I.
Chapter 2: Discovering the Universe for Yourself
a. What does the universe look like from Earth?
i. Constellations
 Constellation: region of the sky with well-defined borders; the familiar
patterns of stars merely help us locate these constellations.
 There are 88 official constellations
 The names were chosen by members of the International
Astronomical Union in 1928
 Sirius, Procyon, and Betelgeuse form the winter triangle
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Winter triangle: spans several constellations and consist of Sirius,
Procyon, and Betelgeuse.
Constellation map of the entire sky.
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Orientated so that the milky way’s center is at the center of the map
and the milky way disk stretches from left to right across the map
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Red lines mark official borders of several constellations near Orion.
Yellow lines connect recognizable patterns of stars.
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Winter Triangle (looking south) on winter evenings from Northern
Hemisphere.
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Star charts
 Round outside edge is the horizon
 Compass directions are marked in yellow
 Turn chart so the edge marked with the direction you’re facing
is down
 The stars above this horizon now match the stars you are facing
 Center of chart is the sky overhead
 A star on the chart halfway from edge to the center is halfway
from the horizon to straight up
ii. The Celestial Sphere
 Some stars (in a constellation) might look like they’re really close to
each other but they could actually be really far apart if they are at
different distances from earth
 This illusion happens because we lack depth perception when it comes
to space, because the stars are so far away.
 Ancient Greeks thought that the illusion was reality and thought that
the stars and constellations are on a great celestial sphere that
surrounds earth.
 Celestial Sphere: The imaginary sphere on which objects in the sky
appear to reside when observed from Earth.
 They do not lie on a celestial sphere, they actually lie at
different distances from earth.
 it is useful for mapping the sky though
 Allows us to map the sky as seen from earth.
 Earth is at the center of the sphere, in line with the horizontal
celestial equator
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Because the center is where we are located as we look
into space
 The north and south poles are at the top and bottom f the
sphere
 A dotted line for the ecliptic runs diagonally and crosses the
celestial equator.
 Two special circles and two points
 North celestial pole: the point directly over Earth’s
North Pole.
i I. Perpendicular to the plane the celestial
equator lies
 South celestial pole: the point directly over earth’s
south pole
i I. Perpendicular to the plane the celestial
equator lies.
 Celestial equator: a projection of earth’s equator into
space, makes a complete circle around the celestial
sphere
i I. Lies in a place that passes through the
equator of earth
 Ecliptic: the path the sun follows as it appears to circle
around the celestial sphere once each year. It crosses
the celestial equator at a 23 ½ degree angle, that it the
tilt of earths axis.
iii. The Milky Way
 Milky Way: a band of light that circles around the celestial sphere,
passing though more than a dozen constellations, and bears an
important relationship to the Milky Way galaxy
 It traces out galaxy's disk of stars, the galactic plane- as it
appears from our location within the galaxy.
 Different than the Milky Way galaxy
 Our view in all directions into the disk of our galaxy
 our Galaxy is shaped like a thin pancake with butter in the middle.
 No matter where we look we see the stars and interstellar
clouds that make up the milky way
 That is why the band of light makes a full circle around
our sky
 The milky way appears wider in the direction of constellation Sagittarius
because it’s in the direction of the central bulge of the galaxy
 We only have a clear view to the distant universe when we look away
from the galactic plane
 Where there are fewer stars and clouds to block our view.
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The dark lanes that run down the center of the Milky Way contain the
densest clouds
 These clouds have made it so we can’t see further than a few
thousand light-years into our galaxy’s disk
 Because of this most of our galaxy has been hidden to us until
new technology like radio waves, infrared light, and x-rays we're
invented and allowed us to peer through the clouds by
observing forms of light that are invisible to our eyes.
iv. The Local Sky
 Local sky: the sky as seen from wherever you happen to be standing,
looks like it takes the shape of a hemisphere or dome.
 The dome shape is because we can only see half of the celestial
sphere at any point from one location
 The ground blocks the other half of the view.
