EQUI STICE SOL NOX. EQUINOX SOLSTICE The Cosmic LandscapeLesson 2: Prehistoric Astronomy3 UICK RITE: You’re a caveman or cavewoman, 500 years B.C. Your

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The Cosmic Landscape Lesson 2: Prehistoric Astronomy 3

UICK RITE:

You’re a caveman or cavewoman, 500 years B.C.Your little caveboy comes to you and asks,

• “Dad (or Mom), why is the sky blue?” Since you have no earthly idea, you ask him

if he has any simpler questions. He comes up with this one:

Mom (or Dad), please explain the east-to-west motion of celestial objects across the sky?”

You have 3 minutes to come up with your best answer


Chapter overview

– Prehistoric astronomy (before 500 B.C.)• What are the cyclic motions that early people saw and

used?• How can we explain these cyclic motions today?• How did early people use the Sun, Moon, and stars to

keep time?


Motivation• By observing the sky early

people could– Keep track of time and the

seasons.

– Know when to plant crops.

• Today’s knowledge helps us explain what they could only wonder about.


Lesson overview• What observations did prehistoric people make

about the sky?• How can we explain the east-to-west motion of

celestial objects across the sky?• How can we explain seasonal changes in

temperature and length of the day?• What are solstices and equinoxes?• Why do constellations in the night sky change

with the seasons?


Introduction

• Our ancestors started studying the sky thousands of years ago.

• They saw– Rising of the Sun, Moon, and stars in the eastern sky and their

setting in the west.

– Seasonal changes in the Sun’s path and star patterns or constellations.

– The Moon’s changing appearance.

– Objects in the sky that didn’t move like the stars.


Introduction (cont’d)

• People learned a lot from these day-to-day and year-to-year patterns and cycles.

• Large structures helped them observe and predict events.


Introduction (cont’d)We can explain what these early people saw.

Stonehenge, a stone monument the ancient Britons built on Salisbury Plain, England. Its orientation marks the Sun’s seasonal rising and setting points. (Courtesy Tony Stone/Rob Talbot.)

A Mayan building oriented to view the summer sunset. (Courtesy of Werner Forman/Art Resource)


How can we explain why celestial bodies “move” from east to west?

• Sun, Moon, planets, and stars seeming to rise in the east, move across the sky, and set in the west

• From Earth’s rotating (spinning) on its axis


How can we explain seasonal changes?

• Seasonal changes on Earth show up in the– temperature– Sun’s path– amount of daylight– star pattern at night


How can we explain seasonal changes? (cont’d)

• Seasonal changes result from two things:– The Earth’s revolving (circling) around the

Sun

– The Earth’s axis of rotation tilting from the plane of its orbit around the Sun.






Solstices and equinoxes

• The solstices and equinoxes are four days important for keeping time.


Solstices and equinoxes (cont’d)

• The solstices: two days when the Sun takes its longest and shortest path across the sky. In the northern hemisphere,– Longest path is on the summer solstice—the day with the longest

daylight.

– Shortest path is on the winter solstice—the day with the shortest daylight.

sol "sun“

+stem of sistere "to come to a stop, make stand still"


Solstices and equinoxes (cont’d)

• The equinoxes—two days when daylight and night are equal:

– Spring equinox

– Fall equinox

"equal" + nox (gen. noctis) "night."

Demo of why the Seasons change

http://www.ioncmaste.ca/homepage/resources/web_resources/CSA_Astro/files/content/multimedia/unit3/reasons_seasons/reasons_seasons.swf


What is the zodiac?• A narrow band that circles the heavens

• Planets move against this background (as seen from Earth)

• Zodiac means “animal sign”; its circle of “animals” includes the constellations.


What are constellations?

– Fixed pattern of stars named for the animal, object, or mythological figure it looks like

– Used by people to track the seasons and navigate on land and sea

– Stars not physically related to one another

– People still seeing constellations prehistoric people saw


Constellations– Constellations Leo (A and B) and Cygnus (C and D)—figures

sketched in to show animals they represent

Photo A and B from Roger Ressmeyer, digitally enhanced by Jon Alpert. Photo C and D, courtesy Eugene Lauria.)


How can we explain seasonal changes in constellations?

• Earth moves around the Sun

• New stars become visible• Ones you saw before are

hidden

Test your understandinghttp://www.nationalgeographic.com/xpeditions/activities/07/popup/cosmic.html

http://www.nationalgeographic.com/xpeditions/activities/07/popup/cosmic.html




Lesson review

• Prehistoric people and observations of sky

• Earth’s rotation and “movement” of bodies across the sky

• Earth’s tilt and seasonal changes in temperature and amount of daylight

• Solstices—two days with the most and least daylight

• Equinoxes—two days with equal day and night

• Constellations—change with the seasons


Activity—Questions for review• Why is it colder in the northern hemisphere in winter

than in summer?

• What season is it in the northern hemisphere when it’s winter in the southern hemisphere? Why?

• How would it affect the seasons if the Earth’s axis of rotation didn’t tilt?

• How is the Earth tilted with respect to the Sun when the equinoxes occur?

• A friend says she can’t see the constellation Aries at night. Can you explain why?


Activity—Test yourself

• Why do objects in the sky appear to rise in the east and set in the west?

• What two things account for changes in the seasons?

• What do we call the longest day of the year (in the northern hemisphere)?


Summary• What observations did prehistoric people make about the

sky?

• How can we explain the east-to-west motion of celestial objects across the sky?

• How can we explain seasonal changes in temperature and length of the day?

• What are solstices and equinoxes?

• Why do constellations in the night sky change with the seasons?


Next• More detail on the Moon’s daily and seasonal

changes

• Moon’s changing phases—most easily observed events in astronomy

History of Astronomy

Arny, 3rd Edition, Chapter 1

Documents

EQUI STICE SOL NOX. EQUINOX SOLSTICE The Cosmic LandscapeLesson 2: Prehistoric Astronomy3 UICK RITE: You’re a caveman or cavewoman, 500 years B.C. Your