Astro 201: Aug. 26, 2010• Turn in HW #1 in front of room• Pick up Telescope Lab Handout• Reading: – Hester et al: Chapter 2– Gleiser: Chapters 1-6

• Wait List students have been added; class limit will be increased, so everyone can be enrolled

• Today:– Discuss Telescope Lab, On-Line quiz– History of cosmology and the scientific method

Which of the following is the largest?

a. the diameter of the Moon

b. the diameter of Earth

c. the diameter of the Milky Way

d. the diameter of the Sun

e. 1 astronomical unit

On-line Quiz, go to d2l:

Open book, 10 questions, available after class today, do it before class next Thursday

Cosmology: Some History

Lascaux Cave Paintings: Cro-Magnons recorded the lunar cycle

15,000 years ago

http://news.bbc.co.uk/2/hi/science/nature/975360.stm

PATTERNS IN THE SKY• Ancient peoples observed the Sun and stars

and created calendars that marked the yearly passage of time

• Earth rotates on its axis once every 24 hours• Earth revolves around the Sun once every 365

days• The stars appear to rise in the east and set in

the west every night, on a big dome called the celestial sphere.

The Earth's rotation is counter clockwise looking down on the N pole, and results in the apparent motion of the celestial sphere -- everything rises in the East and sets in the west.

1 solar daysolar day = time from one noon to the next

= 24 hours

Circumpolar Stars

Always aboveHorizon

In the NorthernHemisphere, theyAppear to rotate around The North Star,Polaris

In the Southern Hemisphere, No bright star at rotation center

Celestial Sphere and Constellations

The sky is divided into 88 "official" constellations, although in reality the stars in any particular constellation are at different distances from Earth.

Figure 2.3: The celestial sphere as a useful fiction

Figure 2.4: As viewed from the Earth’s North Pole

Figure 2.5: Our perspective on the sky depends on our location

Figure 2.9, Right: Celestial sphere at different latitudes

Figure 2.10: Earth’s orbit and constellations along the ecliptic

The Zodiac

Rise and set of the Sun and Starshas 2 possible explanations

• GEOCENTRIC Earth at the center Sun and Stars go around the Earth once per

day

• HELIOCENTRIC Earth rotates on its axis and so the Sun and stars appear to go around the Earth

Ancient Egyptian celestial sphere:the goddess Nut (“noot”) c. 3000 BC

Nut gave birth to the Sun god “RA” every morning

Hindu Rig Veda (1500 BC)• Written cosmology (Sanskrit)• Cyclical or recursive

universe• Universe originates from the Hiranyagarbha, or golden

egg• Timescales eerily coincident

with modern results, e.g. age of the Earth

Hebrew CosmologyAnd God said, “Let there

be a vault in the midst of the waters, and let it divide water from water.” And God made the vault and it divided the water beneath the vault from the water above the vault, and so it was. - Genesis 1:6

Michaelangelo’s depiction of the creationOf the planets and stars

God had divided the waters "above" from the waters "below" by constructing an immense dome that held open the space for dry land.

In the Hebrew Bible the dome is called "raqi'a," meaning a firm substance, and rendered in the King James translation as "the firmament”

The firmament in Biblical times was understood to be firm only by the will of God.

If God were angered, "the windows of heaven" and "the fountains of the deep” could burst open once again and those lovely blue waters would destroy the Earth.

A painting by the American Edward Hicks (1780–1849), showing the animals boarding Noah's Ark two by two.

Aristotle • 4th century BC, Greece• First to give reasons why the Earth is spherical

Aristotle’s 1st reason:

Gravity pulls matter to center of Earth, compressing the Earth into as compact a shape as possible.

…a sphere

Thanks to Barbara Ryden for this discussion

2nd reason:

You see different stars from the

south than from the north.

Big Dipper

Southern Cross

If the Earth were flat:

Big Dipper Southern Cross

During a lunar eclipse, Earth’s shadow is alwaysalways circular.

Only object whose shadow is always circular is a spheresphere.

3rd reason:

The shape of the Earth’s shadow.

Another Greek: Aristarchus of Samos

3rd century BC

Proposed that the Earth rotates on its axis & goes around the Sun.

Rejected by contemporaries

He used geometry to estimate the size of the Sun and Moon, and their relative distances

What was his reasoning?

• The Sun is farther away than the Moon because the Moon comes between the Sun and the Earth during a solar eclipse.

ANIMATION

• The phases of the Moon result from the Moon orbiting the Earth

ANOTHER ANIMATION

• Aristarchus measured the angle between the Moon and the Sun when the Moon was half lit, and deduced the ratio of the Sun-Earth distance to the Moon-Earth distance.

