Page 1
© 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…
Page 2
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 3
© 2005 Pearson Education Inc., publishing as Addison-Wesley
In chapter 2 we will study the motions of the stars, sun, moon and planets across our sky.
Page 4
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 5
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 6
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 7
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 8
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 9
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 10
© 2005 Pearson Education Inc., publishing as Addison-Wesley
The Milky WayA band of light making a circle around the celestial sphere.
What is it?Our view into the plane of our galaxy.
Page 11
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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)
Page 12
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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)
Page 13
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Angle measurements:
• Full circle = 360º• 1º = 60′ (arcminutes) • 1′ = 60″ (arcseconds)
Page 14
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 15
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 16
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Why do stars rise and set?
Earth rotates east to west, so stars appear to circle from west to east.
Page 17
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 18
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Thought QuestionWhat is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator
Page 19
© 2005 Pearson Education Inc., publishing as Addison-Wesley
What is the arrow pointing to?A. the zenithB. the north celestial poleC. the celestial equator
Page 20
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 21
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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)
Page 22
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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!
Page 23
© 2005 Pearson Education Inc., publishing as Addison-Wesley
altitude of the celestial pole = your latitude
Star motion:
Page 24
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 25
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 26
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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!
Page 27
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Special Topic: How Long is a Day?• Solar day = 24 hours• Sidereal day (Earth’s rotation period) = 23hr, 56min
Page 28
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 29
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 30
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 31
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Thought Question
TRUE OR FALSE? Earth is closer to the Sun in summer and farther from the Sun in winter.
Page 32
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 33
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 34
© 2005 Pearson Education Inc., publishing as Addison-Wesley
What causes the seasons?
Sunlight strikes the northern hemisphere at steeper angle(warmer) and it is higher in the sky (longer days).
Page 35
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 36
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 37
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 38
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 39
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 40
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 41
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 42
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Question: Is Polaris really a “North Star”?
Question: are equinoxes always on the same position in the orbit?
Page 43
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Question: Is Polaris really a “North Star”?
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!
Question: are equinoxes always on the same position in the orbit?
Page 44
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Question: Is Polaris really a “North Star”?
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!
Question: are equinoxes always on the same position in the orbit?
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.
Page 45
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 46
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 47
© 2005 Pearson Education Inc., publishing as Addison-Wesley
2.3 The Moon, Our Constant Companion
• Why do we see phases of the Moon?• What causes eclipses?
Our goals for learning:
Page 48
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 49
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Phases of the Moon
Page 50
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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?
Page 51
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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”
Page 52
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 53
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 54
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 55
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 56
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Lunar Eclipse
Page 57
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Do we have lunar eclipse at every full moon?
Page 58
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 59
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 60
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Solar Eclipse
Page 61
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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).
Page 62
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 63
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Paths of totality for solar eclipses till 2030.
Page 64
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 65
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 66
© 2005 Pearson Education Inc., publishing as Addison-Wesley
2.4 The Ancient Mystery of the Planets
Our goals for learning:
• 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?
Page 67
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 68
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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
Page 69
© 2005 Pearson Education Inc., publishing as Addison-Wesley
We see apparent retrograde motion when we pass by a planet in its orbit.
Page 70
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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…
Page 71
© 2005 Pearson Education Inc., publishing as Addison-Wesley
Why did the ancient Greeks reject the real explanation for planetary motion?
• Their inability to observe stellar parallax was a major factor.
Page 72
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 73
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.
Page 74
© 2005 Pearson Education Inc., publishing as Addison-Wesley
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.