 Horizon: boundary between earth and the sky
 Has an altitude of zero degrees
 Zenith: the point directly overhead
 Altitude is 90 degrees
 Has no direction because it is straight overhead.
 Lunar : an imaginary half-circle stretching from the south point on the
horizon, through the zenith, to the north point on the horizon.
 We can pinpoint an object in the local sky by stating its altitude above
the Horizon and direction along the horizon (Aka azimuth- degree
clockwise from due north)
 Direction- one of the two coordinates (the other is altitude) needed to
pinpoint an object in the local sky. It is the direction, such as north,
south, east, or west, in which you must face to see the object. See also
azimuth.
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Altitude- (above horizon) The angular distance between the horizon and
an object in the sky.
Above picture shows a person pointing to a star located in the southeast
direction at an altitude of 60°.
v. Angular Sizes and Distances
 Angular Size: a measure of the angle formed by extending imaginary
lines outward from our eyes to span an object (or the space between
two objects).
 The angle an object appears to span in your field of view
 We cant tell the true sizes or separations of objects in the sky
because of our lack of depth perception.
 Angular size allows us to judge size without knowing how far
away they are.
 The farther away an object is, the smaller its angular size.
 Sun: angular size of 1/2 degree
 Is about 400 times as large in diameter as the moon but it has
the same angular size in our sky because it is about 400 times as
far away
 Moon: angular size of 1/2 degree
 Three parts:
 Angular sizes of the sun and moon are about 1/2 degree
 The pointer stars that point at Polaris and the Big Dipper appear
about 5 degrees part. The top and bottom stars of the southern
cross appear about 6 degrees apart
 To estimate angular sizes or distances, stretch out your arm
forward and above your head. At arm’s length, one finger spans
about 1 degree, a hand with fingers spread spans about 20
degrees, and a fist spans about 10 degrees.
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Angular Distance: a measure of the angle formed by extending
imaginary lines outward from our eyes to span an object (or space
between two objects)
 Angle that appears to separate a pair of objects in the sky
Pointer stars: angular distance between “pointer stars” at the end of the
big dipper’s bowl is about 5 degrees
Southern Cross: the angular length is about 6 degrees
Arcminutes (Symbolized by ‘): 1/60 of 1 degree. used for greater
precision for angular distance. Subdivide each degree into 60 arcminues
and each arcminute into 60 arcseconds.
Arcsecond (symbolized by “): see arcminutes.
Example: 35 degrees 27’15” is read as 35 degrees, 27 arcminutes, 15
arcseconds.
b. Why do stars rise and set?
 Stars move across the sky from east to west
 Ancient people thought this meant that we are in the center of the
universe that rotates us every day.
 We now know that it is the opposite and earth rotates daily and not the
universe.
c. Common Misconceptions: the Moon Illusion
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The moon illusion: the full moon looks like its bigger when it is near the
horizon than it is when it is high in the sky.
 This size change is the illusion made up by our brains.
 It is unknown why our brains do this.
 When compared to another object its angular size stays about
the same throughout the night
 The moon’s angular size depends on its true size and distance
 The distance of the moon changes while it orbits but it
does not change enough to make a noticeable effect on
a single night.
with the view of the celestial sphere it looks like every object on the
sphere is making a daily circle around Earth.
In the local sky it looks more complex because the horizon cuts the
celestial sphere in half
When looking at the figure of a typical Northern Hemisphere location
(latitude 40 degrees north) there are 3 facts about the paths of stars
through the sky.
 Circumpolar: a star that always remains above the horizon for a
particular latitude. They circle counterclockwise around the
north celestial pole every day.
 A star that is too far north to cross the plane of the
observer’s horizon
 It’s daily circle is entirely above the horizon
 Stars near the south celestial pole never rise above the horizon
at all
 Too far south to cross the plane of the observer’s
horizon, always below the observer’s horizon and is
never seen by that observer
 All other stars have daily circles that are partly above the
horizon and partly below, which makes them appear to rise in
the east and set in the west.
 There are four circles on the sphere in the figure that show the
paths of stars due to Earth’s rotation.
