1. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Lecture 3
The Seasons
Phases of the Moon
Eclipses
Constellations
The “Rotating Sky”
This course can be used to satisfy the Physical Science breadth requirement in the College of Letters & Science
3 Units
The Planets
Prof. Geoff Marcy
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2. © 2005 Pearson Education Inc., publishing as Addison-Wesley
MasteringAstronomy
Homework
Due Tomorrow
Friday, Sept. 6pm
Chapter 1 and Chapter 2 Assignments
There will be NO sections this week, but pls. read Ch. 1 & Ch. 2 of text by next Tuesday
You should get an account in Blackboard, where these announcements and assignments are posted
This is not a mathematical course, but there is simple arithmetic such as used for filling out taxes. Also powers of ten: we will be discussing.
The main source of material is the book, but lectures tell you what is important (and fill out detail – the book is brief)
Falling behind in reading or homework makes life very tough for you – It’s really hard to cram this material at the end!
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3. © 2005 Pearson Education Inc., publishing as Addison-Wesley
Next Week’s Homework
Due Friday Sept. 6pm
Chapter 3 Assignment in
MasteringAstronomy
There will be NO sections this week, but pls. read Ch. 1 & Ch. 2 of text by next Tuesday
You should get an account in Blackboard, where these announcements and assignments are posted
This is not a mathematical course, but there is simple arithmetic such as used for filling out taxes. Also powers of ten: we will be discussing.
The main source of material is the book, but lectures tell you what is important (and fill out detail – the book is brief)
Falling behind in reading or homework makes life very tough for you – It’s really hard to cram this material at the end!
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4. . N W 8pm in Sept. Cygnus Venus S Big Dipper Saturn Facing South
(West is to your Right)
8pm
Big Dipper
Cygnus
Vega
Bright white star . W
Arcturus
Bright yellow-orange Star
Venus
Saturn S

5. Cosmic Signposts Map of the Night Sky
Current Night Sky:
Find Vega, Arcturus, Cygnus, and
The Big Dipper on the Map!
Night-time Fun:
Find the four objects in the real night sky.
Do you see anything else interesting
in the night sky?
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6. . N W 8pm in Sept. S Cygnus Venus Find: Vega Arcturus Cygnus
Facing South
(West is to your Right)
8pm
Big Dipper
Cygnus
Vega
Bright white star . W
Arcturus
Bright yellow-orange Star
Venus
Find:
Vega
Arcturus
Cygnus
Big Dipper
Venus
Saturn S

7. Astronomical Observations You’ve Already Made
Sun rises in east, goes overhead, sets in west
Moon and Stars also rise in east, set in west
In Summer: Sun rises higher & Stays up longer.
Seasons occur regularly, every year
Moon goes through phases (new, crescent, quarter, gibbous, full, )
Tides are sometimes high, sometimes low
Separate Phenomena?
Is there a simple theory
that connects these observations ?
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Constellations:
A Pattern or grouping of stars.
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Constellations
No physical significance!
Stars that appear close together
may lie at very different distances. .
Useful only as
rough landmarks.
Names come from
Ancient Arabs and Greeks. .
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The Celestial Sphere
North
Celestial
Pole
The sky above looks like a dome…a hemisphere.
Imaginary “celestial sphere”
This a 2-dimensional representation of the sky (missing distances !)
Earth at center of this sphere. .
Celestial Equator
North Celestial Pole
Celestial Equator
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The Celestial Sphere
North & South celestial poles:
North Celestial Pole
Near North Star
The points in the sky directly above
the Earth’s North and South poles. .
Celestial equator:
The extension of the Earth’s equator
onto the celestial sphere
Ecliptic:
Sun’s path against the star background
Orbital plane of Earth
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12. Section 2 Motions during a Day and Year of the Night Sky
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Earth’s Annual Motion
Earth orbits the Sun, causing the Sun to appear to move eastward with respect to the background stars.
The Sun appears to circle the celestial sphere once every year.
Sun appears in Aquarius on Feb 21
Sun appears in
Capricorn on Jan 21 . . .
Earth
On June 21, the Sun
appears in:
Taurus
Pisces
Cancer
Gemini
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The Milky Way
Band of light across the sky.
What are we actually seeing?
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The
Milky Way
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The Milky Way
Our Galaxy is shaped like a disk
(with spiral arms).
Our solar system is in that disk.
Band of Light:
Looking along that disk.
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The Apparent Daily Rotation of the Sky All Objects rise in the East, set in west
Reason: The Earth Spins on axis.
The sky just appears to rotate in the opposite direction.
The sky appears to rotate around the N (or S) celestial poles.
If you are standing at the poles, objects in sky don’t rise or set.
Standing at the equator, everything rises & sets 90 to the horizon.
Quiz: Sun rises in East.
Does the Earth’s rotation carry you toward the east or the west ?
East
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The Daily Motion
The elevation above horizon of the celestial pole
= [your latitude: 37o].
