Essential information:

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Astronomy 101 Spring 2003
Lecture 1 Jan. 22, 2002
Essential information Lecturers Prof. Deane
Peterson Office ESS 454 Tel [phone]
e-mail deane.peterson _at_ sunysb.edu Office
Hours Tues. 1110 AM 1210 PM
Wed. 1130 AM 1230 PM Prof. Paul
Grannis Office Graduate Physics P101 (in
Main Physics Astronomy office) Tel 632
8088 e-mail paul.grannis _at_ sunysb.edu
Office Hours Mon. 200 300 PM
Wed. 300 400 PM TAs Jinmi Yoon
(AST 101 grading) Daniel Kerr (AST 112
lab)
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Course web page http//www.astro.sunysb.edu/dpe
terso/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!)
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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 doctors 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
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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.
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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 dont 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
dont 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.
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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.
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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. 1.5 x 108
Shift decimal point 8 places to left and get
factor 108
8 7 6 5 4 3 2 1
Examples 1000 103 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
Shift decimal point 9 places right to get factor
10-9
1 2 3 4 5 6 7 8 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.

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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 1 ly
(3 x 105 km/s) x (3.16 x 107 s) 3 x 3.16 x
1057 9.46 x 1012 km Milky Way is
about 100,000 ly 105 ly across.
Cluster
of galaxies is 1 x 106 ly across
( rate x time )
Entire observable universe is about 15 billion ly
(1.5 x 109 ly)

• What is the size of the universe in km?

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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
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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) (arc minutes and arc
seconds have nothing to do with time units !)
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Moons diameter subtends an angle of about 0.5O
30
Orion is 20O across
20O
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)
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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
S2
d2
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?

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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?

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North celestial pole
In addition to the Earths 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 earths
equator (the plane perpendicular to the polar
axis) is the Celestial Equator.
Celestial sphere
Fall equinox
Spring equinox
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
overhead
N
N
overhead
1
3
S
Winter in NY sun low in sky more than ½ day in
night

• Show with a diagram why it is summer in N
  hemisphere when winter in S hemisphere

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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?

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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?

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Moon in Earths shadow causes LUNAR eclipse at
time of full moon only. Can see lunar eclipse
from anywhere on earth.
Lunar eclipse
Moons shadow falling on Earth causes SOLAR
eclipse. Solar eclipse only in limited region
of moons shadow.
Solar eclipse
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Moons 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?