 Earths rotation is drawn with a black arrow
 Two circles are on the northern half of the sphere
 Two circles are on the southern half of the sphere
i i. These circles are parallel to the celestial
equator
ii Ii. They show that stars on the celestial sphere
appear to rotate parallel to the celestial equator
 Stars also look like they move in circles around the
north and south celestial poles.
 The horizon slices through the celestial sphere at an angle to
the celestial equator, causing the daily circles of stars to appear
tilted in the local sky.
 Observer’s horizon is tilted 40 degrees with respect to
the celestial equator
 Horizon circle defines a plane dividing th sphere
diagonally
 A line perpendicular to this plane points to the
observer’s zenith
 It is easier to follow star paths if you rotate the page so that the
zenith points up.
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Time exposure photograph at the beginning of the chapter shows a part
of the daily paths of stars.
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The paths of circumpolar stars are visible within the arch
 The daily circles for these stars are above the horizon but the
picture only shows a portion of the circle.
 The north celestial pole is at the center of the circles
 The larger the circle, the further from the north celestial pole the star is
 If the circles are large enough (far enough from the north celestial pole)
it will cross the horizon so that the stars rise in the east and set in the
west
 The same idea for the southern hemisphere.
 The circumpolar stars are near the south celestial pole
 They circle clockwise instead of couterclockwise
 Earths west to east rotation makes stars appear to move from east to
west through the sky as they circle around the celestial poles
d. Why do the constellations we see depend on latitude and time of year?
 Angular size, physical size, and distance.
 Hold a coin in front of one eye and it blocks your view. Move it
further away and it appears smaller and blocks less of your
view.
 As long as an object is far enough away so that its angular size is
small (less than a few degrees), this formula relates the object’s
angular size (in degrees), physical size, and distance:
 Angular size/360 degrees = physical size/ 2(pi) x
distance
 Ex: the moon’s angular diameter is about 0.5 degrees
and its distance is about 380,000 km. What is the
moon’s physical diameter?
i i. Physical size= angular size x ((2pi x
distance)/360 degrees)
ii ii. Plug in moons angular size and distance
iii iii. physical size= 0.5degrees x( (2pi x
380,000km)/360 degrees) = 3300km
 the moons diameter is about 3300 km. the exact value
(3476km) would be found by using more exact values
 Variation with Latitude
 Latitude- measures north-south position on earth.
0degrees at the equator, 90degreesN at the north pole,
90degreesS at the south pole. 0degrees along the prime
meridian
 Prime meridian- the meridian of longitude that passes
through Greenwich, England; defined to be longitude
0degrees.
 Longitude- measures east-west position on earth
 Latitude + longitude pinpoints a location on earth
i i. ex: Miami is about 26degreeN latitude and
80degreeW longitude.
 Latitude affects the constellations we see because it
affects the locations of the horizon and zenith relative
to the celestial sphere.
 The sky does not vary with longitude though
i i. ex: Charleston, SC and San diego, CA are
about the same latitude so they see the same
set of constellations.
 The picture below shoes how this works for the
latitudes of the north pole (90degreeN) and Sydney,
Australian (34degreeS)
 North Pole: can only see what lies on the northern half
of the celestial sphere and they are all circumpolar
i
i. why the sun remains above the horizon for 6
months. It circles the sky at the north pole just
like a circumpolar star.
 The altitude of the celestial pole in your sky is equal to
your altitude
i i. ex: if the north celestial pole has an altitude of
40degrees above your north horizon, your
latitude is 40degreeN.
 Finding the north celestial pole is easy because it lies
close to the north star Polaris
 In the southern hemisphere you can find the south
celestial pole by using the southern cross.
e. Common misconceptions: star in the daytime
 Stars do not vanish at night and come out during the day
 Stars are always out but their dim light gets overwhelmed by the bright
light of the sun
 You can see stars during the day with a telescope or during a total
eclipse
 Astronauts can see stars during the daytime above earth’s atmosphere
where there is no air to scatter sunlight
 The sun is so bright that astronauts must block its light to see the stars.
ii. Variation with time of year
 The night sky changes throughout the year because earth is changing
position while orbiting the sun
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The annual orbit of earth makes the sun look like its moving steadily
eastward along the ecliptic with stars of different constellations at
different times of the year
Zodiac: the constellations on the celestial sphere through which the
ecliptic passes.
