Astronomy 101:http://people.physics.tamu.edu/dep

Links to lecture notes and other pagLinks to lecture notes and other pag

Astronomy Picture of the Day (AP

http://antwrp.gsfc.nasa.gov/apod/

: The Web Pagepoy/astro101.html

ges of possible interest likeges of possible interest like

OD):

Astronomy 1There is no lab requirement for AS

However, there are “Star Parties” a

Great opportunity to look through a

Confirmed attendance will result inConfirmed attendance will result ingrade

See http://instrumentation tamu eduSee http://instrumentation.tamu.edutimes

101: No Lab!STR101

approximately twice per month

a telescope at celestial objects

n ~1% of extra credit for your finaln 1% of extra credit for your final

u/starparty html for dates andu/starparty.html for dates and

Lecture 2: TThe Skyy

Lecture 2: Moon PLecture 2: Moon PPhases & EclipsesPhases & Eclipses

The sky is divided in

Mesopotamia, circa 30oldest know constella

Pt l 2 d t APtolemy, 2nd century A48 constellations in n

16th to 18th century ADunmapped (mostly sofilled infilled in

nto 88 constellations

00 BC: ations

ADAD: northern skyy

D: outhern) regions of sky

Constellations

Other cultures, other cons

Example: Ursa Major = bgrain scoop wagon plograin scoop, wagon, plo

Stars in a constellation usudistance from us.

At diff t l iAt a different place in ourdifferent star patterns.

s are arbitrary

tellations:

bear, dipper, bull's leg, ow etcow, etc.

ually are not at the same

G l ldr Galaxy, we would see

A more modern star chart of Ursa Major:

Stars are “attached” t

Distances to stars are h

However, we can pretedi t fsame distance from u

celestial sphere.p

Position on the celestiawhen the distance in u

to a “celestial sphere”

hard to measure.

end all stars are at the tt h d t lus, attached to a large

al sphere is known even unknown.

C l ti l S h A l i iCelestial Sphere: A large imagin h t d E thnary sphere centered on Earth

Hipparchus (ca 140 BC) was perhapHipparchus (ca. 140 BC) was perhapof ancient times. He produced a cataclassified them according to their app

Modern usage still retains names of within a constellation although Arabwithin a constellation, although Arabbrightest stars

ps the greatest astronomerps the greatest astronomeralogue of 1000 stars andparent brightness.

stars and rankings by brightnessbic names are also used for thebic names are also used for the

The brightness of alpha OrionisThe brightness of alpha Orionis (Betelgeuse) fromOctober 1979 to August 1996.

It is a slowly pulsating star that will eventually explode as a Type II y p ypsupernova.

We will discuss what a supernova isWe will discuss what a supernova is and how cataclysmic it will be whenalpha Orionis explodes later

Special locations on

North Celestial Pole = poN h P l ( hNorth Pole (near the star

South Celestial Pole = poSouth Celestial Pole poSouth Pole (no nearby b

Celestial Equator = circlEquatorEquator

n the celestial sphere

oint directly above Earth’s P l i )r Polaris)

oint directly above Earth’soint directly above Earth s bright star)

e directly above Earth’s

Distances betweDistances betwecelestial sphere

d i tdegrees, arcminute

360 degrees in a circle360 degrees in a circle

60 arcminutes in a degree

60 arcseconds in an arcmi

een points on theeen points on the are measured in

d des, and arcseconds

inute

½ degree = angular size of½ degree angular size of

Sun & Moon

Celestial navigat

At Earth’s North Pole: P l i i di l hPolaris is directly overh

At Earth’s Equator:At Earth s Equator: Polaris is due north, on t

In Earth’s Northern hemPolaris is due north - hePolaris is due north he(in degrees) is equal to y

tion made simple

h dhead

the horizon

misphere:ight above the horizonight above the horizon your latitude (in degrees)

The celestial sphere appThe celestial sphere appcelestial poles

Observation: Stars, Sumove in counterclocknorth (south) celestianorth (south) celestia

Objects near the celestijwest when above the

t “ tti ” i teast, “setting” in west

What causes these circuWhat causes these circu

pears to rotate about thepears to rotate about the s (1 day cycle)

un, Moon and planets kwise circles around l polel pole.

ial equator move east to qhorizon (“rising” in t)t).

ular motions?ular motions?

Explanations of

HYPOTHESIS #1(P l 2nd )(Ptolemy, 2nd century):Earth is stationary; stars

are attached to a sphere that revolves around

the Earth once per day.

