers:
Prof. Deane Peterson
Office: ESS 454
Tel: 632 – 8223
e-mail: deane.peterson @ sunysb.edu
Office Hours: Tues. 11:10 AM – 12:10 PM
Wed. 11:30 AM – 12:30 PM
Prof. Paul Grannis
Office: Graduate Physics P101
(in Main Physics & Astronomy office)
Tel: 632 – 8088
e-mail: paul.grannis @ sunysb.edu
Office Hours: Mon. 2:00 – 3:00 PM
Wed. 3:00 – 4:00 PM
TAs: Jinmi Yoon (AST 101 grading)
Daniel Kerr (AST 112 lab)
Lecture 1 Jan. 22, 2002
Course web page:
http://www.astro.sunysb.edu/dpeterso/AST101/index.html
Contains course info, homework, lecture notes, grading policies, observing
projects, links to other sites etc. Look for course news here.
Text:
Astronomy Today Chaisson & McMillan. We will cover Chapters 1-4;
a little material on the solar system (Ch. 6 – 15); and Chapters 16 – 27.
Lecture plan on web and in handout.
Course structure:
2 Lectures per week (Mon, Wed) and a 1 hour recitation. Lectures
introduce the material. There are assigned questions and problems from the
book (see web page or handout). See also questions for thought in Lecture.
Recitations review assigned questions and treat some topics in more
detail. There will be about 5-6 quizzes in Recitation based on homework. The
best 3 quiz grades will be used as a component of recitation grade.
There are 3 observing activities required. Two require observations
over the full semester (start now!)
Prerequisites:
No formal prerequisites other than high school algebra and some
acquaintance with high school chemistry. Algebra will be used in describing
astronomy and physics concepts in lecture and recitation.
Exams:
2 Midterm exams (in lecture hour on Feb. 26 and Apr. 7) and one
Final Exam (cumulative) on May 19, 8AM. There will be no makeups for a
missed Midterm; those students with a valid excuse will have the average of
the remaining midterm and final used for the missed exam.
Medical excuse must be signed by Doctor with a phone number indicated and
must specify the nature of the illness. Infirmary notes indicating only that
a student has visited are not accepted (you may ask the infirmary for the
doctor’s note, or get one from a private physician).
Grading:
15 points for each Midterm
30 points for Final
30 points for recitation (Quiz average and Instructor evaluation)
10 points for the observing projects
Disabled policy:
If anyone has a condition which will make it difficult to carry out
the work, or which will require extra time on exams should see the
lecturers in the first week of class, or visit the Disabled Student Service.
Academic honesty:
All work that you hand in must be your own! Copying from others, use of
reference materials on quizzes or exams, fabricating data on the activity
projects will result in zero for that work and will be reported to the College
Committee on Academic Honesty.
Our role in this course:
Our job is to prepare clear lecture and recitation presentations, and to give
you all the information you need about the course. We will make ourselves
available for your questions, and will treat students with respect.
Your role in this course:
This is your education! You will gain in proportion to the effort that you put
in. Much of what you retain from a course like this will be how to think about
the universe we live in. Try to focus on how things work and how we know
what we do – more than the specific facts, for this is what you will remember.
Astronomy, like all science, is based on observation and experiment; if we do
not observe it, we don’t know it! In the case of astronomy, YOU can see many
of the phenomena simply by looking at the sky! Get into the habit of looking
at the sky.
We do not expect you to understand everything that we describe on the first
pass – your job is to ask questions if you don’t understand so we can try again
with a different approach. You have to participate in the learning process!
Come see the faculty in this course with your questions – this is an
essential part of your learning experience.
Astronomical Distances
Astronomical distance scales vary widely. We use
different units for talking about solar system, our
galaxy, the universe at large:
Planet earth: diameter ~ 15,000 km
1 km = 1000 m (1 km = 0.62 miles)
Star (our sun): diameter ~ 1,500,000 km
(100 times larger than Earth)
Distance from Earth to Sun = 150,000,000 km =
1 ASTRONOMICAL UNIT (AU)
Units matter! 10cm, 10m, 10km, 10AU are very
different! Numbers without units are
meaningless.
Writing out very big and very small numbers is a
pain! Use scientific notation.
Large numbers: Scientific notation for 1AU expressed in km:
1 AU = 150,000,000 km = 1.5 x 108 km
150,000,000
8 7 6 5 4 3 2 1
Examples: 1000 = 103 ;
.
= 1.5 x
108
Shift decimal point
8 places to left and
get factor 108
1,000,000 (1 million) = 106
Small numbers: 1 km expressed in AU is
1 km = (1/150,000,000)AU = 0.000 000 0067 AU = 6.7 x 10 -9 AU
.
0 000 000 0067 = 6.7 x 10-9
1 2 3 4 56 7 8 9
Shift decimal point 9
places right to get
factor 10-9
Examples: 0.001 = 10-3 ; 0.000 0001 (1 millionth) = 10-6
 Write diameter of earth (6,378,000 m) and size of atom (0.000 000 015 cm)
in scientific notation.
