I. Our modern View of the UniverseSurvey of the universe and powers of ten

II. The scale of the UniverseAstronomical distances

III. Spaceship EarthMotions of Earth, Sun, Galaxies

Outline of lecture 1 (Ch 1)

Review today at 4:30 in HEC 119by Tony and Zoe

DISTANCE SCALES

• ASTRONOMICAL UNIT (AU):– The average distance between the Earth and the

Sun.• Units used in our solar system

1 AUSun.

Earth

A TOUR OF THE SOLAR SYSTEM

NOT TO SCALE!!!!

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.4 .7 1.0 1.5 5 10 20 30 40Distance

(AU)

• Let’s view it to scale– say the Sun is the size of a large grapefruit

(13.9 cm) in UCF

– then the nearest star would be in California

SCALE OF THE SOLAR SYSTEM

DISTANCE SCALES

• LIGHT-YEAR (LY):– The distance light can travel in one year.– NOTE: this is a distance not a time!

“Celestial Bodies”• Star - A large, glowing ball of gas that generates heat and

light through nuclear fusion• Planets - A moderately large object which orbits a star; it

shines by reflected light. Planets may be rocky, icy, or gaseous in composition.

• Moons - an object which orbits a planet• Asteroids - A relatively small and rocky object which orbits a

star.• Comets -A relatively small and icy object which orbits a star.• Nebula - An interstellar cloud of gas and/or dust• Galaxy - A great island of stars in space, all held together by

gravity and orbiting a common center

A star and all the material which orbits it, including its planets and moons

SOLAR (STAR) SYSTEM

The sum total of all matter and energy; that is, everything within and between

all galaxies

THE UNIVERSE

Age of Universe: about 14 billion years

Age of Solar System: about 4.6 billion years

AGE OF UNIVERSE

Where do we come from?• The first (and simplest) atoms were created during

the Big Bang.• More complex atoms were created in stars.• When the star dies, they are expelled into space….

to form new stars and planets!

Most of the atoms in our bodies were created in the core of a star!

SPEED OF LIGHT

• The speed of light in the vacuum of space is constant! All light travels the same speed!c = speed of light

= 300,000 km/sec

= 3 x 106 km/sec

(no need to memorize)

Looking back in time• Light, although fast, travels at a finite speed.• It takes:

– 8 minutes to reach us from the Sun– 4.3 years to reach us from our nearest star, Alpha Centauri – 1,500 years to reach us from the Orion Nebula– Two million years to reach us from the Andromeda galaxy

• The farther out we look into the Universe, the farther back in time we see!

1.1 A Modern View of the Universe

• What is our physical place in the Universe?We are on a planet, orbiting a star, in a galaxy

• Describe our cosmic origins and why we say that we are “star stuff.”The universe started with an explosion called the “Big Bang” at

that time hydrogen and helium were created, all the other elements were “cooked” (created) inside stars

• Why does looking into space mean looking back in time?Because of the time it takes for light to travel from large

distances back toward us on Earth. For example, the Andromeda galaxy is 2 million light years away

Our goals for learning:

EXPANSION (of the Universe)

• We say that the universe is expanding because the average distance between galaxies is increasing with time.

– NOTE: individual galaxies and star systems are not expanding within themselves!

• Mostly all galaxies appear to be moving away from us.

• The farther away they are, the faster they are moving.– Just like raisins in a raisin

cake; they all move apart from each other as the dough (space itself) expands.

EXPANSION

1. Patterns in The Sky: Stars and constellations

Celestial coordinates: Celestial sphere, poles, equator, ecliptic, right ascension, declination

2. Seasons: Tilt in Earth’s axis (23.5 degrees) Equinoxes and solstices, precession

3. The Moon and Eclipses Lunar and Solar Eclipses

4. Ancient Mystery of the Planets: Apparent Retrograde motion of planets

Outline of Ch 2

• What is a constellation?• A constellation is a region of the sky. The sky is

divided into 88 official constellations.

Constellation: Orion

• What is the celestial sphere?• An imaginary sphere surrounding the Earth upon

which the stars, Sun, Moon, and planets appear to reside.

The Celestial Sphere

North & South celestial polesthe points in the sky directly above the Earth’s North and South poles

celestial equatorthe extension of the Earth’s equator onto the celestial sphere

eclipticthe annual path of the Sun through the celestial sphere, which is a projection of ecliptic plane

Measuring the Sky

We measure the sky in angles, not distances.

• Full circle = 360º

• 1º = 60 arcmin

• 1 arcmin = 60 arcsec

Measuring Angles in the Sky

Moon = 0.5°

Sun = 0.5°

The Local Sky

zeniththe point directly above you

horizonall points 90° from the zenith

altitudethe angle above the horizon

meridiandue north horizon zenith due south horizon

Coordinates on the Celestial Sphere (not in book)

• Latitude: position north or south of equator

• Longitude: position east or west of prime meridian (runs through Greenwich, England)

• Declination: position north or south of celestial equator (in degrees)

• Right Ascension: distance (in hours, 0 to 23h 59 min.) East of vernal equinox

(vernal equinox: where the sun crosses the celestial equator going North)

The Daily Motion

• As the Earth rotates, the sky appears to us to rotate in the opposite direction.

