Astronomy 109 for syllabus see http://www.physics.rutgers.edu/~croft/A109-19.htmlProf. Mark Croft – W113: 848-445-8746: croft AT physics.rutgers.edu

Science-careful observation, reporting, preservation and modeling of nature

Theories & LawsSimple physical rules +

mathematical description

describe experiments

Observation &

Experiment(quantitative)

Prediction(s)- Expand Applicability

- New phenomena

1-1

'Beauty is truth, truth beauty,

that is all ye know on earth, and

all ye need to know.‘ (Keats)

[have a care in unprejudiced application]

Astronomy history case studyCase study of science its tools/methods and their relations to human institutions.

Perhaps first fact-based impressions on humans (historically, individually)

-day/night- the seasons-the moons phases/motion -stars/constellations

Careful longest-possible-time observations and (big) database building

-religion/superstition -navigation -when to plant and when to sow

Astronomy history case study cont.

Application of mathematics to nature

Empirical theories/models: large sets of data gathered into concise

mathematical formalism - great summary but don’t understand the “why”

-Ptolemaic solar system – Kepler’s laws for planetary motion

Underlying mathematical/philosophical formalisms

-Newton’s laws

Instrumental evolution/revolutions (data precision-size-time changes)

Tyco’s observatory, Galileo’s application of telescope, Sputnik…

Astronomy case study cont.

Science coupling to religion, government/politics…(good or bad)

-mysticism/religion motivated millennia of detailed astronomical

observations (good data not so good motivation)

-Religion/government adopts a specific model as dogma

-Galileo’s conviction/punishment for heresy (not so good)

Breakdowns in communication/collective-memory(bad)

-burning of Alexandria Library-fall of Roman Empire - Dark Ages

Revolutions in thinking

Copernicus, Galileo, Newton

Science (internal) impediments to progress

-“theory too beautiful to be wrong” – blinded by love of symmetry

(insistence on perfectly circular orbits)

-reluctance to give up what works good enough

-what is good/ethical for mankind?

“Powers of Ten” Notation

• 1 thousand = 1000

= 103

• 1 million = 106

• 1 billion = 109

• 1 trillion = 1012

⚫ 1 thousandth =

1/1000 = 0.001= 10-3

⚫ 1 millionth = 10-6

⚫ 1 billionth = 10-9

⚫ 1 trillionth = 10-12

Big numbers (>1) Small numbers (<1)

How far is a light-year?

1 light-year = (speed of light) (1 year)

km 365 days 24 hr 60 min 60 s= 300,000 1 yr

s 1 yr 1 day 1 hr 1 min

5 2 1 1 1

5+2+1+1+1

10 12

km 365 days 24 hr 60 min 60 s = 300,000 1 yr

s 1 yr 1 day 1 hr 1 min

=(3 10 3.65 10 2.4 10 6 10 6 10 ) km

=(3 3.65 2.4 6 6)10

=946 10 km=9.46 (10) km

d vt=

10,000 ( )km d equator pole −

C D=40000

3.14159

CD

= =

12,732D km=

100 billion= (1011) stars

100,000 ly

Where we stand in Universe

Earth

Jupiter

Venus

MercuryMars

Sun

8 light min.1 AU

93 106 mi

150 106 km

Local Group

Milky Way

Andromeda 54 galaxies (dwarfs)

8 million ly

Virgo Supercluster

100 million ly

Cosmic microwave background

out to (back to) 379,000 years

Afte Big Bang

13.77 b ly = observable Universe13.77 b years since the Big Bang birth of Universe

Large-scale structure of universe

out to (back to) 3.5 b years

Note

motion

N-star

Finding the celestial pole: what my father taught me

N. Hemisphere looking N

S. Hemisphere

looking S

Duration of photo ~5 hrs

Star tracks trace ~20% of circle

night

sky

rotation

because

Earth

rotates

on axis

Your 1’st observation day/night

Sun rises in E sets in W

24 hrs=day to move 360o

Winter TriangleBetelgeuse-Procyon-

Sirius

WinterEcliptic

Path of

Sun &

planets

across

celestial

sphere

Planets & Moon

move close to ecliptic

Winter

Winter

Rigel (T~11,000 K)Sirius

Orion Belt

Sword

Orion emission

Nebula

(red Balmer line)

Betelgeuse (T~3,200 K)

X-ray

Aldebaran (T~3,910 K)

stars that are close in angle can be at very

different distances

Important in astronomy

Orion

constellation

Earth orbits the Sun (revolves) once every year:average distance of 1 AU ≈ 150 million kilometers.

slightly elliptical orbit 152.1 to 147.1 million km

Earth’s axis tilted by 23.5º (pointing to Polaris)

It rotates in the same direction it orbits, counterclockwise as

viewed from above the North Pole. (right hand rule)

Earth’s Orbital Motion: day as seen from Earth / stars

Daily cycle, noon to noon, is

diurnal motion - solar day

24 hrs

Direction to stars occurs

before 24 hours later, due to

Earth’s rotation around the

Sun; defines one sidereal

day 23 h 56 min 4.1 s

The sky varies as Earth orbits the SunAs the Earth orbits the Sun, the Sun appears to move eastward along the

ecliptic through the constellations of the zodiac.

http://www.physics.rutgers.edu/~croft/A109/IF_02_14_SunPathZodiac.swf

North celestial pole

directly above

North Pole.