WRONG!

the 1 day cycle:

HYPOTHESIS #2(C i 16th )(Copernicus, 16th cent.):

Stars are stationary; Earth rotates about its axis

once per day.p y

RIGHT!RIGHT!

Nicolaus Coperniill h b t Cwe will hear more about Co

icus (1473-1543)i i f l topernicus in a few lectures

The Sun appears to moThe Sun appears to moto stars (1 y

Today the Sun is “in” a panext month in a differen

Sun’s path on the celestialSun s path on the celestial

Constellations through whgzodiac

The ecliptic is NOT the saequator!q

ove west to east relativeove west to east relative year cycle)

articular constellation, nt one, etc.

l sphere = eclipticl sphere = ecliptic

hich the ecliptic runs = p

ame as the celestial

A model of thecelestial spherecelestial sphere,showing theconstellationb d hborders, thecelestial equator,and the apparentpppath of the Sunagainst thebackground ofbackground ofconstellations.

The Sun is in the direction of theS t b i d f th ESeptember, as viewed from the Eborn in September are said to be sign of Virgo.g g

e constellation Virgo inE th Th t i h lEarth. That is why people

born under the astrological

Observation: Sun movto stars (about 1 degr

What causes this annua

ves west to east relative ree per day).

al motion?

Explanations o

HYPOTHESIS #1(P l )(Ptolemy):

Sun revolves around Earth at a slightly

slower rate than the celestial sphere.

WRONG!

of 1 year cycle:

HYPOTHESIS #2(C i )(Copernicus):

Earth revolves around the Sun, once per year.

RIGHT!

The Moon appears tThe Moon appears trelative to stars

Today the Moon is “in” s

In two weeks: something d

In four weeks: first one, ag

Observation: Moon movestars, taking 27.3 days to

h hi hlWhat causes this monthly

to move west to eastto move west to east (1 month cycle)

some constellation

different

gain.

es west to east relative to o complete cycle.

i ? motion?

Explanations of

HYPOTHESIS #1(P l )(Ptolemy):

Moon revolves around Earth at a significantly

slower rate than the celestial sphere.

WRONG!

f 1 month cycle:

HYPOTHESIS #2(C i )(Copernicus):

Moon revolves around Earth, once per month.

RIGHT!

Daily path of the Sun in the skyDaily path of the Sun in the skyy at latitude 23 5 deg Ny at latitude 23.5 deg N.

The Sun rises dueeast and sets duewest only on the firstwest only on the firstday of spring and thefirst day of autumn.

Note that theSun at noontimeon June 21st is47 degrees higherin the sky thanin the sky thanon December 21st.

The first day of spring occurs aor March 21st (That's in the nor March 21st. (That s in the n

The first day of autumn occurs22nd. (This would be the first the southern hemisphere.)

The key thing is that the equinothe declination of the Sun is zel d h l i llocated on the celestial equator

about March 20thnorthern hemisphere )northern hemisphere.)

s about Septemberday of spring in

oxes occur whenero degrees (i.e.)r).

Daily path of the Sun in the sky aDaily path of the Sun in the sky aat the Earth's equator.at the Earth s equator.

Note that theNote that theSun rises due eastand sets due westonly on the first dayof spring and thefirst day of autumnfirst day of autumn(about March 21st

and September 22nd).

Daily path of the Sun in the skyDaily path of the Sun in the sky On June 21st the Sun is 47 degreat noontime than on December 2

at latitude 66 5 deg Nat latitude 66.5 deg N.ees higher in the sky21st.

Note that theSun rises dueeast and setsdue west onlydue west onlyon the first dayof spring and thefirst day of autumn (aboutMarch 21st andMarch 21st andSeptember 22nd).

The Earth's axis of rotation ito the plane of its orbit about

s tilted 23.5 degreesgt the Sun.

When the northern hemisphere Sun we have summer in the norwinter in the southern hemisphe

When the northern hemisphere the Sun we have winter in in the

d i h h hand summer in the southern hem

This is the cause of the season

is tilted towards therthern hemisphere andere.

is tilted away frome northern hemisphere

i hmisphere.

ns.

ImImwoSunonebetlatiattheat n

age of the Earth as itage of the Earth as itould appear from then over the course ofe year. Locationstween +23 and -23 deg itude see the Sun attude see t e Su ate zenith twice a yearnoontime.