 Traditionally there are 12 constellations along the zodiac but
with the official borders there a thirteenth constellation called
Ophiuchus.
 Ophiuchus: the 13th constellation of zodiac
 There’s a different background of zodiac constellations at
different times of the year
 August 21: sun appears to be in leo but we cannot see leo
because it is in the daytime sky. But at this time we can see
aquarius because its in the opposite location from leo on the
celestial sphere
 In February we see leo because aquarius is in the day time sky
f.
II.
Common misconceptions: what makes the north star special?
 Polaris is not the brightest star in the sky
 There are more than 50 other stars that are just as bright or brighter
 It is special because it is close to the north celestial pole and useful for
navigation
The Reason for seasons
a. What causes the seasons?
 seasons are caused by the combination of earths rotation and orbit
 the tilt of earths axis causes sunlight to fall differently on earth at
different times of year
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Step 1: (for the picture above) the tilt of the earths axis remains pointed
towards Polaris throughout the year
 The orientation of the axis relative to the sun changes over the
course of each orbit
 Northern hemisphere is tipped toward the sun in June and away
in December
 In the southern hemisphere is tipped toward the sun in
December and away from the sun in June.
Step 2: ( for the picture above) earth in June the axis tilt causes sunlight
to strike the northern hemisphere at a steeper angle and southern
hemisphere at a shallower angel
 This makes it summer in the northern hemisphere
 Steeper angle means more concentration of sunlight
and makes it warmer
 Steeper angle also means more hours of daylight (while
its being warmed by the sun) because earths rotation
means the sun follows a longer and higher path through
the sky
 This makes it winter in the southern hemisphere
 Shallower sunlight angle means a lower concentration
of sun and a shorter, lower path through the sky (for
the sun)
 Step 3: (in the picture above) both hemispheres are illuminated equally
in march and September. Making it spring for the hemisphere that is on
the way from winter to summer and fall for the hemisphere moving
from summer to winter.
 Step 4: (in the picture above) shows that it has become winter in the
northern hemisphere and summer for southern hemisphere
 Seasons are not affected by the distance between earth and the sun
 At its furthest distance the earth is only 3% farther from the sun
(in July) than it is in the nearest (January)
 The difference made by the strength of sunlight due to change
in distance is overwhelmed by effects caused by the axis tilt
 If earth did not have axis tilt we would not have seasons.
 Jupiter has small axis tile of about 3degree
 Saturn has axis tilt of about 27degree
 Both planets have almost circular orbits around the sun
b. Common Misconceptions: The Cause of Seasons
 earths varying orbital distance has no effect on the weather
ii. Solstices and Equinoxes
 There are four special moment in the year that correspond to one of
four special positions in earth’s orbit. They help us mark the seasons
 June solstice: summer solstice in the northern hemisphere
occurs around June 21
 The moment when the northern hemisphere is tipped
most directly toward the sun and gets the most direct
sunlight
 December solstice: winter solstice in northern hemisphere
around December 21st
 When northern hemisphere gets the least amount of
sunlight
 March equinox: spring (vernal) equinox in the northern
hemisphere around march 21st
 When northern hemisphere moves from tipped slightly
away from the sun to slightly toward the sun.