Our galaxy (Milky Way) diameter = 1018 km = 6.7 x 109 AU across. -- so big we
use a new unit LIGHT YEAR (ly). 1 ly is distance that light travels in 1 year
at speed of light = 3 x 105 km/s. There are 3.16 x 107 seconds in a year, so
( rate
x
time )
1 ly = (3 x 105 km/s) x (3.16 x 107 s) = 3 x 3.16 x 105+7 = 9.46 x 1012 km
Milky Way is about 100,000 ly = 105 ly across.
Cluster of galaxies is ~ 1 x 106 ly across
Entire observable universe is about 15 billion ly (1.5 x 109 ly)
 What is the size of the universe in km?
We observe objects on the sky as if pasted on CELESTIAL SPHERE. But
objects that look close to each other may be at vastly different distances
along our line of sight.
Constellation ORION (the hunter) is a
collection of stars that are at quite
different distances.
 Find Orion high in the sky at 10 PM
The familiar constellations group stars that are
typically very far apart and quite unrelated
Determine positions on the sky in
ANGULAR MEASURE
1 full circle on sky = 360 degrees (O)
Each 1O has 60 arc minutes (60’)
Each 1’ has 60 arc seconds (60’’)
30’
(arc minutes and arc seconds have
nothing to do with time units !)
Orion is
~ 20O across
Moon’s diameter
subtends an angle
of about 0.5O = 30’
Scientists measure angles in Radians
2p radians make a full circle (360O)
Thus 360O = 2p radians (1 radian = 57.2O)
Angle in radians = Angle in degrees x (2p / 360)
Angular size and real size (in meters, km, AU, ly etc) are related.
We must know the distance to the object to relate them.
S1
d2
S2
q
d1
Object 1 with size S1 at distance d1 has same
angular size q as object 2 with size S2 = 2S1 at
distance d2 = 2d1
Simple ratio for objects of equal angular size:
S1/S2 = d1/d2
Relationship of angle and size:
S = d q
(if q in radians, and q small – less than about 0.1 radians or 6O)
 What is the angle subtended by a penny (diameter = 2cm) held at a distance
of 4 m (400 cm) from your eye? (in radians and in degrees)
 What is the difference in the angle from your left and right eyes to a point
on the wall that is 10 m away?
Sky coordinates:
The earth REVOLVES around the sun in 1 year in a nearly circular orbit.
(To us it looks like the sun orbits the earth – and the ancients thought this
to be the case). The line traced by the sun on the celestial sphere is the
ECLIPTIC. Each one-twelfth of the ecliptic (360O /12 = 30O) is one of the
signs of the zodiac. Today, the Sun is entering the “house of Aquarius”
Earth location
today
 Does astrology
(foretelling fortune
by location of sun at
your birth) make any
scientific sense?
Why or why not?
In addition to the Earth’s revolution
around the sun, it ROTATES on its axis
from N to S poles in one day.
The axis of rotation is inclined at 23.5O
with respect to the axis of revolution.
The intersection of the line from Earth
center through N or S pole with celestial
sphere are the Celestial poles. The
projection of the earth’s equator (the
plane perpendicular to the polar axis) is
the Celestial Equator.
Planes of celestial equator and the ecliptic
intersect at EQUINOXES. Happens at points in
orbit on ~Mar. 21 and Sept. 21.
Summer in NY sun high in
sky; more than ½ day in
sunshine
1
North celestial pole
Celestial
sphere
Fall
equinox
Spring
equinox
N
S
 Show with a diagram why it is summer in
N hemisphere when winter in S hemisphere
N
3
Winter in NY; sun low in sky;
more than ½ day in night
Time for earth to rotate to take the sun from overhead one day to
overhead the next day is SOLAR DAY.
Time for earth to rotate to take the fixed stars from a given location to
same location the next day is SIDEREAL DAY.
Since Earth moves 1/365th of way around its orbit in 1 day, Solar day is
longer than Sidereal day. (by about 4 minutes)
To the
‘fixed’ stars
 What would be
relation between Solar
and sidereal day if
Earth rotated in
opposite sense to its
revolution on orbit?
Moon is in approximately circular orbit around the
earth – so travels in a circle whose center moves
around the sun.
The moon ‘shines’ by reflected sunlight, so in 1 ‘lunar
month’, the fraction of its surface seen illuminated
on earth varies.
Lunar sidereal month is
shorter than SYNODIC
month (time from full moon
to full moon)
 What is the phase of
moon today??
 At what time does full
moon rise? When does 1st
quarter moon rise?
Moon in Earth’s shadow causes
LUNAR eclipse – at time of full
moon only. Can see lunar eclipse
from anywhere on earth.
Moon’s shadow falling on Earth causes
SOLAR eclipse. Solar eclipse only in
limited region of moon’s shadow.
Lunar eclipse
Solar eclipse
Moon’s orbit is not exactly in the ecliptic plane. Can only get solar eclipse
when Earth, Sun and Moon line up exactly, so not an eclipse every month.
No eclipses if moon is above
the Earth-Sun line.
 Why is a solar eclipse more rare than a lunar eclipse?