E

M S

Here’s a picture more to scale

3°

Earth – Sun distance (E-S) is 19 times19 times Earth – Moon distance (E-M). (actual value 400x)

90°

87°

Thus, Aristarchus correctly discovered that the Sun is much farther away from the Earth than the Moon.

He then concluded that the Sun was MUCH bigger than the Earth or Moon.He reasoned that it was more plausible that the smaller body would orbit the bigger body: thus the Earth orbits the Sun

Despite Aristarchus, for 2000 years, a GEOCENTRIC model of the Universe was favored

EARTH at the center

STARS affixed to the celestial sphere

Moon, Planets and the Sun are between the EARTH and STARS

PTOLEMY2nd century AD

Ptolemy’s ALMAGEST was translated into Latin in 1496

GEOCENTRIC: Earth at center

Copernicus (1473-1543)Heliocentric Model

• In order to understand the motion of the planets in the sky Ptolemy’s model had to be modified

• In terms of the heliocentric model, the planets orbit the Sun in ellipses, not circles

• Retrograde motion of the planets with respect to the stars required EPICYCLES

Ptolemy’s Epicycles

Eclipticthe fact that the planets orbit the

Sun in a plane means that they always appear to

lie on a great circle on the sky, called the ecliptic

Retrograde motionThe planets sometimes appear to be traveling west

to east, unlike the stars and Sun which always go east to west

Next slide: animationAstronomy Picture of the Day for Dec. 20, 2001. Jupiter and Saturn showing retrograde motion.Combining 23 pictures taken at 2 week intervals

from June 2000 - May 2001.

Planet = “wanderer”

Ptolemy’s explanation for retrograde motion of the planets

Earth Equant

Epicycle

Deferent

The planet moves along its epicycle as the epicycle moves along the deferent around the Earth. To make the observations as accurate as possible, it was necessary to place the Earth slightly off center of the orbits, but to preserve symmetry that meant that there was an equal place (“Equant”) opposite the Earth from the center. The combined motion of the planet and the resulting retrograde motion are shown.

Earth Off Center

Copernicus’ explanation for retrograde motion

Copernicus:

“On the Revolution of the Heavenly Spheres”

Published at the time of his death, in 1543

Major Conclusions:1.The planets orbit the Sun2. The apparent daily motion of the Sun and stars is the result of the Earth’s rotation 3.The stars are much farther away than the Sun

Heliocentric model: distance from Sun to stars must be much greater than distance from Sun to Earth.

Since Earth orbits Sun, stars should show parallax (a shift in apparent position) over the course of half a year.

Implication: distance to stars is several thousand times Earth – Sun distance.

OBSERVATION: Parallax of stars is TOO SMALL to be seen by the naked eye.

Not observed until 1800s.

Parallax

Unit of distance: the PARSEC A star which is one parsec from Earth shows a parallax of one arcsecond

€

d =1

p

Remember: 360 degrees in a circle, 60 arcminutes per degree, 60 arcseconds per arcminute

Reactions to Copernicus:

On March 5, 1616, Copernicus' work was banned from being taught and discussed by the Congregation of the Index "until corrected."

It stayed on this list of prohibited books and teachings until 1822.

Martin Luther (1483-1546): [Copernicus] “is a fool who wishes to reverse the entire scheme of astronomy; but sacred scripture tells us that Joshua commanded the Earth to stand still, not the Sun.”Giordano Bruno (1548-1600): burned at the stake for advocating that stars are suns in their own right, and that there is a plurality of worlds like the Earth.

v. 1.0

v. 2.0

v. 3.0

Cosmological Models:

Version 1.0: “Superdome” model

Version 2.0: Geocentric model

Version 3.0: Heliocentric model

Which is right?

Occam's Razor: Pluralitas non est ponenda sine neccesitate

"plurality should not be posited without necessity."

Given a set of otherwise equivalent models of a phenomenon, the simplest one is the best.

Keep it simple, stupid.

English philosopher from OckhamFranciscan Monk

Got into trouble with the Pope for advocating “apostolic poverty”

Died of the Black Death while in exile

Advocated

Epistomological Parsimony

c.f. Ontological parsimony

Epistomology = theory of knowledgeOntology = what exists?

William of Occam 1285-1349

1610 “Siderius Nuncius” (The Starry Messenger)

• “Spots” on the Sun; the Sun rotates

• The Moon has mountains, craters, rocky surface with imperfections

• The “planet” Jupiter is not a pinpoint star – but a disc in the sky WITH MOONS that orbit it

• Venus has “PHASES” like the MOON

Galileo Galilei (1564 – 1642)

ItalianFirst to use a telescope to look at the sky

Spots on the SunThe Sun rotates

Eventually went blind

Galileo Galilei (1564 – 1642)

The Moon Has Mountains and Valleys

Galileo Galilei (1564 – 1642)

Phases of Venus

Galileo observed that Venus showed phases entirely like those of the moon from full to crescent, which it must do if the Copernican theory was correct.