 September equinox: fall (autumnal) equinox in northern
hemisphere around September 22
 When northern hemisphere starts to tip away from the
sun
 Dates and times of solstices and equinoxes may vary by a couple of days
from above dates
 Leap years add one day on feb 29 every four years
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Modern calendars include leap years to keep the solstices and
equinoxes around the same days
Leap year pattern based on fact that the true length of the year is close
to 365 ¼ days. (referring to tropical year)
Tropical year: time from one march equinox to the next
 Axis precession causes the tropical year to be slightly shorter
(by about 20 mins) than earth’s orbital period
Sidereal year: earth’s orbital period
Equinoxes and solstices are marked by observing changes in the sun’s
path through the sky
Equinox: during the only two days of the year when the sun rises exactly
due east and sets exactly due west
 Also the days when the sun is above and below the horizon for
the same time of 12 hours
 Means equal night
June Solstice: on the day the sun follows its longest and highest path
through the northern hemisphere sky and shortest and lowest path
through the southern hemisphere sky
 When the sun rises and sets farthest to the north of due east
and due west
 Day when northern hemisphere has the most hours of daylight
and the sun is the highest in the midday sky
December solstice: the sun rises and sets farthest to the south
 Northern hemisphere has least hours of daylight and lowest
midday sun
 First photo shows suns path for solstices and equinoxes for
northern hemisphere sky (latitude 40degreeN).
 Paths are different for different latitudes
 Latitude 40degreeS paths look similar but are tilted to
the north instead of the south
 Second photo shows images of the sun taken in 7 to 11 day
intervals over a year at the same mean solar time and in the
same spot in the northern hemisphere
 Photo looks eastward so north is to the left and south to
the right
 Images that are high to the north shoe a time near June
solstice
 Image that are low and south are near December
solstice
 Analemma: the figure 9 shape created by the photos
i Happen from a combination of earths axis tilt
and earths varying speed as it orbits the sun
iii. Special Topic: How long is a day?
 A rotation period is actually 23 hours and 56 minutes (23 hours, 56m,
4.09 seconds), 4 minutes short of the 24 hours we think of
 Sidereal day: (sy-dear-ee-al) means related to the stars. The time it
takes any star to make one full circuit through our sky
 Solar day: our 24 hour day. The average time it takes the sun to make
one circuit through the sky
 Explanation: point at an object that represents the sun and an object
behind it that represents a star. Stand a few steps away to represent
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earth. Rotate counterclockwise while standing in place and you will
point at both the sun and the star at the same time again.
 But the earth also orbits the sun.
 Sidereal day: take a couple of steps around the sun as you
rotate counterclockwise. One full rotation and you will be
pointing at the distant star
 Solar day: you have to rotate a little extra to point back at the
sun. this extra rotation makes the solar day longer
Figure out how long the extra rotation takes
 Earth does a full 360degree around the sun in about 365 days, a
rate of about 1 degree per day
 Extra rotation takes about 1/360 of earths rotation period,
about 4 minutes.
iv. First Days of Seasons
 Equinox and solstice is said to mark the first day of a season
 June solstice is the first day of summer but in the northern hemisphere
it has its max tilt toward the sun
 Why is it the first of summer instead of the midpoint of summer?
 1. It was easier for ancient people to recognize the highest point as
summer solstice than days in between
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2. We think of seasons in terms of weather.
 Warmest weather takes 1-2 months after solstice because it
takes a while for the sun to reach and warm the earth
 Midsummer in terms of weather would be in late July and early
august making June a good choice for first day of summer
v. Seasons around the world
 Seasons are different around the world because of different latitudes.
 High latitudes have more extreme seasons
 Vermont has longer summer days and longer winter nights than florida
 Artic circle (latitude 66 ½) the sun is out all day on the june solstice and
never rises on December solstice
 “what you actually see is also affected by the fact that the
atmosphere bends sunlight, so that the sun can appear to be
above the horizon even when it is actually slightly below it)
 This shows the progression of the sun around the horizon on
June solstice in the artic circle. The sun skims the northern
horizon at midnight, slowly rises higher, its highest point when
due south at noon
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Seasons are also different in the equatorial regions
 They get the most direct sunlight on the equinoxes and least on
the solstices
 They tend to have rainy and dry seasons instead of the four
seasons that higher latitudes experience
 Rainy seasons come when the sun is higher in the sky
c. Common Misconceptions: High Noon
 High noon: highest point the sun is in the sky each day when it crosses
the meridian (rarely at exactly noon)
 If you don’t live in the tropics (between latitudes 23.5degreeS
and 23.5degreeN) the sun is never directly overhead
d. How does the orientation of Earths axis change with time?
