Discovering the Universe for Yourself

© 2005 Pearson Education Inc., publishing as Addison-Wesley

From the previous class…• What is smaller a typical planet or a light

second?Light second correspond approximately to a Moon-Earth

distance. Therefore, the answer is a typical planet. ( by the way radius of the Earth is 6,378 km)

• Why the plane travel to Europe is shorter?As you guessed the answer is the wind…


Chapter 2 Discovering the Universe for Yourself

We had the sky, up there, all speckled with stars, and we used to lay on our backs and look up at them, and discuss about whether they was made, or only just happened.

Mark Twain (1835 – 1910)American author, from Huckleberry Finn


In chapter 2 we will study the motions of the stars, sun, moon and planets across our sky.


2.1 Patterns in the Night Sky

Our goals for learning:

• What are constellations?• How do we locate objects in the sky?• Why do stars rise and set?• Why don’t we see the same constellations

throughout the year?


What are constellations?A constellation is a region of the sky.(every point in the sky belongs to some constellation; it is to the

sky as a state is to the US)

88 constellations fill the entire sky.

Official borders set in 1928, by IAU.

Most of the names of northern constellations date back to ancient middle eastern civilizations; the names of southern are mostly given by European explorers in 17th

century.


Thought QuestionThe brightest stars in a constellation…

A. All belong to the same star cluster.B. All lie at about the same distance from

Earth.C. May actually be quite far away from each

other.


The brightest stars in a constellation…

A. All belong to the same star cluster.B. All lie at about the same distance from

Earth.C. May actually be quite far away from

each other.


The Celestial SphereWe lack depth perception when we look into space…it seems like all the stars are surrounding us on a sphere.

The ancient Greeks believed that stars actually lie on a celestial sphere.

There are four special elements on the celestial sphere:

north celestial polesouth celestial polecelestial equatorecliptic


The Celestial Sphere

Question: where would the Earth be on this sphere?Question: Explain the shape of Milky way.

The sphere shows:

star patterns,borders of 88 constellations,ecliptic,celestial equatorand poles.


The Milky WayA band of light making a circle around the celestial sphere.

What is it?Our view into the plane of our galaxy.


How do we locate objects in the sky?Now we move from celestial sphere to the local sky.Question: why does the local sky look like a dome?

Key features on the local sky:Horizon: boundary between earth and skyZenith: the point overhead.Meridian: half circle which contains south, zenith and north.

We can locate any object by itsaltitude (above horizon) and direction (along horizon)


If we want to measure sizes of objects and distances we do it in angles...

Question: why we do not measure actual sizes and distances on the sky? (hint: angular sizes of Moon and Sun are about the same)


Angle measurements:

• Full circle = 360º• 1º = 60′ (arcminutes) • 1′ = 60″ (arcseconds)


Thought QuestionThe angular size of your finger at arm’s length is

about 1°. How many arcseconds is this?

A. 60 arcsecondsB. 600 arcsecondsC. 60 × 60 = 3,600 arcseconds


The angular size of your finger at arm’s length is about 1°. How many arcseconds is this?

A. 60 arcsecondsB. 600 arcsecondsC. 60 × 60 = 3,600 arcseconds


Why do stars rise and set?

Earth rotates east to west, so stars appear to circle from west to east.


Our view from Earth (local sky):• 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 east and

set in west.Question: would you see more circumpolar stars if you would live in Canada or in the US?

Celestial Equator

Your horizon


Thought QuestionWhat is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator


What is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator


Why don’t we see the same constellations throughout the year?

1. Depends on whether you stay travel: Constellations vary with latitude.

2. Depends on time of year: Constellations vary as Earth orbits the Sun.


1) Let’s review how we locate coordinates on the Earth• Latitude: position north or south of equator• Longitude: position east or west of prime meridian (runs

through Greenwich, England)


The sky varies with latitude but not longitude.

If the south celestial pole appears in your sky at an altitude of 34°above your south horizon, your latitude is 34° - important for navigation!


altitude of the celestial pole = your latitude

Star motion:


Thought QuestionThe North Star (Polaris) is 50° above your horizon, due north. Where are you?

A. You are on the equator.B. You are at the North Pole.C. You are at latitude 50°N.D. You are at longitude 50°E.E. You are at latitude 50°N and longitude

50°E.


The North Star (Polaris) is 50° above your horizon, due north. Where are you?

A. You are on the equator.B. You are at the North Pole.C. You are at latitude 50°N.D. You are at longitude 50°E.E. You are at latitude 50°N and longitude 50°E.


2) The (night) 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 the in the sky.

zodiac: constellations along the ecliptic!


