Ch. 2.pdf

8/14/2019 Ch. 2.pdf

1/65

Chapter 2

Discovering the Universe for Yourself

8/14/2019 Ch. 2.pdf

2/65

2.1 Patterns in the Night Sky

What does the universe look like from

Earth?

Why do stars rise and set?

Why do the constellations we see

depend on latitude and time of year?

Our goals for learning:

8/14/2019 Ch. 2.pdf

3/65

What does the universe look

like from Earth?

With the naked

eye, we can see

more than 2,000stars as well as

the Milky Way.

8/14/2019 Ch. 2.pdf

4/65

Constellations

A constellation isa region of the

sky.

88 constellations

fill the entire sky.

8/14/2019 Ch. 2.pdf

5/65

Thought Question

The brightest stars in a

constellation

A. All belong to the same star cluster.

B. All lie at about the same distance fromEarth.

C. May actually be quite far away from

each other.

8/14/2019 Ch. 2.pdf

6/65

The Celestial Sphere

Stars at different

distances all appear

to lie on thecelestial sphere.

Ecliptic is Suns

apparent path

through the

celestial sphere.

8/14/2019 Ch. 2.pdf

7/65

The Celestial Sphere

The 88 official

constellations

cover thecelestial sphere.

8/14/2019 Ch. 2.pdf

8/65

The Milky Way

A band of lightmaking a circle

around the

celestial sphere.

What is it?

Our view into theplane of our

galaxy.

8/14/2019 Ch. 2.pdf

9/65

The Milky Way

8/14/2019 Ch. 2.pdf

10/65

The Local Sky

An objects altitude (above horizon) and

direction (along horizon) specifies its location

in your local sky

8/14/2019 Ch. 2.pdf

11/65

The Local Sky

Zenith: The pointdirectly overhead

Horizon: All points

90 away fromzenith

Meridian: Line

passing throughzenith and

connecting N and S

points on horizon

8/14/2019 Ch. 2.pdf

12/65

We measure the sky using angles

8/14/2019 Ch. 2.pdf

13/65

Angular Measurements

Full circle = 360 1 = 60 (arcminutes)

1 = 60 (arcseconds)

8/14/2019 Ch. 2.pdf

14/65

Thought QuestionThe angular size of your finger at arms length

is about 1 . How many arcseconds is this?

A. 60 arcseconds

B. 600 arcsecondsC. 60 60 = 3,600 arcseconds

8/14/2019 Ch. 2.pdf

15/65

Angular Size

angular size = physical size360 degrees

2 distance

An objects angular

size appears smaller ifit is farther away.

8/14/2019 Ch. 2.pdf

16/65

Why do stars rise and

set?

Earth rotates west to east,

so stars appear to circle

from east to west.

8/14/2019 Ch. 2.pdf

17/65

Our view from Earth:

Stars near the north celestial pole are circumpolar

and never set. We cannot see stars near the south celestial pole.

All other stars (and Sun, Moon, planets) rise in eastand set in west.

Celestial Equator

Your Horizon

A circumpolar

star never

sets

This star

never rises

8/14/2019 Ch. 2.pdf

18/65

Thought QuestionWhat is the arrow pointing to?

A. the zenith

B. the north

celestial pole

C. the celestial

equator

8/14/2019 Ch. 2.pdf

19/65

Why do the constellations we see

depend on latitude and time of year?

They depend on latitude because your

position on Earth determines which

constellations remain below the horizon. They depend on time of year because Earths

orbit changes the apparent location of the

Sun among the stars.

8/14/2019 Ch. 2.pdf

20/65

Review: Coordinates on the Earth

Latitude: position north or south of equator

Longitude: position east or west of primemeridian (runs through Greenwich, England)

8/14/2019 Ch. 2.pdf

21/65

The sky varies with latitude

but not longitude.

8/14/2019 Ch. 2.pdf

22/65

Altitude of the celestial pole =

your latitude

8/14/2019 Ch. 2.pdf

23/65

8/14/2019 Ch. 2.pdf

24/65

The sky varies as Earth orbits the Sun

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

eastward along the ecliptic. At midnight, the stars on our meridian are opposite the

Sun in the sky.

8/14/2019 Ch. 2.pdf

25/65

What have we learned?

What does the universe look like fromEarth?

We can see over 2,000 stars and the Milky

Way with our naked eyes.

Each position on the sky belongs to one of88 constellations.

We can specify the position of an object in

the local sky by its altitude above the

horizon and its direction along the horizon.

Why do stars rise and set?

Because of Earths rotation.

8/14/2019 Ch. 2.pdf

26/65

8/14/2019 Ch. 2.pdf

27/65

2.2 The Reason for Seasons

Our goals for learning:

What causes the seasons?