All stars located at an angle less than your latitude away from:
your celestial pole never set. (circumpolar)
the other celestial pole is never seen by you.
Other stars, (& Sun, Moon, planets)
rise in East and set in West
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The Daily Motion
Daily circles of Stars --- CCW looking north, W N E
Looking North
Looking South
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20. Time Lapse Photography
Quiz:
Exposure Time:
1 min
15 min
2 hr
6 hr
6 hours
Arcs are ~10% of a circle:
10% of 24 hours = 2.4 hours
From Hawaii, looking toward South:
Which direction did
stars rotate, CW or CCW ?
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Seasons
What is the cause of the seasons on Earth?
Earth’s axis is tilted
Earth’s distance from Sun varies during the year
Sun’s brightness varies during the year
The Greenhouse effect varies during the year
Observation:
When it’s winter in the northern hemisphere,
it’s summer in the southern hemisphere.
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The Earth orbits around the Sun once every year.
Elliptical Orbit (not circle)
The Earth’s axis is tilted by 23.5º!
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Tilt of Earth’s Axis
The Earth’s axis is tilted 23.5° from being perpendicular to the ecliptic plane (Earth’s orbital plane).
So, the celestial equator is tilted 23.5° to the ecliptic.
As seen from Earth, the Sun spends 6 months north of the celestial equator and 6 months south of the celestial equator.
Seasons are caused by the Earth’s axis tilt, not the distance from the Earth to the Sun!
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24. The Cause of the Seasons
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25. Axis tilt causes uneven heating by sunlight throughout the year.
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When is summer?
Summer solstice occurs June 21:
The first day of summer and longest day of year.
Earth’s spin axis tilted toward Sun
It takes time for the direct sunlight to heat up the land and water.
Therefore, July & August are typically hotter than June.
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27. Why distance to the Sun Doesn’t Matter for Earth :
Small distance variation for Earth — about 3%
Distance does matter for some other planets, notably Mars, Mercury, and Pluto).
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28. Section 3 Phases of the Moon & Eclipses
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The Moon
Why do we see phases of the Moon?
What conditions are necessary for an eclipse?
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Phases of the Moon:
29.5 day cycle
Time:
One Month
new
crescent
first quarter
gibbous
full
last quarter
Waxing
(getting bigger)
Waning:
(Getting Smaller)
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Why do we see phases?
Half the Moon is illuminated by Sun and half is dark
We see some combination of the bright and dark faces
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32. Demo: Phases of the Moon
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Phases of the Moon
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Quiz
Suppose this evening at 7:30pm (sunset), you saw the moon above the western horizon. What was its phase?
Waning Quarter
Waxing Quarter
Waning Crescent
Waxing Crescent
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Eclipses
Solar Eclipse: Moon blocks the Sun
Sun is behind the moon:
Only at new moon
You are in the Moon’s shadow.
within umbra: total solar eclipse
within penumbra: partial solar eclipse
Lunar eclipse: Earth blocks Sunlight to Moon
Happens only at full moon
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Solar Eclipse
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Lunar Eclipse
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Eclipses
The Earth & Moon cast shadows.
When either passes through the other’s shadow, we have an eclipse.
Why don’t we have an eclipse every full & new Moon?
Sun
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Eclipse Predictions
Eclipses recur every18 yr, 11 1/3 day saros cycle
But even then, eclipse location and type (e.g., partial, total) may vary
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End of Lecture 1
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41. Mysterious Backward Motion of Planets
Why do planets sometimes seem to move backwards relative to the stars?
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42. 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
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43. Apparent retrograde motion of Planets
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Retrograde Motion
Like the Sun, planets usually appear to move eastward relative to the stars.
But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months.
Noticeable over many nights; on a single night, a planet rises in east and sets in west…
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Parallax Angle
Apparent shift of a star’s position due to the Earth’s orbiting of the Sun
The nearest stars are much farther away than the Greeks thought.
So the parallax angles of the star are so small, that you need a telescope to observe them.
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Patterns and Motions
in the Sky
The Circling Night Sky Overhead
the rotation of the Earth about its axis
The Reason for Seasons
Tilt of Earth’s axis as Earth orbits around Sun
“Precession” of the Earth’s Axis
the wobbling of Earth’s axis
The Moon
Orbits around the Earth
Phases of the Moon: We see it illuminated by Sunlight
> Eclipses of the Moon and Sun
day
year
moonth
month
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47. Possible reasons why stellar parallax was undetectable:
Stars are so far away that stellar parallax is too small for naked eye to notice
Earth does not orbit Sun; it is the center of the universe
Unfortunately, with notable exceptions like Aristarchus, the Greeks did not think the stars could be that far away, and therefore rejected the correct explanation (1)…
Thus setting the stage for the long, historical showdown between Earth-centered and Sun-centered systems.
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A Universe in motion
Contrary to our perception, we are not “sitting still.”