Special Topic: How Long is a Day?• Solar day = 24 hours• Sidereal day (Earth’s rotation period) = 23hr, 56min


What have we learned?• What are

constellations?• A region of the sky;

every position on the sky belongs to one of 88 constellations.

• How do we locate objects in the sky?• By its altitude above

the horizon and its direction along the horizon.


What have we learned?• Why do stars rise and set?

• Because of Earth’s rotation. • Why don’t we see the same

constellations throughout the year?• The sky varies with latitude. • The night sky changes as Earth

orbits the Sun.


2.2 The Reason for SeasonsOur goals for learning:• What causes the seasons?• How do we mark the progression of the

seasons?• Does the orientation of Earth’s axis change

with time?


Thought Question

TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter.


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.


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 Earth’s axis tilt.


What causes the seasons?

Sunlight strikes the northern hemisphere at steeper angle(warmer) and it is higher in the sky (longer days).


Axis tilt causes uneven heating by sunlight throughout the year.

Question: Jupiter has an axis tilt of about 3° and nearly circular orbit around the Sun. Does it have seasons?


Summary: The Real Reason for Seasons

• Earth’s axis points in the same direction (to Polaris) all year round, so its orientation relative to the Sun changes as Earth orbits the Sun.

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

• AXIS TILT is the key to the seasons; without it, we would not have seasons on Earth.


Why doesn’t distance matter?• Only small variation of Earth-Sun distance — about 3%;

this small variation overwhelmed by effects of axis tilt.• Note: Some planets have greater distance variation that DOES affect their

seasons — notably Mars, Pluto.

•Question: If the southern and north hemisphere get the same amount of sunlight over the year, how come that seasons are milder in the southern part?


How do we mark the progression of the seasons?Ancient men used to track the change of seasons by observing the Sun’s position in the sky…

To do it now, we define four special points:summer solsticewinter solsticespring (vernal) equinoxfall (autumnal) equinox

Jun 21st December 21st

around March 21st

September 22nd

Question: at what latitude the Sun is overhead during the summer solstice? (hint: tropic of cancer.


We can recognize solstices and equinoxes by Sun’s path across sky:

Equinoxes: Sun rises precisely due east and sets precisely due west. The day is exactly 12 hours long.

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.

The beginnings of seasons coincide with the days of equinox and solstice, as they are seen from northern hemisphere.

Question: How about the Southern hemisphere? And the region of Equator, how do the seasons look like there?


Seasonal changes are more extreme at high latitudes

Path of the Sun on the summer solstice at the Arctic Circle – Sun becomes circumpolar. Question: how does the winter solstice look like at the Arctic Circle?


Does the orientation of Earth’s axis change with time?

Although the axis seems fixed on human time scales, it actually precesses over about 26,000 years.All spinning objects precess, i.e. spinning top.


Question: Is Polaris really a “North Star”?

Question: are equinoxes always on the same position in the orbit?



No! Polaris has been a good North Star only for couple of centuries. In 13,000 years Vega will be a North Star. The axis does not point near any bright star during most of its cycle!




No! Polaris has been a good North Star only for couple of centuries. In 13,000 years Vega will be a North Star. The axis does not point near any bright star during most of its cycle!


No. For example summer equinox used to be in Aries (2000 years ago), but now it is in Pisces. So, if you are born on March 21st, your astrological sign would appear to be Aries, even though, the Sun was really in Pisces when you were born. (horoscope is based on the star sky mapped by Ptolemy, in old Greece).Also, summer solstice is not in Cancer anymore, it is in Gemini.


What have we learned?• What causes the seasons?

• The tilt of the Earth’s axis causes sunlight to hit different parts of the Earth more directly during the summer and less directly during the winter.

• How do we mark the progression of the seasons?• The summer and winter solstices are when the

Northern Hemisphere gets its most and least direct sunlight, respectively. The spring and fall equinoxes are when both hemispheres get equally direct sunlight.


What have we learned?• Does the orientation of the Earth’s axis change with

time?• Yes. The tilt remains about 23.5 degrees (so the season

pattern is not affected), but Earth has a 26,000 year precession cycle that slowly and subtly changes the orientation of the Earth’s axis.


2.3 The Moon, Our Constant Companion

• Why do we see phases of the Moon?• What causes eclipses?



Why do we see phases of the Moon?The Moon is always

half illuminated by Sun and half dark

Depending on the angle under which see the moon we see some combination of the bright and dark faces


Phases of the Moon


Moon Rise/Set by Phase

Different phases of the Moon rise and set at different times above our horizon.

Question: Is it true that we see moon only during the night?


Phases of the Moon: 29.5-day cyclenew

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.

