9/15/16
Today’s Lecture
! More info about LaunchPad Quizzes and Assignments
! Discovering the Night Sky (Chapter 1)
! There will be no observing session this evening
! the first observing session is scheduled for Thursday evening next week (Sep. 22)
! please remember to sign up on LearningSystem if you would like to attend
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Getting Started on LaunchPad
! All PHYS 1510H students should now have gained access
to LaunchPad
! if you have not yet done so, you should do so ASAP
! Once in LaunchPad, I recommend doing the following:
! check out the e-book
! complete the first LearningCurve
! try the Practice Assignment
! try the Practice Quiz (as many times as you want to)
! The first LearningCurve was due earlier today
! Assignment 1 and Quiz 1 will be available later today
! both will be due on Tue. Sep. 20 at 2 p.m.
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Grading of LaunchPad Assignments
! Tutorial components of the assignments
! you will only be graded on the final “Quick Check” page of
multiple choice questions within a given tutorial
! you can only complete this page once!
! you will not be graded on the earlier pages of the tutorial
! you can visit these pages as many times as you like
! Image Map components of the assignments
! full marks on a question if you get it right the first time
! half marks on a question if you get it correct on a
subsequent attempt
! Interactive Exercises
! graded based on all of your answers
! Animations, Videos, etc.
! full marks are awarded upon completion
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Grading of LaunchPad Quizzes
! LaunchPad quizzes consist of 15 multiple choice
questions
! you are allowed 3 attempts on each quiz
! you will get a different quiz on each attempt, since
questions are drawn at random from a large list of questions
! this is to encourage you to master the material
! only your best score (on a given weekly quiz) will be counted, so you have nothing to lose by trying again
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Academic Integrity
! Online assignments and quizzes are to be completed by
you alone
! just as with an in-class quiz in another course, you should
not be asking classmates or friends for help
! It is acceptable to talk to your classmates when
learning the material
! in fact, I would encourage you to find classmates with
whom to discuss the material you encounter in this course
! However, you should not be asking for help when you
are being graded (i.e., when completing assignments and quizzes)
! Please see the course syllabus and Trent’s Academic Integrity Policy (or ask me) for further information
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DISCOVERING THE NIGHT SKY
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Topics To Be Covered in this Lecture
! I am assuming that you have read Chapter 1 in advance
of this lecture
! I will not attempt to cover every topic from Chapter 1 during this lecture
! I will focus on the following in this lecture:
! constellations and asterisms
! the motions of stars in the night sky
! the celestial sphere
! the seasons
! phases of the moon
! lunar eclipses
! solar eclipses
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Stars in the Night Sky
! On a clear dark night, several
thousand celestial objects can be seen with the naked eye
! almost all of these objects are
Milky Way stars
! Most of these stars are similar
to the Sun, but much further away
! this is why they appear to be
much fainter than the Sun
! this also explains why they don’t move noticeably with respect to one another
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Patterns in the Night Sky
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Constellations and Asterisms
! Many ancient civilizations identified and named
various patterns of stars in the sky
! these patterns are known as constellations or asterisms
! such patterns are very useful for getting to know your
way around the night sky
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Modern Day Constellations
! In the past, many different names were assigned to various
patterns in the sky
! There are now 88 internationally agreed-upon
constellations
! these constellations cover the entire sky
! they include some recognizable patterns as well as the
surrounding regions of the sky
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Asterisms
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The Motion of Stars in the Sky
! While stars do not move
noticeably with respect to one another, they do appear to move across the sky during the night
! This is because the Earth rotates on its axis
! each rotation takes 23 hours
and 56 minutes
! this is called a sidereal day
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Star Motions Explained
! The east-to-west motions of stars at the equator
results from the west-to-east rotation of the Earth
! The circular motion of stars above Earth’s poles results from these positions being aligned with the Earth’s rotation axis
! The Earth’s rotation also explains the rising and setting of the Sun and Moon
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The Celestial Sphere
! The motion of stars due to
Earth’s rotation makes it more difficult to map the sky
! We will therefore use a map which is not affected by the Earth’s rotation
! Imagine a giant empty sphere that surrounds the Earth and rotates around us
! We will refer to this as the celestial sphere
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Orientation of the Celestial Sphere
! Earth’s rotation axis gives rise
to a natural orientation for the celestial sphere
! The celestial north and south poles are the outward extensions of the Earth’s poles
! they are directly overhead at
the Earth’s poles
! The celestial equator is the
outward extension of the Earth’s equator
! it is overhead everywhere
along the Earth’s equator
! The celestial sphere rotates with
the stars
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Polaris: The North Star
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Locating Objects on the Sky
! Since most objects appear to
be stationary on the celestial sphere, we can identify their positions on this new map
! Geographers use coordinates of longitude and latitude to identify positions on the Earth’s surface
! Astronomers use coordinates called right ascension and declination to identify positions on the inside of the celestial sphere
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Earth’s Orbit Around the Sun
! The Earth travels around the Sun once every year
! its path through space is called its orbit
! This motion means that the Sun appears to move with
respect to the distant stars
! its position on the celestial sphere changes
! it traces out a circle on the celestial sphere, taking one
year to travel around the circle
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The Ecliptic
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The Zodiac
! As the Sun appears to
move along the ecliptic, it travels through 13 constellations
! They are called constellations of the Zodiac
Note: precession (see section 1-10 from text) causes these dates to differ from those used by astrologers ...
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The Tilt of the Earth’s Axis
! The Sun does not travel along
the celestial equator, as you might have expected
! this is because the Earth’s
rotation axis is not lined up with the ecliptic plane
! Earth’s rotation axis is tilted by
about 23.5 degrees with respect to the ecliptic plane
! all of the other planets in our
solar system are also tilted with respect to their orbits (some much more than others)
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The Seasons
! The tilt of the Earth’s rotation axis is responsible for
the phenomenon of seasons on the Earth
! As the Earth orbits the Sun, its orientation with respect to the stars stays the same
! However, its orientation with respect to the Sun changes throughout the year
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What If Earth’s Axis Wasn’t Tilted?
