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Astronomy 311

Professor Menningen January 2, 2014

• Syllabus overview

– books & supplies

– course goals

– assignments &

grading

• About the professor

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How to Learn Astronomy

• Stay curious

• Interact with the same material several

times

• Work together with someone

• Try extra homework questions

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How to Succeed in College

• Set a GPA goal

• Treat college like an 8 to 5 job

• Attend class

• Keep up

• Relate to your professors

• Be a "well-rounded square"

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Lecture 1

Introduction &

The Night Sky

January 2a, 2013

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What is Astronomy?

• Study of all things outside of the

atmosphere of the Earth.

– Planets

– Stars

– Galaxies

– Structure and Evolution of the Universe

(Cosmology)

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The Constellations

• Groupings of prominent stars which appear

nearby in the sky

• Includes the whole area in the sky- not just

the stars

• 88 constellations

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Constellation of Orion

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Stars are at

different

distances

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Units of Measure • We’ll use mainly the metric system, based on the

meter, kilogram, and second

• We’ll often use scientific notation:

• The speed of light is

• You should know the metric prefixes and scientific

notation

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1 light-year 9,461,000,000,000,000 m

9.461 10 m

8299,792,458 m/s 3.00 10 m/sc

Using your calculator • Many times we’ll be using very large or very small numbers. We’ll

write them in scientific notation, such as:

• Use the EXP or EE key on your calculator to save time and avoid

errors: or

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191.6 10 J instead of 0.00000000000000000016 J

1 . 6 EE ( ) 1 9 1 . 6 EXP 1 9

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Converting Units

• Use conversion factors

• Move decimel point (requires practice, and

good familiarity with metric prefixes)

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Convert meters to parsecs

1.0 ly 1.0 pc1.85 10 m 6.00 pc

9.46 10 m 3.26 ly

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If 1.0 AU = 1.5×1011 m, what is a speed

of 3.0×108 m/s in units of AU/hr?

200

0 AU/h

r

500

AU/h

r

7.2

AU/h

r

0.1

2 AU/h

r

0% 0%0%0%

0 of 5

A. 2000 AU/hr

B. 500 AU/hr

C. 7.2 AU/hr

D. 0.12 AU/hr

120

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If 1.0 AU = 1.5×1011 m, what is a speed

of 3.0×108 m/s in units of AU/hr?

A. 2000 AU/hr

B. 500 AU/hr

C. 7.2 AU/hr

D. 0.12 AU/hr

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3.0 10 m 1.0 AU 3600 s7.2 AU/hr

s 1.5 10 m h

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Measuring Angles • Apparent distances in the sky are determined by

measuring the angle between two objects.

• There are 360 degrees in a circle.

• Astronomers measure angles in degrees and

arc minutes (60 per degree) and

arc seconds (60 per arc minute or 3600 per degree)

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Angular Size

q

As distance increases, angular size decreases

q

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Angular Size

• The size of an object is related to its distance

from Earth and its observed angular diameter:

physical diameter distance angular diameter 180

180 57.3

exact: tan

r rD

D r

q q

q

q

r

D

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Linear Size from Angular Size

180

rD

q

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If the distance between Earth and Jupiter is cut to a

third (due to their orbital motions) the angular size

of Jupiter as seen from Earth is _____.

cut t

o a n

inth

cut t

o a th

ird

trip

led

nin

e tim

es la

rger

0% 0%0%0%

0 of 5

A. cut to a ninth

B. cut to a third

C. tripled

D. nine times larger

120

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If the distance between Earth and Jupiter is cut to a

third (due to their orbital motions) the angular size

of Jupiter as seen from Earth is _____.

A. cut to a ninth

B. cut to a third

C. tripled

D. nine times larger

new

new new old

oldold new

old

new oldnew ol

new ol

d1old old3

d

180°

180°

beca

3

us

3

e

D

r r

D r

r

r

D

r

D

q

q

qq q

q

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On a day when Jupiter is 6.3×108 km away from

the Earth, the Great Red Spot has an angular

diameter of 7.9 arc seconds. What is the diameter

of the Great Red Spot in km?

24,

000

km

1.4

×106

km

500

km

1,2

50 k

m

0% 0%0%0%

0 of 5

A. 24,000 km

B. 1.4×106 km

C. 500 km

D. 1,250 km

120

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On a day when Jupiter is 6.3×108 km away from

the Earth, the Great Red Spot has an angular

diameter of 7.9 arc seconds. What is the diameter

of the Great Red Spot in km?

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1.0°7.9 arcsec 0.00219°

3600 arcsec

6.3 10 km 0.00219°

180° 180°

24,000 km

rD

q

q

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Estimating Angles

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Viewing the Sky

S

W

E

N

Zenith: point directly overhead

Meridian: Line running North-

South. Sun and stars reach their

highest point in the sky when they

cross the meridian each day.

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Positions in the Sky • Altitude = the angular distance of an object above

the horizon.

• Azimuth = the angle measured eastward along the

horizon from North to the point directly below the

object.

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The Celestial Sphere

• Need a coordinate system in the sky (like

latitude and longitude).

• Locations of the stars, Sun and planets can

be specified on the celestial sphere.

• Stars appear fixed on the celestial sphere.

• Sun and planets move on the sphere

throughout the year.

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North Celestial Pole

South Celestial Pole

North Pole

South Pole

Equator

Celestial Equator

Polaris, the North Star

The Celestial

Sphere

Our View of The Celestial Sphere 27

Celestial pole (Polaris)

Pole is same

angle above

N horizon

as your

latitude on

Earth

Equator is

90° − latitude

above S horizon

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Daily Motions

• The Earth spins on its axis one time every

day.

– Causes day and night.

– The Sun, Moon, and stars all move from east to

west across the sky during the course of a day.

– The North Star, Polaris, does not move. Its

angle above the northern horizon is equal to the

observer’s latitude on the Earth’s surface.

– The stars that we can see depend on our latitude

on Earth.

Observers can only see stars that are

North of the Celestial Equator.

The North Star is directly overhead

Stars will never rise or set (the

stars are circumpolar). They

appear to circle the North Star.

View from North Pole 29

Celestial Equator and

Observer’s horizon

Observers can see all of the stars

during the course of a year.

The North and South Celestial

Poles are on the horizon.

All stars appear to rise and set

straight up from the horizon.

None are circumpolar.

View from the Equator 30

Observer’s horizon

View from Northern Hemisphere

Some stars will rise and set. The

most northern stars will never set

(they are circumpolar).

We cannot see the most southern

stars on the Celestial Sphere.

Circumpolar star

North Star

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Observer’s horizon

Star trails in the Northern Hemisphere

Motions of stars for part of a night. Center star is Polaris, the

North Star. Computer Animation

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