How do we mark the progression of the

seasons?

How does the orientation of Earths axis

change with time?

8/14/2019 Ch. 2.pdf

28/65

TRUE OR FALSE?

Earth is closer to the Sun in summer

and farther from the Sun in winter.

Hint: When it is summer in the U.S.,

it is winter in Australia.

Thought Question

8/14/2019 Ch. 2.pdf

29/65

TRUE OR FALSE!

Earth is closer to the Sun in summer

and farther from the Sun in winter.

Seasons are opposite in the N and S

hemispheres, so distance cannot be the reason.

The real reason for seasons involves Earths

axis tilt.

Thought Question

8/14/2019 Ch. 2.pdf

30/65

What causes the seasons?

Seasons depend on how Earths axis affects

the directness of sunlight.

8/14/2019 Ch. 2.pdf

31/65

Direct light causes more heating.

A i tilt h di t f

8/14/2019 Ch. 2.pdf

32/65

Axis tilt changes directness of

sunlight during the year.

S ltit d l h ith

8/14/2019 Ch. 2.pdf

33/65

Suns altitude also changes with

seasons

Suns position at noon in

summer: higher altitude

means more direct

sunlight.

Suns position at noon inwinter: lower altitude

means less direct

sunlight.

8/14/2019 Ch. 2.pdf

34/65

Summary: The Real Reason for

Seasons Earths axis points in the same direction (to

Polaris) all year round, so its orientationrelative to the Sun changes as Earth orbits

the Sun.

Summer occurs in your hemisphere whensunlight hits it more directly; winter occurswhen the sunlight is less direct.

AXIS TILT is the key to the seasons; withoutit, we would not have seasons on Earth.

8/14/2019 Ch. 2.pdf

35/65

Why doesntdistance matter?

Variation of Earth-Sun distance is small about 3%; this small variation is overwhelmed

by the effects of axis tilt.

8/14/2019 Ch. 2.pdf

36/65

How do we mark the progression of the seasons?

We define four special points:

summer solsticewinter solstice

spring (vernal) equinox

fall (autumnal) equinox

8/14/2019 Ch. 2.pdf

37/65

We can recognize solstices and equinoxes by

Suns path across sky:

Summer solstice:

Highest path, rise and

set at most extreme

north of due east.

Winter solstice: Lowest

path, rise and set at

most extreme south of

due east.

Equinoxes: Sun rises

precisely due east and

sets precisely due

west.

8/14/2019 Ch. 2.pdf

38/65

Seasonal changes are more

extreme at high latitudes

Path of the Sun on the summer solstice

at the Arctic Circle

How does the orientation of Earths

8/14/2019 Ch. 2.pdf

39/65

How does the orientation of Earth s

axis change with time?Although the axis seems fixed on human time

scales, it actually precesses over about 26,000years.

Polaris wont always be the North Star.

Positions of equinoxes shift around orbit; e.g.,

spring equinox, once inAries, is now in Pisces!

Earths axis

precesses

like the axis

of a spinning

top

8/14/2019 Ch. 2.pdf

40/65

What have we learned?

What causes the seasons? The tilt of the Earths axis causes

sunlight to hit different parts of the Earth

more directly during the summer and less

directly during the winter.

We can specify the position of an object

in the local sky by its altitude above the

horizon and its direction along thehorizon.

8/14/2019 Ch. 2.pdf

41/65

What have we learned? How do we mark the progression of the

seasons? The summer and winter solstices are when

the Northern Hemisphere gets its most and leastdirect sunlight, respectively. The spring and fallequinoxes are when both hemispheres get

equally direct sunlight.

How does the orientation of Earths axischange with time? The tilt remains about 23.5 degrees (so the

season pattern is not affected), but Earth has a26,000 year precession cycle that slowly andsubtly changes the orientation of the Earthsaxis.

2 3 The Moon

8/14/2019 Ch. 2.pdf

42/65

2.3 The Moon,

Our Constant Companion

Why do we see phases of the Moon?

What causes eclipses?

Our goals for learning:

8/14/2019 Ch. 2.pdf

43/65

Why do we see phases of the

Moon?

Lunar phases are

a consequence of

the Moons 27.3-

day orbit aroundEarth

8/14/2019 Ch. 2.pdf

44/65

Phases of Moon

Half of Moon is

illuminated by Sun

and half is dark

We see a

changing

combination of the

bright and darkfaces as Moon

orbits

8/14/2019 Ch. 2.pdf

45/65

Phases of the Moon

8/14/2019 Ch. 2.pdf

46/65

Phases of the Moon: 29.5-day cycle

new

crescent

first quarter

gibbous

full

gibbous

last quarter

crescent

waxing Moon visible in afternoon/evening.

