1

Looking at the Night Sky

How to find your way around:• Position -> where is that object?• Distance -> how much space between these

two things?• Motion -> where will that object be later

tonight?• Bright/faint objects -> magnitude!

Using AnglesDistances on the sky are measured in_______, ___-________, and ___-_______

• 1 degree = __ arc-min

• 1 arc-min = __ arc-sec

• Width of index finger = __º

• Width of three fingers =__º

• Width of entire hand = __º

Earth’srotation

causes theSun,

Planets,Moon and

stars toappear to

move whenviewed from

Earth

Rising and Setting Stars• The Earth’s eastward rotation

causes stars to appear to movewestward.

• Stars near the _____ ____________ move in small circles, andare called ____________

• Stars far from the pole move inlong arcs

Nightly Motion of the Stars

• Imagine looking toward the North.What do stars appear to do overthe course of an evening?

• What about stars in the South?East? West? Directly overhead?

Nightly Motion of the Stars

For stars (and Moon and planets) thatappear in the southern sky: Stars firstrise near the _______ horizon, moveupward and toward the south, and thenmove down and set near the _______horizon.

2

Nightly Motion of the Stars

• Looking North: Starsappear to move counter-clockwise around thestationary North Star(_______) – we call these_____________ stars.

Looking North: CircumpolarStars

– Circumpolar stars seem to move counter-clockwise around the stationary _____ ____.

– These constellations and stars are visible anynight of the year in the NORTHERN sky becausethey never rise or set!

– Examples: Ursa Major, Ursa Minor, Draco,Cepheus, and Cassiopeia

How long did ittake to get thispicture?

Motion Lecture Tutorial – pg. 3

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with

one another.• Come to a consensus answer you both agree

on.• If you get stuck or are not sure of your answer,

ask another group.• If you get really stuck or don’t understand what

the Lecture Tutorial is asking, ask me for help.

•Ancient people classified stars into 6 groups:•“_____ Magnitude” stars were the brightest•“_____ Magnitude” stars were the faintest

•A modified version of this system is still used

Measuring Stars: Magnitudes

Constellations help us find our way in the sky,but we want to know more…

Magnitudes System

• Way to group stars by intensity/brightness• Brighter star = ______ magnitude• Fainter star = ______ magnitude

• Difference of 1 in magnitude < -- > factor of 2.5 inbrightness

• EX: Mag. 3 star vs. mag. 6 star :– Mag. 6 star is _______– 2.5 x 2.5 x 2.5 times fainter = ____ times fainter

3

Examples of Magnitudes What can you see?

What we can see in the sky(constellations) at any given time isdetermined by three factors:– Time of night/day– ____________________– ____________________

Review: Coordinates on the Earth• ________: position north or south of equator• ________: position east or west of prime meridian (runs

through Greenwich, England)

The sky varies with ________ but not_________.

Altitude of the celestial pole =____ ________

Why do the constellations we seedepend on latitude and time of year?

• They depend on latitude because yourposition on Earth determines _________________________________________.

• They depend on time of year because Earth’s________________________________________________________________________.

4

Why can’t we see the sameconstellations all year round?

The sky varies as Earth orbits the Sun

• As the Earth orbits the Sun, the Sun appears to move_________ along the ecliptic.

• At midnight, the stars high in the sky are ________ theSun in the sky.

Constellations along theSun’s path • The path of the Sun

(_______) was animportant part ofancient astronomy

• The planets and Sunrepresented deities,and their positionswere significant

• Most ancientastronomers werealso in the astrologybusiness

Zodiac -The 13 Zodiacal constellations thatour Sun covers-up (blocks) in the

course of one year(used to be only 12)

• Aquarius• Pisces• Aries• Taurus• Gemini• Cancer

• Leo• Libra• Virgo• Scorpius• _________• Sagittarius• Capricornus

The Zodiacal Constellations that our Suncovers-up (blocks) in the course of one year

(only 12 are shown here)

North Star

365 days

1 dayAries

Pisces

Taurus

Gemini

Cancer LeoVirgo

Libra

Scorpius

Sagittarius

Capricornus

Aquarius

Figure 1

5

Whichconstellation is theSun in front of for

the situationshown?Pisces

Aries

Taurus

Gemini

Cancer

Figure 2 – What itwould look like if youwere the observer inFigure 1

South

North Star

365 days

1 dayAries

Pisces

Taurus

Gemini

Cancer LeoVirgo

Libra

Scorpius

Sagittarius

Capricornus

Aquarius

Figure 1

Pisces

Aries

Taurus

Gemini

Cancer

Figure 2 – What itwould look like if youwere the observer inFigure 1

South

North Star

365 days

1 dayAries

Pisces

Taurus

Gemini

Cancer LeoVirgo

Libra

Scorpius

Sagittarius

Capricornus

Aquarius

Figure 1

E W

W

E

What time is itfor the

observer?

What is the nameof the

constellation thatwould appear on

the observersEastern Horizon?

Western?

Pisces

Aries

Taurus

Gemini

Cancer

Figure 2 – What itwould look like if youwere the observer inFigure 1

South

North Star

365 days

1 dayAries

Pisces

Taurus

Gemini

Cancer LeoVirgo

Libra

Scorpius

Sagittarius

Capricornus

Aquarius

Figure 1

E W

W

E

12 hours laterwhat object will

be at theposition thatTaurus is in

now?18 hours laterwhere will the

Sun be?Where will

Scorpius be?

Lecture-Tutorial: SeasonalStars (page 7)

• Work with a partner!• Read the instructions and questions carefully.• Discuss the concepts and your answers with one

another.• Come to a consensus answer you all agree on.• If you get stuck or are not sure of your answer, ask

another group.• If you get really stuck or don’t understand what the

Lecture Tutorial is asking, ask me for help.

The changing position of the Sun during the year! How can we describe the motion of the sunin the sky?

westsouth

meridian

east

Zenith or overheadX

6

How can we describe the motion of the sun in the sky?

east westsouth

zenithmeridian

PM or postmeridian

AM or antemeridian

LUNCH!!

Where is the Sun at noon today?

east westsouth

zenith

here orthere?

What about the path of the Sun?

east westsouth

zenith

there

What abouttomorrow atnoon?

JUST ALITTLELOWER !!

Where is the Sun throughout the year?

east westsouth

zenith

Dec 21WinterSolstice

June 21SummerSolstice

Mar 21(Vernal)

andSept 21

(Autumnal)Equinox

Position of the Sun when photographedduring the day of the winter and summer

solstice