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Lecture #2: The Sky & Some History

4General Introduction to the Sky

4Coordinates: Getting your bearings

4Earth's spin and orbital motion

4Constellations & Astrology

4Classical astronomy

4Modern astronomy

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The Main Point

The movements of the Sun, Moon,planets, and stars that we observe

every day and night are apparent, notactual motions, because we are

moving as well!

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General Introduction to the Sky

4Watching and knowing the sky is the key tounderstanding our place in the Cosmos

4For most of human history, knowing the skywas critical for survival

4Today, hardly anyone ever looks up...

4The sky moves! (or does it...)

4Actual vs. Apparent motion

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Coordinates: Getting your bearings

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Northern Sky Motion Animation

Simulation using "Sky Gazer"planetarium software

(you can do this too!)

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Key concepts:

• Zenith• Horizon• Celestial Sphere• Celestial Pole• Rotation Axis• Celestial Equator• Latitude

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People living in different places on the Earth see differentmotions of the night sky...

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Earth's Spin and Orbital Motion

4Earth spins once per day on its axis

4Earth orbits around the Sun once per year– Not obvious! The Sun appears to orbit us!

4Earth's orbital plane is called the ecliptic

4Earth's axis is tilted relative to its orbit– The tilt angle (obliquity) is about 23°

– The tilt is the reason for the seasons! (Lec. 4)

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Key Concepts:

• Earth's axis is tilted• Tilt angle changes the angle of sunlight striking the Earth's surface• At a fixed location on the Earth, the angle of the sunlight varies with time• Seasons!

• Other planets have different tilts, and thus different types of seasons

More details in Lecture 4…

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Constellations & The Zodiac

4 Constellations are patterns of stars in the sky, usedby ancient peoples for navigation and folklore

4 They provide a "roadmap" to the sky, but no realscientific information (the stars happen to appearadjacent in the sky, but most are far apart in space)

4 During one year, the Sun appears to travel througha group of 12 constellations along the ecliptic: thiszone is called the zodiac

4 The planets also move through the same zone

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Lookingeast onon earlywinterevening...

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OrionThe Hunter

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The Sun's apparent motion through the zodiac

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Classical Astronomy

4 Many ancient civilizations had sophisticatedconcepts of astronomy and cosmology– Babylonians, Assyrians, Egyptians, Chinese, Mayans,

Polynesians, Druids, ...

– Calendars, navigation, crop & river cycles, comets,supernovae, Venus, ...

4 Much of modern astronomy derives from theGreeks and Romans (~2000-2500 years ago)

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What did the ancients know?

4 The Earth is round– Pythagoras ("perfect spheres")

– Aristotle (eclipse shadows)

4 Earth's diameter ~12,000 km– Eratosthenes (~ 200 B.C)

– Sticks and Shadows!

4 Earth's axis precesses– Hipparchus' data (~ 150 B.C.)

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What did they think they knew?

4 Earth is the center of the Universe– Simplest explanation for apparent motions!

– "Warm and fuzzy" result

– No stellar parallax observed

4 Sun, Moon, & Planets orbit Earth– Ptolemy (~ 140 A.D.)

– Complex circular paths

– Model could predict positions!

4 Ptolemaic (geocentric) cosmology would be thedominant paradigm for 1000 years!

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Astrology

4 Ancient system of beliefs that claimed that the positions ofthe Sun, Moon, and planets governed the actions andevents in our lives

4 Astrology's ability to predict actions or personality traitshas been repeatedly tested--and it fails.

4 Yet astrology and horoscopes continue to pervade societyand the media. Why?– Therapeutic power of guidance and advice?

– Connection to a simpler, less technological time?

4 Astrology is entertainment, not science

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Group DiscussionAlthough astronomical mythology no longer holds a powerfulsway over the modern imagination, we still find proof of the

power of astronomical images in our culture.

Break up into groups of 2-4 and with one person acting as thedocumentarian, list as many products, songs, movies, etc. that

have astronomical names and/or themes.

Write the names and section numbers of the members of yourgroup and turn in your lists at the end of this exercise.

5 minutes

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"Modern" Astronomy Pioneers

4 Nicolaus Copernicus (1473-1543)– Proposed a heliocentric cosmology

– Earth and planets circle the Sun

– Only the Moon circles the Earth

4 A simple model; it fit the data

4 But a controversial model– Difficult to test at the time

– Any experiments were widely mistrusted anyway

– Inconsistent with contemporary philosophy, theology

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"Modern" Astronomy Pioneers

4 Galileo Galilei (1564-1642)–Used math to describe acceleration

–First astronomer to use a telescope!

–Discovered moons of Jupiter (1609)

–Explored features on the Moon

–Discovered phases of Venus

4 Provided data and tests needed to validate theCopernican heliocentric model of the solar system

4 De-throning of the Earth still met fierce resistance

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Summary

4 Actual vs. Apparent motion– Earth spins on its axis

– Earth's axis is tilted: Seasons!

– Motion of the sky depends on latitude

– Earth, planets orbit around the Sun

4 The need to know the sky was the foundation of astronomy

4 Greek, Roman astronomers provided the first pieces

4 Astrology is the historical relic of ancient astronomy– Astrology ≠ Astronomy !!!

4 Copernicus and Galileo started modern astronomy

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Next Lecture...

4Orbits and Gravity

4Kepler and Newton

4Reading:– Chapter 5