According to the Ptolemaic theory Venus would have to be a perpetual crescent.

Galileo Galilei (1564 – 1642)

Observations consistent with Copernicus, inconsistent with Ptolemy.

Jupiter Has Moons

Galileo Galilei (1564 – 1642)

Galilean Moons – 4 Largest “moons” of JupiterIo, Europa, Callisto, & Ganymede

1632: "Dialogo Dei Massimi Sistemi” Dialogue on the Two Chief World Systems

In Italian, Not Latin -- For the common people

Two people, one representing the view of Ptolemy and other the view of Copernicus, present their arguments before an intelligent layman.

Galileo Galilei (1564 – 1642)

"Eppur Si Muove“"(And, yet it moves!")

The Pope Urban II thought that Simplico, the character who upheld the views of Ptolemy in the book, was a deliberate and insulting caricature of himself

1633: Trial by Church (threatened with instruments of torture), forced to recant views

1633: Galileo is ordered to his house in Arcetri, under house arrest, where he remains for the rest of his life.

The book remained on the index of forbidden books until 1822

Pope John Paul II finally gave an address that admitted that there had been errors in the treatment of Galileo by the church, in 1992.

ASIDE: Contrast Galileo with Nicolas Steno (1638-1686), who laid the groundwork for the interpretation of the fossil record. See The Seashell on the Mountaintop by Alan Cutler.

Modern Scientific Method

1. Observations and Experiments Data which describes the physical world

2. Theory, Hypotheses, Models: -> Organize Facts from experiment & observations Unifying principles Make testable predictions

Creationism and Intelligent Design vs. Evolution.

The example of gravity as a scientific theory: Newton’s Law c. 1700 AD

Einstein’s theory of General Relativity c. 1916

Photon versus wave description of light

The most interesting scientific questions to work on for a professional scientist are those we don't know the answer to.

Interesting theories to work on are those which fail to explain an observation or fact.

Many modern astronomers are deeply religious people.

The Astronomical Basis for Calendars

The Earth rotates on its axis once a day.The Earth orbits the Sun with a period of about 365.242190 daysThe Moon orbits the Earth, such that the period between new moon and new moon is 29.5305 days

These periods vary a little bit because of the gravitational forces exerted by the other planets and other factors.

Every 4 years, we take into account the .242190 days by added a leap day, February 29.

Otherwise we would keep slipping, and eventually January would be summer in the northern hemisphere, etc.

Now, the thing is that there are NOT an integral number of months (29.5305 days) in one 365.242190 year.

So if for example today is a new moon, next year it will not be.

The Gregorian Calendar (Christian, one we use)

is based on the motion of the Earth around the Sun.

The length and number of months have no connection to the motion of the Moon

It is based on the Julian Calendar, which was introduced by Julius Caesar in 45 BC.

He made Jan. 1 the start of the Year.

During the middle ages, different groups in Europe adopted different start dates for the new years.

In 1582, Pope Gregory reformed the calendar, which is basically what we use today.

The French recognized a different pope for a while and had a different calendar.

The Islamic Calendar

is based on the motion of the Moon, with no connection to the motion of the Earth around the Sun.

Hijri Calendar, Based on the Qur'an.

Used in many countries around the Gulf, e.g. Saudi Arabia.

Purely lunar: 12 months, each with 29.53 days.

Therefore the Islamic year has 12x29.53 = 354.36 days

The Islamic year is shorter than the time it takes the Earth to go around the Sun.

Thus, a particular Islamic month falls during different times of the year , e.g. Ramadan.

Years are counted since the Hijra, which is is the time that Mohammed emigrated to Medina in AD 622.

So AD 2009 is Islamic year 1430.

The Jewish and Chinese Calendars

combine both, so years are linked to the period of the orbit of the Earth around the Sun, AND months are linked to the motion of the Moon around the Earth.

19 x365.24 = an integral number of 29.53 day months.

Jewish Year: A "ordinary" year has 355 days, and 12 months. A "leap" year has 385 days and 13 months.

The length of a particular month varies from year to year by a day, so that if New Year's Day (Rosh HaShannah) is, say, a new moon, then the first day of every month is a new moon.

Years are counted since the creation of the world, taken to be 3761 BC. Thus 1998 is Jewish year 5759.