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EE8086 Astronomy: Stars, Galaxies & Cosmology p.1-1

Overview of the Universe

Astronomical Terminology Scale of the Universe

Motion of Earth in the Universe

Tour of our Sky The Cause of Seasons

Further Reading:The Essential Cosmic Perspective, Chapters 1 & 2

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Astronomical Terminology

Star

large, glowing ball of gas thatgenerates heat & light through

nuclear fusion

Planet

moderately large object orbiting a

star & shines by reflected light

own gravity makes it round cleared its orbital path

may be rocky, icy, or gaseous in

composition

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Astronomical Terminology, contd

Moon object orbiting a planet

Asteroid

relatively small & rocky objectorbiting a star

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Comet relatively small & icy object that

orbits a star

Nebula

an interstellar cloud of gas and/ordust

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Solar System

The Sun & all the

material that

orbits it, including

its planets &

moons

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Star System

star(s) & all the material that orbits

it, including its planets & moons

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How Large is the Solar System?

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How Large is the Solar System?

SunJupiterEarth

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Galaxy

great island of stars in space, all held together by gravity &

orbiting a common center

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Milky Way Galaxy in our Sky

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Our Address in the Milky Way Galaxy

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Universe

sum total of all matter & energy, i.e. everything within &

between all galaxies

origin of Universe explained by the Big Bang Theory

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Our Cosmic Address
http://0.0.0.1/ZoomUniverse.swfhttp://0.0.0.1/ZoomUniverse.swf

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Our Cosmic Origin

The Universe hasbeen expanding since

its hot & densebeginning in BigBang. Each of the 3cubes represents thesame region of theuniverse, which hasexpanded with time.

Within a few billion years after the BigBang, gravity caused local concentrations ofmatter to collapse into galaxies while theuniverse continued to expand.

Galaxies like theMilky Way act ascosmic recyclingplants. Stars aremade from materialin gas clouds & dustwithin the galaxy.Stars return materialto interstellar spacewhen they die.

A star forms at the center of acollapsing cloud of gas & dust, &planets may form in the spinningdisk surrounding the young star.

Massive stars explodewhen they die, scatteringthe elements theyproduced into space.

Stars shine with the energyproduced by the nuclear fusion intheir cores. The fusion also createsheavier elements from lighterones.

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How Big is the Universe?

There are as many

stars in the

observable universeas there are grains of

dry sand on all the

beaches on Earth.

Observable universe

is ~13.9blight-years.

Definition of a light-year:

the distance traveled by light in one year

unit for distance, not time!

corresponds to ~10 trillion km

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Our Lifetimes compared to the Age of the Universe

The universe is ~13.9byears

old.

On a cosmic calendar:

entire history of the universe

compressed into 1 year

solar system forms in early Sep

life on Earth started by late Sep

dinosaurs appeared on 26-Dec

& became extinct on 30-Dec

On 31-Dec:

human evolve @9pm

modern human @11:58pm

human civilization only occupy

the last half-min

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Spaceship Earth

Contrary to our perception, we are not sitting still.

we are moving with the Earth in several ways & at surprisingly

fast speeds!

The Earthrotatesaround

its axis once

every day.

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Earths Motion in the Solar System

Earth is racing around the Sun.

Earth is at an average distance of 150mkm from the Sun

Earths axis tilted by 23.5 (pointing to Polaris)

orbits around the Sun in the same direction as its rotation

counter-clockwise as viewed from above the North pole

The Earth orbits

around the Sunonce every year.

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Earths Motion in the Solar System, contd

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Solar Systems Motion in the Milky Way Galaxy

The Solar System is racing around the Milky Way Galaxy.

the Sun moves randomly relative to other nearby stars at typical

speed of more than 70,000 km/hr

the Sun orbits the

galactic center

once every 230m

years at speed of800,000 km/hr

The Solar System orbits

the galactic center once

every 230myears.

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Milky Way Galaxys Motion in the Universe

The Universe is expanding.

galaxies are carried along with the expansion & are generally

moving away from each other (like expanding raisin cake)

those in Local Group can move towards or away from us, e.g.

Milky Way moving towards Andromeda @ 300,000 km/hr

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Summary of Earths Motion in the Universe

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Patterns in the Night Sky

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Constellations

A constellation is a

regionof the sky.

defined in 1928 by

the International

Astronomical

Union

often recognizable

by a pattern or

grouping of stars

total of 88 official

constellations, mostnames come from

antiquity

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Star Charts

Planisphere make your own at

http://www3.ntu.edu.sg/eee/eee6/astroclub/articles.asp

Planetarium software (PC) http://www.stellarium.org/ (free open source )

Planetarium for mobile phones (Java-Enabled) http://mobilestarchart.sourceforge.net/

Satellite Observations http://www.caski.com/cs.cgi

http://www.skysatellite.com

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The Celestial Sphere

The sky above us looks like

a dome (hemisphere).

perception when looking into space, as the stars are too faraway

The patterns of stars have no physical significance!

stars that appear close together may lie at different distances

celestial sphere =

lower +upper half of

the dome

we see half the sphere

at any moment

All stars appear to lie

on the celestial sphere.

we lack the depth

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Model of the Celestial Sphere

The idea of a celestial sphere is used to map the sky.

shows how stars are

arranged in the sky

This is a 2-D

representation of the sky

as viewed from Earth.