Th i i iThe reason it is warmer in sis that the sunlight is more cground when the Sun is highg g

h i iummer than in winterconcentrated on theher in the sky.y

The word planet to the ancient GStar” These objects move diffeStar . These objects move diffe

We now know that they are otheThe orbital planes of the other psimilarly to that of the Earth. Thfound within a few degrees of thfound within a few degrees of th

Greeks meant “wanderingerently in the sky than the starserently in the sky than the stars.

er worlds.planets are oriented quitehus, the planets are usuallyhe ecliptic.he ecliptic.

Hipparchus compared the coordirecords made by the Babyloniansecliptic longitudes of the stars weabout 1 degree per century (Theabout 1 degree per century. (The1 degree in 72 years.) This is calbe confused with the word “preci

i i i lik i ion its axis is like a spinning top.

nates of some stars withs and discovered that theere increasing with time,e modern value is aboute modern value is aboutlled precession (not toision”). The Earth turning

While Polaris is close to the north cthe case. Due to the 26,000 year pemany stars take their turns being th

celestial pole now, it was not alwayseriod of precession,

he pole star.

Nutation (“nodding”)( g )of the axis of rotationis due to the tidalforces not beingforces not being constant over time.R = rotation ofEarth. P = precession.N = nutation.

Moon PhaseMoon Phasees & Eclipseses & Eclipses

Astronomic

Lunation (Phases of thhttp://antwrp.gsfc.nasa.gov/apod/a

Sh d f th E th (Shadow of the Earth (http://antwrp.gsfc.nasa.gov/apod/image/0

Shadow of the Moon (http://antwrp.gsfc.nasa.gov/apod/image/9

al “movies”

he Moon)ap051113.html

(L li )(Lunar eclipse)0311/112003lunareclipse_koehn.gif

(Solar eclipse)9803/moonshad_goes_big.gif

The Moon’s phases chaThe Moon s phases chaless of the Moo

Fu

Waning Gibbous

Last Quarter

W i CWaning Crescent

NeNe

ange as we see more orange as we see more or on’s sunlit half.

ull

Waxing Gibbous

First Quarter

W i C Waxing Crescent

ewew

The Moon is a sphere i

Half closest to

Half furthest fro

illuminated by the Sun.

o Sun is light.

om Sun is dark.

Gibbous and cGibbous and cresult from p

rescent shapesrescent shapes perspective.

Example: Crescent

We see Moon in almost thesame direction as Sunsame direction as Sun.

We see only a sliver of li idMoon’s sunlit side.

We see crescent Moon close to Sun in sky.

“Horns” of crescent pointHorns of crescent point away from Sun.

Moon

e

The Moon rotates abouThe Moon rotates abouaround th

On Earth, we always see t(“near side”)( near side ).

The “far side” is always tu

ut its axis as it revolvesut its axis as it revolves he Earth.

the same side of the Moon

urned away from us.

No rotation oof the Moon:

Observer on distant star always seesstar always sees same side.

Observer on Earth sees all sides.

One rotation pper revolution:

Observer on distant star sees all sidesstar sees all sides.

Observer on Earth always sees same yside.

The length of the SIDER

Sidereal month = time fod E haround Earth.

The view from Earth:The view from Earth:

Early afternoon of Thursdfull Moon in Pisces.

L t i f W d dLate evening of WednesdaMoon in Pisces again (b

REAL month = 27.3 days

or one revolution of Moon

day, Sept 7, 2006:

O t 4 2006ay, Oct 4, 2006: but now it is gibbous).

The length of the SYNOD

Synodic month = time beh ( Mthe next (or one new Mo

The view from Earth:The view from Earth:

Early afternoon of Thursdfull Moon in Pisces.

L t i f F id OLate evening of Friday, Ofull Moon again (but no

DIC month = 29.5 days

etween one full Moon and d h )oon and the next).

day, Sept 7, 2006:

O t 6 2006Oct 6, 2006: ow it is in Aries).

Why is SYNODICWhy is SYNODIC SIDEREA

Synodic – measured relati

Sidereal – measured relati

i iSun is a moving target; 2.3 ex

month longer thanmonth longer than AL month?

ive to Sun

ive to stars

d hxtra days to catch up.