• The sky appears to rotate around the N (or S) celestial poles.

• If you are standing at the poles, nothing rises or sets.

• If you are standing at the equator, everything rises & sets 90 to the horizon.

Annual Motion• As the Earth orbits the Sun, the Sun appears to move

eastward with respect to the stars.• The Sun circles the celestial sphere once every year.

Annual Motion• The Earth’s axis is tilted 23.5° from being

perpendicular to the ecliptic plane.

• Therefore, the celestial equator is tilted 23.5° to the ecliptic.

• As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator.

• Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!

Annual Motion

eclipticthe apparent path of the Sun through the sky

equinoxwhere the ecliptic intersects the celestial equator

solsticewhere the ecliptic is farthest from the celestial equator

Annual Motion

Summer Solstice (~June 21)When Sun is farthest North (23.5 degrees) from celestial equator

When Sun crosses celestial equator going South Autumnal Equinox (~September 21)

Vernal Equinox (~March 21)When Sun crosses celestial equator going North

Winter Solstice (~Dec. 21)When Sun is farthest South (23.5 degrees) from celestial equator

2.4 Precession

• What is the Earth’s cycle of precession?

Precession of the Earth’s Axis• The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years.• Precession changes the positions in the sky of the celestial poles and the equinoxes. Polaris won't always be the north star. However the tilt in the axis is the same (23.5 degrees) as the Earth’s axis precesses

2.5 The Moon, Our Constant Companion

• Why do we see phases of the Moon?

• What conditions are necessary for an eclipse?

Lunar Motion

Phases of the Moon’s 29.5 day cycle• new

• crescent

• first quarter

• gibbous

• full

• gibbous

• last quarter

• crescent

waxing

waning

Eclipses

• The Earth & Moon cast shadows.

• When either passes through the other’s shadow, we have an eclipse.

• Why don’t we have an eclipse every full & new Moon?

EclipsesWhen the Moon’s orbit intersects the ecliptic (node):

at new moon solar eclipse

at full moon

• everyone on the nighttime side of Earth can see it

lunar eclipse

you must be in Moon’s shadow to see it

Solar Eclipse

Lunar Eclipse

What have we learned?

• Why do we see phases of the Moon?• At any time, half the Moon is illuminated by the Sun

and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon.

• What conditions are necessary for an eclipse?• An eclipse can occur only when the nodes of the

Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon.

2.6 The Ancient Mystery of the Planets

• Why do planets sometimes seem to move backwards relative to the stars?

Retrograde Motion

• Planets usually appear to move eastward relative to the stars.

• But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months.

Noticeable over many nights

Explaining Apparent Retrograde Motion

• Easy for us to explain: occurs when we “lap” another planet (or when Mercury or Venus lap us)

• But very difficult to explain if you think that Earth is the center of the universe!

Why did the ancient Greeks reject the notion that the Earth orbits the

sun?

• Mainly because ancient Greeks knew that we should see stellar parallax if we orbited the Sun – but they could not detect it.

Parallax Angle

Apparent shift of a star’s position due to the Earth’s orbiting of the Sun

The nearest stars are much farther away than the Greeks thought.

So the parallax angles of the star are so small, that you need a telescope to observe them.

Possible reasons why stellar parallax was undetectable:

1. Stars are so far away that stellar parallax is too small for naked eye to notice

2. Earth does not orbit Sun; it is the center of the universe

Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)…

Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.

Ch 3 (Histrory of Astronomy) “Does the World Turn?”

1. Archeoastronomy2. Astronomy of Greece: good records & more

rational approacha. Thales and Pithagoras: roots of science

Thales: Universe is rational Pithagoras: nature is governed by musical (mathematical)

principles

b. Plato and Aristotle: Geocentric universe Geocentric universe Earth is imperfect Heavens are perfect “uniform circular motion” is perfect. Hence, heavenly

bodies must follow uniform circular motionc. Ptolemy: mathematical models of geocentric views

3. The Copernican revolution “The Church Strikes back”

4. The Nature of Science

Ch. 3

Histrory of Astronomy PART II: “The Church Strikes Back”

1. Problems with Ptolemy’s models, lots of errors accumulated over the centuries

2. Copernicus: publishes heliocentric model and dies

3. Tycho observes planetary motions and dies4. Kepler: uses Tycho’s observations & writes 3

laws (see book):a. Each planet moves in ellipse with the Sun at one focus.b. The line between the Sun ……. (faster near Sun and vice versa)c. p2 = a3

5. Galileo:a.Uses telescope to discover moons of Jupiter, study sunspots and phases of Venus b.Supports Copernicus’ theory & gets in trouble with Church

3.3 The Copernican Revolution

Copernicus’ Heliocentric Model

•Sun is at center•Earth orbits like any other planet•Inferior planet orbits are smaller•Retrograde motion occurs when we “lap” Mars & the other superior planets

Tycho Brahe • Greatest observer

of his day

• Charted accurate positions of planets

• Hires Kepler

Johannes Kepler

• Greatest theorist of his day

• Uses Tycho’s observations to come up with three laws of planetary motion

Kepler’s Laws1 Each planet’s orbit around the Sun is an

ellipse, with the Sun at one focus.