Earth is placed in the

center of the sphere

Special points & circles:

north & south celestialpoles

celestial equator

ecliptic

Th L l Sk

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The Local Sky

zenith :point directly overhead

horizon : boundary between Earth & sky (90from zenith)

meridian : line from northern to southern horizon through zenith

azimuth : angledirection along horizon, clockwise from due north

altitude : angle above the horizon

Our local sky appears

to take the shape of a

hemisphere define

location of a star by

its altitude &azimuth.

R i C di E h

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Review: Coordinates on Earth

Latitude:position north or

south of equator Singapore: 122N

Longitude:position east orwest of prime meridian

(Greenwich, England)

Singapore: 10348E

M i Si & Di t i th Sk

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Measuring Size & Distance in the Sky

True sizes or separations of objects in the sky cannot be

determined due to lack of depth perception

describe using angular size & angular separation instead

H d Sk M

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Handy Sky Measures

For more

precise

astronomicalmeasurement:

eachdegree

subdivided into

60arcminutes

eacharcminute

subdivided into

60arcseconds

D il M ti f C l ti l Obj t i th Sk

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Daily Motion of Celestial Objects in the Sky

Earth rotates from west to east.

celestial sphere appears to rotate around us from east to west

Stars make daily circles around the celestial poles

Stars at the north or south

celestial poles will appear

stationary.

N th H i h Vi

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Northern Hemisphere View

Other stars (& Sun, Moon, planets) generally

have daily circles partly above & below horizon.

appear to rise in the east & set in the west

Stars near north

celestial pole (at

angle less than Afrom celestial pole)

do not rise or set,

but remain above

the horizon circumpolar star

Stars near south

celestial pole never

rise above horizon.

A

S th H i h Vi

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Southern Hemisphere View

Similarly, stars near south

celestial pole do not rise

or set, but remain abovethe horizon

circumpolar star

Stars near north celestial

pole never rise abovehorizon.

Other stars (& Sun,

Moon, planets) generally appear to

rise in the east & set in the west.

If you stand at the poles, nothing rises or sets.

If you stand at the equator, everything rises & sets 90to the horizon.

Wh t C t ll ti ?

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What Constellations can you see?

It depends on your latitude& time of the year.

due torotation&orbitof Earth

Variation with Latitude

The constellations you see depend on your latitude but not longitude.
http://0.0.0.1/MotionNightSky.swfhttp://0.0.0.1/MotionNightSky.swf

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Daily circles CCW looking north, CW looking south

Northern Hemisphere Southern Hemisphere

Variation with Time of Year
http://0.0.0.1/MotionNightSky.swfhttp://0.0.0.1/MotionNightSky.swfhttp://0.0.0.1/SunPathZodiac.swfhttp://0.0.0.1/SunPathZodiac.swf

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Variation with Time of Year

As the Earth orbits the Sun, the Sun appears to move eastward along

the ecliptic with respect to the stars.

Constellations along the ecliptic make up the zodiac.

How Long is a Day?
http://0.0.0.1/SunPathZodiac.swfhttp://0.0.0.1/SunPathZodiac.swf

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How Long is a Day?

The Seasons

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The Seasons

Four Special Moments in the Year

Summer Solstice (21 June) when the northern hemisphere receives its most direct sunlight

Winter Solstice (21 December)

when the northern hemisphere receives its least direct sunlight Spring Equinox (21 March)

when northern hemisphere just starts to tip towards the Sun

Fall Equinox (22 September) when northern hemisphere just starts to tip away from the Sun

What causes the Seasons?
http://0.0.0.1/SunlightVaryOrbit.swfhttp://0.0.0.1/SunlightVaryOrbit.swf

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What causes the Seasons?

Earths axis tilted at 23.5from normal to ecliptic plane.

celestial equator tilted at 23.5to the ecliptic plane

The Sun spends 6 months north & south of the celestialequator in a year.

Seasons are caused by Earths axis tilt& not the distance

from the Earth to the Sun!

Summer in the Northern Hemisphere
http://0.0.0.1/SunlightVaryOrbit.swfhttp://0.0.0.1/SunlightVaryOrbit.swf

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Summer in the Northern Hemisphere

Why Distance is not the cause of Seasons?

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Why Distance is not the cause of Seasons?

Variation of Sun-Earth distance is only about 3%. small variation overwhelmed by effects of axis tilt

However distance does matter for some other planets, notablyMars and Pluto

Seasons are more extreme in the northern hemisphere. due to more land, less ocean

Seasonal Change in the Suns Path

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Seasonal Change in the Sun s Path

Days are longer &

warmer in summer.

In winter, days are

shorter & cooler.

