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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