We are moving with the Earth.
and not just in one direction
The Earth rotates around it’s axis once every day!
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What have we learned?
What is a constellation?
A constellation is a region of the sky. The sky is divided into 88 official constellations.
What is the celestial sphere?
An imaginary sphere surrounding the Earth upon which the stars, Sun, Moon, and planets appear to reside.
Why do we see a band of light called the Milky Way in our sky?
It traces the Galactic plane as it appears from our location in the Milky Way Galaxy.
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What have we learned?
Describe the basic features of the local sky.
The horizon is the boundary between Earth and sky. The meridian traces a half circle from due south on your horizon, through the zenith (the point directly overhead), to due north on your horizon. Any point in the sky can be located by its altitude and direction.
How does the sky vary with latitude?
As the celestial sphere appears to rotate around us each day, we see different portions of the paths of stars from different latitudes. The altitude of the celestial pole (north or south) is the same as your latitude (north or south).
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What have we learned?
Why are some stars above the horizon at all times?
All stars appear to make a daily circle. Circumpolar stars are those for which their entire daily circles are above the horizon, which depends on latitude.
What is the cause of the seasons on Earth?
As the Earth orbits the sun, the tilt of the axis causes different portions of the Earth to receive more or less direct sunlight at different times of year. The two hemispheres have opposite seasons. The summer solstice is the time when the northern hemisphere gets its most direct sunlight; the winter solstice is the time when the southern hemisphere gets its most direct sunlight. The two hemispheres get equally direct sunlight on the spring and fall equinoxes.
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What have we learned?
Why are the warmest days typically a month after the beginning of summer?
The summer solstice is usually considered the first day of summer, but the warmest days come later because it takes time for the more direct sunlight to heat up the ground and oceans from the winter cold.
How does the night sky change through the year?
The visible constellations at a particular time of night depend on where the Earth is located in its orbit around the Sun.
What is the Earth’s cycle of precession?
A roughly 26,000 year cycle over which the earth’s axis traces a cone as it gradually points to different places in space.
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What have we learned?
Why do we see phases of the Moon?
At any time, half the Moon is illuminated by the Sun and half is in darkness. The face of the Moon that we see is some combination of these two portions, determined by the relative locations of the Sun, Earth, and Moon.
What conditions are necessary for an eclipse?
An eclipse can occur only when the nodes of the Moon’s orbit are nearly aligned with the Earth and the Sun. When this condition is met, we can get a solar eclipse at new moon and a lunar eclipse at full moon.
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What have we learned?
Why were eclipses difficult for ancient peoples to predict ?
There are 3 types of solar eclipse and 3 types of lunar eclipse. Although the pattern of eclipses repeats with the approximately 18-year saros cycle, they do not necessarily repeat with the same type of eclipse and are not necessarily visible from the same places on Earth.
Why do planets sometimes seem to move backwards relative to the stars?
Apparent retrograde motion occurs over a period of a few weeks to a few months as the earth passes by another planet in its orbit.
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What have we learned?
Why did the ancient Greeks reject the idea that the Earth goes around the Sun, even though it offers a more natural explanation for planetary motion?
A major reason was their inability to detect stellar parallax --- the slight shifting of nearby stars against the background of more distant stars that occurs as the Earth orbits the Sun. To most Greeks, it seemed unlikely that the stars could be so far away as to make parallax undetectable to the naked eye, even though that is in fact the case. They instead explained the lack of detectable parallax by imagining the Earth to be stationary at the center of the Universe.
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Measuring the Sky
We measure the sky in angles, not distances.
Full circle = 360º
1º = 60 arcmin
1 arcmin = 60 arcsec
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57. The Local Sky zenith horizon altitude meridian
the point directly above you
horizon
all points 90° from the zenith
altitude
the angle above the horizon
meridian
due north horizon zenith due south horizon
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58. Review: Coordinates on the Earth
Latitude: position north or south of equator
Longitude: position east or west of prime meridian (runs through Greenwich, England)
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Annual Motion
ecliptic
the apparent path of the Sun through the sky
equinox
where the ecliptic intersects the celestial equator
solstice
where the ecliptic is farthest from the celestial equator
zodiac
the constellations which lie along the ecliptic
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60. Why do we see the same face?
Rotation period = orbital period
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61. Measuring Angles in the Sky
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62. Seasonal changes are more extreme at high latitudes
Path of the Sun on the summer solstice at the Arctic Circle
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63. Precession of Earth’s Axis
The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years.
Precession changes the positions in the sky of the celestial poles and the equinoxes.
Polaris won't always be the north star.
The spring equinox,
seen by ancient Greeks in
Aries, moves westward
and is now in Pisces!
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November at 10pm
(West is to your Right) .
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Facing South
(West is to your Right)
10pm
Jupiter .
Pleiades
Sirius
Brightest Star
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Pleiades
Taurus .
Orion
Belt
South
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