The Changing View from Earth

Chapter 13

Cartoon of the Day

What our Ancestors Saw

For thousands of years the sky has been a source of informationTimeDateWeather

Farmers took their cues from the celestial bodies (Sun, moon, stars) to know when to plant, and harvest their crops

Sailors used stars as a guidance system to cross oceans

Astrologers were in demandPeople believed their destinies could be

foretold by the stars. Ancients used the stars and motion of

planets to Predict planetary motion, seasons, and

eclipses

Stories from Cultures• Hindu mythology

– Seven wise men married seven sisters. Six of the women divorced their husbands and moved to another location in sky. They became the Pleiades (distinct star pattern)

Asterism – a distinctive star pattern– The seven husbands became

the seven stars of the big dipper. The wife became Alcor. A star in the crook of the big dipper.

Stories from Cultures

• Algonquin, Iroquois, and Narragansett – saw the constellation Ursa Major as a bear

running away from hunters– In the fall because the bear is low enough to

brush the trees … the blood from its wounds turns the leaves red.

• Another legend tells of 3 hunters chasing 4 elk as the 7 stars of the big dipper.– One hunter is accompanied by a dog

Celestial motion (Lunar)

The moon traces a westward path across the sky.Each night it rises in the

east an hour later than the previous night

It’s shape appears to change in phases, waxing from thin crescent to half and full moon. Then waning to a sliver again.

http://www.astro.wisc.edu/~dolan/java/MoonPhase.html

Solar MotionThe Sun has no phases

It also rises earlier and farther north each day from December 22 to June 22 and sets later

Through the summer and fall it rises later and set earlier.

– http://www.earth.uni.edu/~morgan/ajjar/SolarMotion/solarzenith.html

Stellar Motion

Stars and Planets also follow the same pattern

They rise 4 minutesEarlier each night

Planetary Motion And Retrograde

Greeks noticed 5 objects wandering through the starsCalled planetsMercury, Venus, Mars, Jupiter, Saturn

Venus and Mercury seemed to stay close to the sun

Mars, Jupiter, Saturn wandered Eastward Once a year they appeared to go backwards (westward)

Retrograde motion

Retrograde motion is the backwards motion that planets appear to trace across the sky.

Modelling Celestial Motion

• Noticing the patterns is only half the battle– Come up with basic ideas or theories– Check theories using models

Two early modelsGeocentric

Earth centered modelHeliocentric

Sun centered model ( the one accepted today)

Geocentric

AristotlePlaced stars on outer

circle (firmament or fixed stars)Also called the celestial

sphereInside the sphere he

arranged more concentric spheres on which he placed the Sun, Moons and planets

Explaining Epicycles

Aristotles model didn’t explain epicyclesPtolemy created a model which included

an additional level of circles called EpicyclesGood for predicting astronomical events

Heliocentric1500’s Nicholas Copernicus

Proposed a new modelFixed sunPlanets (including Earth rotate around the sun)Arranged the planets orbits in a solar plane

Imaginary disk extending out the suns Equator

Galileo

Italian Astronomer Discovered evidence supporting heliocentric

model. Used a telescope

Saw that Venus had phases like the moon did Spots on surface of sun Mountains on the moon Rings around saturn Four moons orbiting Jupiter (actually has 16)

Galileo• Published his ideas in Dialogue

– Galileo was required to recant his heliocentric ideas, which were condemned as "formally heretical";.

– He was ordered imprisoned; the sentence was later commuted to house arrest.

– His offending Dialogue was banned; and in an action not announced at the trial, publication of any of his works was forbidden, including any he might write in the future.

The solution

The answers were discovered by a German mathematician Johannes Kepler According to calculations predictions

would be more accurate if planetary orbits were ellipses (rather than circles)

Ideas strengthened by Newton’s Gravitational laws – All objects in the universe are attracted to

one another

New Planets

In 1781 Uranus was discovered Later using sun-centered model and

Newton’s laws they predicted where another planet should be Pointed their telescopes in it’s direction

and discovered Neptune. Now knew of 8 planets in total

(missing Pluto)

Today’s Views

• Scientist constructed geocentric model based on observations with unaided eye

• When Ideas were challenged a new theory was formulated

• This process of formulating new ideas continues even today.

• Thanks to technology we now have information on solar system, sun, 9 planets and their moons, meteors, asteroids, comets

The Sun Made of Hydrogen gas• Diameter 1.4 million km (110 times earth)• Surface is constantly writhing/churning Solar prominance

Streamers of hot gas that arch into space. Cooler regions appear darker in color

(Sunspots) Near them violent outbursts/eruptions occur –

Solar flares Solar flares send high energy subatomic

particles into space. (creates solar wind which can affect Earths activities)

The Sun 330,000 times more massive than earth

Hydrogen and helium Three layers

Core – 15,000,000 ۫ C Photosphere – region of suns light

6,000 ۫ C Corona

1,000,000 ۫ C Closest star to the Earth

Because Sun has planets orbiting it scientist predict other stars might have planets orbiting

The Planets

Inner planets Mercury, Venus, Mars Called terrestrial planets

Because of their rocky composition

Outer planets Jupiter, Saturn, Uranus, Neptune Have a gaseous composition

Pluto Class all by itself because of strange orbit and

tiny size

Scale

Earth diameter = 12,750 km (1 Earth-Diameter)

• Venus = 12,100 km (0.95 Earth-diameter)

• Jupiter = 11.2 Earth-Diameter– 143,200 km / 12,750 km = 11.2

Scale

• Other scales measured– Mass– Density– Rotational period (time around sun)– Orbital period

Measuring Distance

Distances in astronomy are so immense they are “astonomical” Scale used = astronomical units (AU) 1 AU = average distance from Earth to sun

(149,599,000 km)– Mars distance = 1.5 AU

• Why is AU expressed as an average distance?

Other Solar System Bodies Asteroids

Known as minor planets (1m – 100’s of km)– Cere’s 1000 km Irregular shaped bodies of rock (silicate) Some cross path of earth (potential Collision)

Comets Made of dust and ice Orbit at large distances Some fall towards the sun (evaporate) form tails

Meteors, and Meteorites Dust and rock particles that heat up an vaporize in

earths atmosphere (shooting stars) - Meteors Some remain large enough to hit the earth -

Meteorites