T t l S lTotal Sola E liar Eclipse:

T t l LTotal Luna E liar Eclipse:

The shadow of the EarthThe shadow of the Earthextended object) has tw

Umbra: inner part of shadow

Penumbra: outer part of shPenumbra: outer part of sh

h (or any otherh (or any other wo parts:

w, Sun completely hidden.

hadow Sun partially hiddenhadow, Sun partially hidden.

The Earth's shadow consists of aumbra (Latin for “shadow”), ancalled the penumbra.

a darker part, called thend a not-so-dark part

Earth’s S

Earth’s umbra stretches 1.i E h Mtimes average Earth-Mo

At the Moon’s orbit, Earthkilometers wide (2.6 tim

Th M fit i id thThe Moon can fit inside throom to spare.

Shadow:

.4 million kilometers (3.7 di )oon distance).

h’s umbra is 9000 mes Moon’s diameter).

h E th’ b ithhe Earth’s umbra, with

A lunar eclipse occursA lunar eclipse occurs through the Ea

Lunar eclipses occur whandand

Lunar eclipses occu

when the Moon passeswhen the Moon passes arth’s shadow.

hen Earth is between Sun MoonMoon.

ur at FULL MOON.

Three types of

(1) Penumbral:

None of Moon enters umb

(2) Total:(2) Total:

All of Moon enters umbra1 hour 40 minutes.

(3) Partial:(3) Partial:

Part of Moon enters umbr

f lunar eclipse

bra. Boring.

a. Totality lasts up to

ra.

A solar eclipse occursA solar eclipse occurs through the M

Moon umbra stretches for

Minimum Earth-Moon disMinimum Earth-Moon dis

= 363,000 kilometers.

Maximum Earth-Moon di

= 405 000 kilometers 405,000 kilometers.

when the Earth passeswhen the Earth passes oon’s shadow.

r 380,000 kilometers.

stance =stance =

stance =

Solar eclipses ocSolar eclipses ocis between S

Solar eclipses occuSolar eclipses occu

ccur when Moonccur when Moon Sun and Earth.

ur at NEW MOONur at NEW MOON.

Three types of

(1) Total: Observer is in Ml l hid Scompletely hides Sun.

(2) Annular: Moon umbr(2) Annular: Moon umbrMoon is surrounded by

(3) Partial: Observer is inumbra; Moon takes ‘bumbra; Moon takes b

f solar eclipse

Moon’s umbra; Moon

ra falls short of Earth;ra falls short of Earth; y a ring of Sun.

n penumbra, to one side of bite’ out of Sunbite out of Sun.

Moon’s S

The Moon’s orbital motioidl h E hrapidly over the Earth.

Total solar eclipse is visibTotal solar eclipse is visibto 270 km wide) for a sh

Total lunar eclipse is visi

F T t t t l lFrom Texas, next total luntotal solar: Apr 8, 2024

Shadow:

on makes its umbra sweep

ble from a narrow path (upble from a narrow path (up hort time (up to 7.5 min).

ble from half the Earth.

D 21 2010 tnar: Dec 21, 2010; next

Few qu

1) Why are eclipses not fr

2) During a total lunar eclobserver on the surfacobserver on the surfac

3) For an observer on the phases? (easy)

4) A th l4) Are there more solar or

estions:

requent? (medium)

lipse, what is seen by an ce of the Moon? (easy)ce of the Moon? (easy)

Moon, does Earth show

l li ? (h d)r lunar eclipses? (hard)

At the average distance of the MAt the average distance of the MEarth's umbral shadow is 2.65 tof the Moon, or about 1.37 degrdoes not occur every full MoonMoon's orbit with respect to the

Moon from the Earth theMoon from the Earth, thetimes the angular diameterrees. But a lunar eclipse

n because of the tilt of thee ecliptic.

In the previous millennium (100were 681 total lunar eclipses aneclipses. Only 5.5 percent of thclose enough to one of the nodeto produce a total lunar eclipseto produce a total lunar eclipse.

If the Moon is in the Earth's shathe Earth's hemisphere facing thchance to view the eclipse.

01 to 2000 AD) therend 859 partial lunarhe full Moons occures of the Moon's orbit

adow, everyone onhe Moon has a

Because the Moon's orbit around tthe angular diameter of the Moon Similarly because the Earth's orbiSimilarly, because the Earth s orbielliptical (but less so), the Sun's an

the Earth is elliptical,changes somewhat.

it about the Sun is alsoit about the Sun is alsongular diameter changes.