Kepler’s Laws2 A planet moves along its orbit with a speed that

changes in such a way that a line from the planet to the Sun sweeps out equal areas in equal intervals of time. (the closer to the Sun, the faster it moves)

Kepler’s Laws 3 The ratio of the cube of a planet’s average

distance from the Sun to the square of its orbital period is the same for each planet.

a3 / P2 = 1

a3 = P2

a in AU

P in years

Galileo Galilei

• First man to point a telescope at the sky

• wanted to connect physics on earth with the heavens

• Dialogue Concerning the Two Chief World Systems

This book got him in trouble with the Church!

Galileo’s Observations• Galileo discovered

that Jupiter had four moons of its own.

• Jupiter was the center of its own system.

• Heavenly bodies existed which did not orbit the earth.

Galileo’s observation of the phases of Venus was the

final evidence which buried the geocentric model. Geocentric Heliocentric

No gibbous or full phases! All phases are seen!

Galileo observed all phases!

3.5 The Nature of Science

• How can we distinguish science from nonscience?

Our goals for learning:

I. Science:1. Based on observations and theory2. Open to criticism and constant challenge in

light of new evidence3. Not perfect (human), not always right, but

most successful discipline at predicting the way nature works.

II. Pseudoscience1. Generally holds “absolute truth”2. Considers only part of evidence available3. Tends to play on emotions and fears instead

of logic

Science and Pseudoscience

Empirical Discipline (no theoretical explanation)

Easily abused by charlatans Big problem: astrologers do not agree on

what any given celestial configuration means

In other words, who will you believe if one astrologer tells you this is a great time to travel and another tells you to stay home?

Astrology

I. Newton and Galileo Galileo’s experiments with falling objects:

1. g = 9.8 m/sec2

2. Objects fall together3. Inertia (motion in absence of force)

II. Newton’s Laws:1. 3 laws of motion: a. Inertia b. F=ma c. Action =

Reaction 2. Gravitation: F= GMm/R2 (Inverse-square law)

III. Orbits: 1. Closed: circles (circular velocity) & ellipses (v > vc)

2. Open: parabolas and hyperbolas (escape velocity)

Ch 4 Newton and Gravity(soap opera’s final episode)

How does Newton’s law of gravity extend Kepler’s laws? (some not in book)

• Ellipses are not the only orbital paths. Orbits can be:bound

• Circle (v = vc)

• Ellipse (v > vc)

unbound

• Parabola (v = ve)

• Hyperbola (v > ve)

• Circular and Escape velocities (vc and ve)

vc = GM/R

ve = 2GM/R

circular and

Tides

• Gravitational force decreases with (distance)2

– The Moon’s pull on Earth is strongest on the side facing the Moon, and weakest on the opposite side.

• The Earth gets stretched along the Earth-Moon line.

• The oceans rise relative to land at these points.

What have we learned?• How do gravity and

energy together allow us to understand orbits?• Gravity determines orbits• Orbiting object cannot

change orbit without energy transfer

• Enough energy -> escape velocity -> object leaves.

•How does gravity cause tides?•Gravity stretches Earth along Earth-Moon line because the near side is pulled harder than the far side.

5.1 Basic Properties of Light and Matter Light: electromagnetic waves

1. Velocity (c = speed of light), wavelength and frequency (colors), energy.2. Electromagnetic spectrum, visible spectrum, atmospheric windows

Matter: Atoms. How do light and matter interact? 5.2 Learning from Light: Origin of Starlight

1. How photons are produced2. Relation temperature motion of atoms 3. Blackbody Radiation (hot iron example). Wien’s Law:

hotter brighter, cooler dimmer

hotter bluer, cooler redder (max ~1/T)

4. Colors of Stars: redder are cooler, bluer are hotter5. Types of spectra (Kirchhoff’s 3 laws ): Continuous, Absorption and Emission6. Radial Velocity: Doppler effect

5.3 Telescopes: reflecting and refracting, ground, airborne, space.

Outline Ch 5 Light: The Cosmic Messenger

1. Continuous Spectrum (thermal radiation spectrum)

2. Emission Spectrum3. Absorption spectrum

Kirchhoff’s Laws

1 3

2

• Radial Velocity• Approaching stars: more

energy, • Receding stars: less energy,

Doppler Effect

• Approaching stars: more energy, spectral lines undergo a blue shift

• Receding stars: less energy, spectral lines undergo a red shift

/ = v/c

Radial Velocity

Basic Telescope Design• Refracting: lenses

Refracting telescope Yerkes 1-m refractor

Basic Telescope Design• Reflecting: mirrors• Most research telescopes

today are reflecting

Reflecting telescopeGemini North 8-m

3. Atmosphere absorbs most of EM spectrum, including all UV and X-ray, most infrared