! Every place on Earth would
always have equal length days and nights (12 hrs each)
! It would still be warmest at the equator, and coolest at the North and South pole
! There would be no seasons!
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Heat From the Sun
There are two main factors which determine the overall temperature at a given latitude on the Earth:
! The higher the Sun is above the horizon, the more effective it is at heating that part of the Earth’s surface
! The longer the Sun is above the horizon, the more time it has to heat the surface
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Special Places in Earth’s Orbit
See LaunchPad Animation 1.4
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The Summer Solstice
! Occurs around June 21 in the northern hemisphere
! the southern hemisphere has its winter solstice at this time
! All locations in the northern hemisphere have their
longest day of the year at this time
! all locations north of the Arctic Circle have 24 hours of
daylight at this time
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Equinoxes
! Midway between the summer and winter solstices lie
two times when there is equal length day and night everywhere on Earth
! The vernal equinox occurs when the Sun moves north across the celestial equator
! this is also called the spring equinox
! this occurs on around March 20
! The autumnal equinox occurs when the Sun moves south
across the celestial equator
! this occurs on around Sept. 22
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Important Things to Remember
! If the Earth’s axis was not tilted, we would not have
seasons
! The seasons do not result from the Earth being closer to the Sun during the summer
! if this were true, both hemispheres would experience
summer at the same time
! the Earth is actually closest to the Sun during the month of January
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The Phases of the Moon
! Moonlight is simply reflected
sunlight
! The illumination of the Moon
as seen from the Earth changes as the Moon orbits the Earth
! If the Moon is opposite the Sun in our sky, it will be fully illuminated (a Full Moon)
! If the Moon is in the same direction as the Sun, it will not be illuminated by the Sun (a New Moon)
See LaunchPad Animation 1.5
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The Moon’s Phases
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Locating the Moon in the Sky
! If you know the current phase of the Moon, you can
predict where it will be in our sky
! A full Moon will be opposite the Sun
! e.g., a full moon rises in the east at sunset
! A thin crescent Moon will always be close to the Sun in
the sky
! e.g., low in the west at sunset or low in the east at dawn
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What Is the Moon’s Phase In This Image?
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Earth and Moon Seen From Space
34 This image was taken by the DSCOVR spacecraft during a New Moon phase.
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Why Can We See the Dark Side of the Moon in this Image?
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Earthshine
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Lunar Eclipses
! If the Moon orbited the Earth in the ecliptic plane, the
Earth-Moon-Sun would line up twice during each lunar month
! Instead, the Moon’s orbit is inclined by 5 degrees with respect to the ecliptic
! it spends half of its time above the ecliptic and half of its
time below the ecliptic
! when it passes through the ecliptic plane, it may line up
directly with the Earth and Sun, causing a lunar eclipse
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Earth’s Umbra and Penumbra
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3 Types of Lunar Eclipse
! A total eclipse occurs when the entire Moon passes
through Earth’s umbra
! A partial eclipse occurs when only part of the Moon passes through Earth’s umbra
! A penumbral eclipse occurs when the Moon passes through the penumbra only
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The Eclipsed Moon
A partial eclipse A total eclipse
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Solar Eclipse
! A solar eclipse occurs when
the Moon is directly between the Sun and Earth
! this can occur only if the
Moon crosses the ecliptic during the new Moon phase
! In this case, the Earth moves
through the Moon’s shadow, temporarily reducing the amount of sunlight it receives
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The Angular Size of the Sun and Moon
! The Sun and Moon have a similar angular (or apparent)
size in the sky
! this coincidence results from the Sun being about 400
times larger than the Moon and also about 400 times further away
! The Moon’s distance varies somewhat during its orbit
! sometimes it is close enough to us that it appears to be slightly larger than the Sun
! most of the time it appears to be a bit smaller
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The Moon at Apogee & Perigee
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The Moon’s Umbra & Penumbra
! If the Moon is close enough to us, it is just able to
completely cover the Sun
! this phenomenon is called a total eclipse
! in this case, the Moon’s umbra barely reaches Earth
! Locations outside the umbra but within the penumbra
see a partial eclipse of the Sun
! More commonly, the Moon is unable to completely cover the Sun
! only the penumbra reaches the earth
! this is called an annular eclipse
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Total Solar Eclipse
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A Solar Eclipse
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The Moon’s Umbra
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The Earth During a Total Solar Eclipse
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A Recent Solar Eclipse
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Annular Eclipse
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An Annular Eclipse of the Sun
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Two Different Total Solar Eclipses
Note how the size of the umbra is different from one eclipse to another.52
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An Annular Eclipse on Mars
These three images show the Martian moon Phobos passing in front of the Sun, as seen by the Mars Curiosity Rover on the surface of Mars.
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Summary
The main cycles we see in the sky and experience on the Earth are caused by three celestial motions:
! Earth’s rotation (~24 hours) is responsible for night and
day, and is largely responsible for motions of stars and other objects through the sky
! Earth’s orbit around the Sun (~365 days) defines our year and the seasons
! The Moon’s orbit around the Earth (~27 days) causes the Moon’s phases and eclipses
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Before Our Next Lecture
! Finish reading Chapter 1 (if you haven’t already)
! Complete Assignment 1 and Quiz 1
! due on Tue. Sep. 20 at 2 p.m.
! Read Chapter 2 (we will cover it in class next week)
! Complete LearningCurve: Gravitation and the Motion of
the Planets
! due on Thu. Sep. 22 at noon
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