The distance between Earth anThe distance between Earth an356,400 and 406,700 km. Thaorbit is elliptical, not perfectly M ' di f 3476 k iMoon's diameter of 3476 km, ifrom 29.39 to 34.14 arc minute

The distance between the Sun 147.1 and 152.1 million km. Telliptical than the Moon's orbitelliptical than the Moon's orbitthe Sun's diameter of 1,392,00varies from 31.46 to 32.53 arc

In order for a total solar eclipsmust be aligned with the Sunmust be aligned with the Sun, a larger angular size.

nd Moon varies betweennd Moon varies betweenat's because the Moon's

circular. Given thei l i iits angular size varieses.

and Earth varies betweenThe Earth's orbit is lesst around the Earth Givent around the Earth. Given

00 km, its angular sizeminutes.

e to occur, the new Moonand the Moon must haveand the Moon must have

If the Sun and Moon are aligangular diameter is greater t

e ha e a t t l l liwe have a total solar eclipse

gned and the Moon'sthan that of the Sun,e.

Note coronaand prominences.

If the Sun and Moon are alignedtoo far away from the Earth to coSun's disk, we have an annular sSun s disk, we have an annular scase we do not get to see the Sun

d, but the Moon isompletely cover thesolar eclipse. In thissolar eclipse. In thisn's corona.

If the Moon is not quite at one oalso get a partial solar eclipsealso get a partial solar eclipse.

of its nodes, we can

Some solar eclipses are such thaat the start and end of the track,

iddl Th di f h E hmiddle. The radius of the Earthmake a total solar eclipse in the

at they are annularand total in the

h i j bi hh is just big enough tomiddle.

In the previous millennium (1following number of solar ec

625 total

837 partial837 partial

767 annular

156 annular

1001 to 2000 AD) theclipses occurred:

r

r-total

One big difference between totaltotal lunar eclipses, is that you uto be situated in the path of totaliOn average, the path of the solarlocation in the northern hemisphlocation in the northern hemisphso, while in the southern hemispevery 450 years. On August 21, h hthe Moon passes across the easte

States. That is our next good oppspectacle of Nature without travespec c e o N u e w ou ve

The maximum duration of a totaover 7 minutes. Some only last

l solar eclipses andsually have to travelity of a solar eclipse.r eclipse crosses a here every 350 years orhere every 350 years orhere it occurs on average2017, the shadow of

h lf f h i dern half of the Unitedportunity to view thiselling too far.e g oo .

al solar eclipse is a littlea couple seconds.

Eclipse prEclipse pr

It Mprecm

edictionediction

is said that Thales ofMiletus (ca. 624-546 BC)

di t d th t t l lredicted the total solarclipse of 585 BC. Howight he have done this?g

The Babylonians discovered to the occurrence of solar eclwith a period of 18 years 11 a Leap Day). This is called t

The basic rule is this: if a toton a given date at a given plathat another total solar eclipsMoons later, or 6585 1/3 daythe cycle will occur 1/3 of ththe cycle will occur 1/3 of thEarth to the west.

that there was a patternlipses. They repeat1/3 days (give or takethe saros cycle.

al solar eclipse occursace, odds are very goodse will occur 223 newys. The next eclipse inhe way around thehe way around the

The saros cycle that includes the of August 11, 1999, began with apartial solar eclipse visible near tpartial solar eclipse visible near tJanuary 4, 1639. After 14 partialannual eclipse occurred on June 6by an annular-total eclipse on Juncycle includes 41 total solar eclipmoving south on the Earth. Thenmoving south on the Earth. Thensolar eclipses of decreasing fractiApril 17, 3009, with a partial solath S th P l Th l l t 1the South Pole. The cycle lasts 1

Other saros cycles can progress fy p g

total solar eclipsea very unimpressivethe North Pole onthe North Pole onl solar eclipses, an6, 1891, followedne 17, 1909. Then thepse whose tracks keepn there are 20 partialn there are 20 partialional cut, ending onar eclipse visible near370370 years.

from south to north.

The number of active saros cyc48 with a period of roughly 600to say that with good eclipse recto say that with good eclipse recit is possible to predict solar eclThis is how eclipse “prediction”beginning of the 18th century. Athe orbit of the Moon in enoughmake these predictions on the bmake these predictions on the bmechanics and gravitational the

les varies from 42 to0 years. Suffice itcords from the pastcords from the past,lipses in the future.” went until theAfter that we understoodh detail that we couldbasis of Newton'sbasis of Newton seory.