Seasonal Change in the Suns Path contd

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Seasonal Change in the Sun s Path, cont d

Northern

HemisphereEquator

Seasonal Change in the Suns Altitude

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North SouthSun rising on 3 particular days

Equinoxes

Summer

solstice

Winter

solstice

Seasonal Change in the Sun s Altitude

Photograph taken at

8~10 day intervalsover a year.

same place & time

Figure 8

observed due to the

combination of

Earths axis tilt &

varying speed as it

orbits the Sun.

Suns Path in the Arctic Circle

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Sun s Path in the Arctic Circle

Latitude at 663344N of the equator.

On summer solstice, the Sun does not set but skims the

northern horizon at midnight.

Earth's Precession

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Earth s Precession

The direction of Earths rotation axis is

not fixed in space but executes a slow

precession (like a top) with a period of26,000 years.

axis currently pointed at Polaris

13,000 years later: axis will point to

Vega (within a few degree)

Amount of axis tilt stays close to 23.5.

pattern of season not affected

positions of solstices & equinoxes in

Earths orbit gradually shift with cycle

of precession

Precession due to gravitys effect on

a tilted, rotating object that is nota

perfect sphere.

The Moon Our Constant Companion

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The Moon, Our Constant Companion

The Moon is the

brightest & most

noticeable objectin our sky.

orbits the Earth

in 271/3days

rise in the east &sets in the west

appears to move

eastward from

night to nightYou can also see

it in the day!

Phases of the Moon
http://0.0.0.1/PhasesOfMoon.swfhttp://0.0.0.1/PhasesOfMoon.swf

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Phases of the Moon

Half the Moon is illuminated by the Sun.

we see a

combinationof the bright &

dark faces

The phase of

the Moondepends on its

position relative

to the Sun as it

orbits Earth.

Phases of the Moon, contd
http://0.0.0.1/PhasesOfMoon.swfhttp://0.0.0.1/PhasesOfMoon.swf

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Phases of the Moon, cont d

Each complete cycle of phases takes about 291/2days.

from one new moon to another

~2 days longer than Moons orbital period of 271/3days due to Earths motion around the Sun during the time the Moon

is orbiting around Earth

new

crescent

first quarter

gibbous

full

gibbous

third quarter

crescent

Waxing means increasingMoon visible in afternoon/ evening

gets fuller & rises later each day

Waning meansdecreasingMoon visible in late night/morning

gets less and sets later each day

}}

Eclipses

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Eclipses

Earth & Moon cast shadows.

When one passes through the others shadow, we have an

eclipse.

Two types of eclipses:

Lunar Eclipse Earth is between Sun & Moon

Solar Eclipse Moon is between Sun & Earth

Conditions for Eclipses
http://0.0.0.1/MoonOrbitTilt-Ecliptic.swfhttp://0.0.0.1/MoonOrbitTilt-Ecliptic.swf

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Co d t o s o c pses

Moons orbit is inclined at 5to the ecliptic plane.

we do not get a lunar & a solar eclipse every month (~twice/year)!

Moon only crosses the ecliptic plane at 2nodes

eclipse possible only when full or new moon occurs near nodes

Solar Eclipses
http://0.0.0.1/MoonOrbitTilt-Ecliptic.swfhttp://0.0.0.1/MoonOrbitTilt-Ecliptic.swfhttp://0.0.0.1/TotalSolarEvolution.swfhttp://0.0.0.1/TotalSolarEvolution.swf

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p

You will see it when you

are in Moons shadow.

There are three types ofsolar eclipses.

Total solar eclipse

within umbra

Partial solar eclipse

within penumbra

Annular solar eclipse

Moon is relatively further

away & its umbralshadow does not reach

Earth

Progression of a Total Solar Eclipse
http://0.0.0.1/TotalSolarEvolution.swfhttp://0.0.0.1/TotalSolarEvolution.swf

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g p

Lunar Eclipses

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p

Lunar eclipse begins

when the Moon enters

Earths penumbra.After that, one of the 3

types of lunar eclipse

can be seen:

penumbral lunar eclipse Moon only passes

through penumbra

partial lunar eclipse

part of full moon passesthrough umbra

total lunar eclipse

Moon passes entirely

through umbra

Predicting Eclipses

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g p

Moons node slowly move around its orbit around Earth.

eclipse do not occur every 6 months

Eclipses recur in the ~18 years 111/3days Saros Cycle.

same relative geometry & a nearly identical eclipse

but type (partial or total) & location may vary

Planets in Our Sky

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y

Five planets easy to find with the

naked eye (all looked like stars):

Mercury difficult to see,always close to Sun

Venus very bright when

visible, morning or evening

starMars noticeably red

Jupiter very bright

Saturn moderately bright

In a particular night, planets rise

in the east & set in the west like

stars.

Apparent Retrograde Motion of Planets

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pp g

Planets usually move

eastwardfrom night

to night relative tothe stars.

Sometimes they go

westwardfor a few

weeks or months:

apparent retrograde

motion

easily explained by a

Sun-centered solar

system

Composite of 29 photos of Mars from Jun03 to

Nov03 showing apparent retrograde motion.

ee 8086 lecture 01

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