Gets fuller and rises later each day.

waning Moon visible in late night/morning.

Gets less and sets later each day.

}

}

Thought Question

8/14/2019 Ch. 2.pdf

47/65

Thought Question

A. First quarter

B. Waxing gibbousC. Third quarter

D. Half moon

Its 9 am. You look up in the sky and see

a moon with half its face bright and half

dark. What phase is it?

8/14/2019 Ch. 2.pdf

48/65

We see only one side of Moon

Synchronous rotation:

the Moon rotates

exactly once with eachorbit

That is why only one

side is visible fromEarth

8/14/2019 Ch. 2.pdf

49/65

What causes eclipses?

The Earth and Moon cast shadows.

When either passes through the others

shadow, we have an eclipse.

L E li

8/14/2019 Ch. 2.pdf

50/65

Lunar Eclipse

8/14/2019 Ch. 2.pdf

51/65

When can eclipses occur?

Lunar eclipsescan occur only at

full moon.

Lunar eclipsescan be

penumbral,

partial, or total.

S l E li

8/14/2019 Ch. 2.pdf

52/65

Solar Eclipse

8/14/2019 Ch. 2.pdf

53/65

When can eclipses occur?

Solar eclipses canoccur only at newmoon.

Solar eclipses can be

total, partial, orannular.

Wh d h li d f ll

8/14/2019 Ch. 2.pdf

54/65

Why dont we have an eclipse at every new and full

moon?

The Moons orbit is tilted 5 to ecliptic plane

So we have about two eclipse seasons each year, with

a lunar eclipse at full moon and solar eclipse at newl

moon.

8/14/2019 Ch. 2.pdf

55/65

Summary: Two conditions must be

met to have an eclipse:1. It must be full moon (for a lunar eclipse) or new

moon (for a solar eclipse).

AND

2. The Moon must be at or near one of the two pointsin its orbit where it crosses the ecliptic plane (itsnodes).

Predicting Eclipses

8/14/2019 Ch. 2.pdf

56/65

Predicting Eclipses Eclipses recur with the 18 yr, 11 1/3 day

saros cycle, but type (e.g., partial, total)and location may vary.

Wh t h l d?

8/14/2019 Ch. 2.pdf

57/65

What have we learned?

Why do we see phases of the Moon? Half the Moon is lit by the Sun; half is in

shadow, and its appearance to us is

determined by the relative positions of

Sun, Moon, and Earth What causes eclipses?

Lunar eclipse: Earths shadow on the

Moon Solar eclipse: Moons shadow on Earth

Tilt of Moons orbit means eclipses occur

during two periods each year

2 4 The Ancient Mystery of the Planets

8/14/2019 Ch. 2.pdf

58/65

2.4 The Ancient Mystery of the Planets

What was once so mysterious aboutplanetary motion in our sky?

Why did the ancient Greeks reject the realexplanation for planetary motion?

Our goals for learning:

8/14/2019 Ch. 2.pdf

59/65

Planets Known in Ancient Times

Mercury

difficult to see; alwaysclose to Sun in sky

Venus very bright when visible;

morning or evening star Mars

noticeably red

Jupiter very bright

Saturn moderately bright

8/14/2019 Ch. 2.pdf

60/65

We see apparent retrograde motion

8/14/2019 Ch. 2.pdf

61/65

We see apparent retrograde motion

when we pass by a planet in its orbit.

8/14/2019 Ch. 2.pdf

62/65

Explaining Apparent Retrograde Motion

Easy for us to explain: occurs when we

lap another planet (or when Mercury

or Venus laps us) But very difficult to explain if you think

that Earth is the center of the universe!

In fact, ancients considered but rejected thecorrect explanation

8/14/2019 Ch. 2.pdf

63/65

Why did the ancient Greeks reject the

real explanation for planetary motion?

Their inability to observe stellar parallax was a major factor.

8/14/2019 Ch. 2.pdf

64/65

The Greeks knew that the lack of observable

parallax could mean one of two things:

1. Stars are so far away that stellar parallax is toosmall to notice with the naked eye

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

universe

With rare exceptions such as Aristarchus, the Greeksrejected the correct explanation (1) because theydid not think the stars could be thatfar away

Thus setting the stage for the long, historical showdownbetween Earth-centered and Sun-centeredsystems.

What have we learned?

8/14/2019 Ch. 2.pdf

65/65

What have we learned?

What was so mysterious about planetarymotion in our sky?

Like the Sun and Moon, planets usually drift

eastward relative to the stars from night to night;

but sometimes, for a few weeks or few months, aplanet turns westward in its apparent

retrograde motion

Why did the ancient Greeks reject the real

explanation for planetary motion? Most Greeks concluded that Earth must be

stationary, because they thought the stars could

not be so far away as to make parallax