The Celestial Sphere

88 constellations cover

the celestial sphere.

Ecliptic - path of sun

South celestial pole

directly above South Pole.

Celestial equator is a

projection of Earth’s

equator onto sky.

Earth’s Orbital Motion: seasons

Ecliptic is plane of Earth’s path around the Sun; at 23.5° to

celestial equator

Northernmost point of path (above celestial equator) is

summer solstice; southernmost is winter solstice; points

where path crosses celestial equator are vernal and autumnal

equinoxes

Combination of day

length and sunlight

angle gives seasons

• Time from one

vernal equinox to

next is tropical year

Seasons: e.g. NYC 40.7o N longitude

Summer solstice

Winter solstice

Dec. 21

June 21

http://www.physics.rutgers.edu/~croft/A109/IF_02_15_ReasonForSeasons.swf

Seasons: e.g. NYC 40.7o N longitude

22–23 Sep.

Autumnal or Fall Equinox

Vernal or Spring Equinox

Vernal or Spring Equinox

Mar. 21

http://www.physics.rutgers.edu/~croft/A109/IF_02_15_ReasonForSeasons.swf

Earth’s axis precession

Precession: rotation of Earth’s axis itself; makes

one complete circle in 25,772 years

Cause: gravitational forces of the Moon &

Sun on Earth's equatorial bulge

The Local SkyAn object’s altitude (above horizon) and direction

(along horizon) specify its location in your local sky.

Zenith: the point

directly overhead

Horizon:

all points

90° away

from

zenith

Meridian:

line passing

through

zenith and

connecting N

and S points

on horizon

The Local Sky angle estimation

Angular Measurements

1 = 60

(arcseconds)

Full circle = 360º

1º = 60 (arcminutes)

angular size vs physical size

360 degreesangular size = physical size

2 distance

An object’s angular size

appears smaller if it is

farther away.

Coordinates on the EarthLatitude:

Coordinates on the

Celestial Sphere

position E or W

of prime meridian

(through Greenwich, England)

position N or S of equatorLongitude: Project Latitude up into sky

“Declination”

Project Longitude up into sky

“Right Ascension”

[Don’t let it rotate.

“0” fixed by Vernal Equinox

of 1950.

* Must correct for slow

precession of equinoxes]

Triangulation: Measure baseline and angles, can

calculate distance

Distance measurement: Parallax

The longer the baseline

the better

2 AU

Only see with telescope ~1806

Moon ~29.5 day

cycle of phases—

synodic month

Phases due to

different amounts of

sunlit portion being

visible from Earth

360° rotation around

Earth, sidereal

month, ~2 days

shorter

http://www.sumanasinc.com/webcontent/animations/content/moonphase.html

Waxing• Moon visible in afternoon/evening

• Gets “fuller” and rises later each day

Waning• Moon visible in late night/morning

• Gets “less full” and sets later each day

new

crescent1’st

quartergibbous

full

gibbouslast

quarter

crescent

Lunar 29.5-day cycle

Phases of the Moon: 29.5-day cycle

Waxing• Moon visible in afternoon/evening

• Gets “fuller” and rises later each day

Waning• Moon visible in late night/morning

• Gets “less full” and sets later each day

http://www.sumanasinc.com/webcontent/animations/content/sidereal.html

Similar effect got

Lunar cycle

sidereal month

synodic or lunar month27.322 d

29.531 d

New Moon

New Moon

When can eclipses occur?

• Lunar eclipses

can occur only at

full moon.

• Lunar eclipses can

be penumbral,

partial, or total.

What causes eclipses?

• The Earth and Moon cast shadows.

• When either passes through the other’s shadow, we

have an eclipse.

Solar eclipse: The

Moon is between

Earth and the Sun

• Partial when only

part of the Sun is

blocked

• Total when all of it

is blocked

• Annular when the

Moon is too far from

Earth for total

(eccentric orbit)

Why don’t we have an eclipse at every new and full

moon?

– The Moon’s orbit is tilted 5.2°to ecliptic plane.

– So we have about two eclipse seasons each year, with a lunar

eclipse at new moon and solar eclipse at full moon.

Eclipses occur when Earth, Moon, and Sun form

a straight line

When can eclipses occur?

• Solar eclipses can occur only at new moon.

• Solar eclipses can be partial, total, or annular.

Planets Known in Ancient Times- known as “wanders”

• Mercury

– difficult to see; always close to Sun in sky

• Venus

– very bright when visible; morning or evening “star”

• Mars

– noticeably red

• Jupiter

– very bright

• Saturn

– moderately bright

What was once so mysterious

about planetary motion in our sky?

• Planets usually move slightly eastward from night to

night relative to the stars.

• But sometimes they go westward relative to the stars

for a few weeks: apparent retrograde motion.

retrograde motion

retrograde motion

For superior planet (orbit beyond Earth e.g. Mars )

Occures when Earth catches up to and passes

More on planetary motion