Months is actually a “moonths”


Thought QuestionIt’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?

A. First quarterB. Waxing gibbousC. Third quarterD. Half moon


It’s 9 am. You look up in the sky and see a moon with half its face bright and half dark. What phase is it?

A. First quarterB. Waxing gibbousC. Third quarterD. Half moon


What causes eclipses?• The Earth and Moon cast shadows.• When either passes through the other’s shadow, we have an

eclipse.

Lunar eclipse: when Earth lies directly between Sun and Moon.Solar eclipse: when Moon lies between Sun and Earth, then people living where the Moon shadow falls can not see the Sun.

Light partially blocked

Light completely blocked


Lunar eclipses• Lunar eclipses can occur only at full moon.• Lunar eclipses can be penumbral, partial, or total.

The full moon darkens only slightly.

Lasts about one hour. The Moon appears red because Earth’s atmosphere bends some of the red light toward the Moon.


Lunar Eclipse


Do we have lunar eclipse at every full moon?


No!– The Moon’s orbit is tilted 5° 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.


Solar Eclipses• Solar eclipses can occur only at new moon.• Solar eclipses can be partial, total, or annular.

Moon’s umbra touches the Earth, covering about 270 km in diameter and traveling at a speed of 1,700 km/hr. Eclipse lasts for few minutes.

If the Moon is farther and the shadow does not reach the Earth we see annular eclipse - a ring of sunlight surrounds the disk of the Moon.


Solar Eclipse


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).

AND2. The Moon must be at or near one of the two points in its

orbit where it crosses the ecliptic plane (its nodes).


Predicting EclipsesWe have about two solar eclipses each year, but since positions of the

eclipse seasons slowly move around the orbit, eclipses do not occur in fixed parts of the year.

Eclipses actually recur with the 18 yr, 11 1/3 day saros cycle, but type (e.g., partial, total) and location may vary.

Any two eclipses separated by one saros cycle share very similar geometries. They occur at the same node with the Moon at nearly the same distance from Earth and at the same time of year. Because the saros period is not equal to a whole number of days, its biggest drawback is that subsequent eclipses are visible from different parts of the globe.

Since there are two to five solar eclipses every year, there are approximately forty different saros series in progress at any one time.


Paths of totality for solar eclipses till 2030.


What have we learned?• Why do we see phases of the Moon?

• Half the Moon is lit by the Sun; half is in shadow. • The appearance of the Moon to us is determined by

the Sun, Earth, and Moon positions.• What causes eclipses?

• Lunar eclipse: Earth’s shadow on the Moon. Can be penumbral, partial, or total.

• Solar eclipse: the Moon’s shadow on Earth. Can be partial, total, or annular.

• Tilt of Moon’s orbit means eclipses occur during two periods each year.

• Eclipses recur with the 18 yr, 11 1/3 day saros cycle


Astronomy Picture of the Day for October 16th, 2006.

“The robotic Casini spacecraft now orbiting Saturn recently drifted in giant planet's shadow for about 12 hours and looked back toward the eclipse Sun. Cassini saw a view unlike any other. First, the night side is seen to be partly lit by light reflected from its own majestic ring system. Next, the rings themselves appear dark when silhouetted against Saturn, but quite bright when viewed away from Saturn and slightly scattering sunlight, in the above exaggerated color image.”

Taken from the Nasa site: http://antwrp.gsfc.nasa.gov/apod/ap061016.html


2.4 The Ancient Mystery of the Planets


• What was once so mysterious about the movement of planets in our sky?

• Why did the ancient Greeks reject the real explanation for planetary motion?


Planets Known in Ancient Times• 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 eastward from night to night relative to the stars. – You cannot see this motion on a single night; rather,

planets rise in the east and set in the west.

• But sometimes they go westward for a few weeks or months: apparent retrograde motion


We see apparent retrograde motion when we pass by a planet in its orbit.


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!

• In fact, ancients considered but rejected the correct explanation…


Why did the ancient Greeks reject the real explanation for planetary motion?

• Their inability to observe stellar parallax was a major factor.


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 too small 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 Greeks rejected the correct explanation (1) because they did not think the stars could be that far away…

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


What have we learned?• What was so mysterious about

planetary motion 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, a planet turns westward in its apparent retrograde motion.

• Easy for us to explain: occurs when Earth passes a planet by (“laps it”) in its orbit. But difficult to explain if you think Earth is the center of the universe.


What have we learned?• Why did the ancient Greeks reject the real

explanation for planetary motion?• They could not detect stellar parallax. • Most Greeks concluded that Earth must be

stationary, because they thought the stars could not be so far away as to make parallax undetectable.

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Discovering the Universe for Yourself