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The constellations associated with the solstices and equinoxes change
slowly over time
Precession: the gradual wobble of the axis of a rotating object around a
vertical line. Alters the orientation of earths axis in space
 When spinning a top its axis will sweep out circles at a slower
rate
 The top axis processes
 Earth does this more slowly
 A cycle of earths procession take about 26,000 years
 Changes direction the axis points in space
 Affects orientation of axis, not the amount of tilt
 Tilt is about 23 ½ degrees
 And doesn’t affect pattern of the seasons
 Changes point in earths orbit that solstices and equinoxes
happen and changes the constellations that are seen at those
times
 Caused by gravity
 Earth gravity tries to pull over the lopsided, tilted spin axis but
precesses the axis instead
 Gravity from sun and moon try to straighten out earths equator,
which has the same tilt as the axis
 Ex: thousands of ears ago the June solstice happened when the sun was
in the constellation cancer, now it happens when the sun is in gemini
 This explains why the latitude the sun is directly overhead on
the June solstice (23 1/2degreeN) on a map is called the tropic
of cancer
e. Common Misconceptions: Sun Signs.
 Astrology began a few thousand years ago
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III.
Sun signs are supposed represent the constellation that the sun
appeared on your birth date
 Procession makes it so that is no longer the case for most
 These signs are based on the positions of the sun among the stars that
were almost 2000 years ago
 Earths axis has moved about 1/13 through its 26,000 year precession
sine then, the sun signs are off by almost a month from the actual
position of the sun and constellations today
 Ex: march 21 sun sign is aries but the sun is now in pisces
The Moon, Our Constant Companion
a. Why do we see phases of the moon?
 The moon is the brightest and most noticeable object in the sky (except
for the sun) and orbits with us around the sun
 The moon orbits the earth and returns to the same position relative to
the sun (the earth-sun line) about every 29 ½ days
 Lunar Phases: the time the moon takes to orbit the earth. The
appearance of the moon changes as its position to the sun changes
 Month (moonth): originated from the 29 ½ day period of the moon
 The cycle of phases is about 2 days longer than the moons orbital period
(27 1/3 days) because of earths motion around the sun. the reason is
the same as why the solar day is longer than the sidereal day
ii. Understanding phases
 Sunlight comes to both earth and the moon from about the same
direction
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Two basic facts about the lunar phases
 1. Half of the boon always faces the sun and is bright and the
other half always faces away and is dark
 2. When you look at the moon in different positions of its orbit,
you see different combinations of bright and dark faces
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The new moon is in line with the sun and hidden in the bright daytime
sky
 Moon phase determines when we will see it
 Full moon will rise around sunset because it is opposite the sun
 Reaches highest point at midnight and sets around sunrise
 First-quarter moon: rise around noon, reach high point around sunset,
set around midnight
 Is when the moon is 90degree east of the sun
 Waxing: phases from new to full. Means increasing
 There is no half moon phase
 First-quarter: we see half moon, one quarter through monthly cycle
 Third-quarter: half moon, three quarter through monthly cycle, a week
after full moon
 New cycle stars at new moon
 Crescent: phases just before and after new moon
 Gibbous: (hard g “gift”) phases just before and after full moon
iii. The moon’s synchronous Rotation

Synchronous rotation: the rotation of an object that always shows the
same face to an object that it is orbiting because its rotation period and
orbital period are equal
 Moon rotates o axis in same time it orbits earth
 This happens because of earths gravity in the same way the
moons gravity causes tides on earth.
 Because we see the same face of the moon means the moon
must rotate only once during its orbit around earth
iv. The view from the moon
 If on the moon during a new moon you would see a full earth because
the moon is between the sun and earth so it would be daytime on
earth.
 On a full moon you would see the night side of earth, a new earth
 If on the moon looking at the earth, you will always see the opposite
phase from the moon
b. Common Misconceptions: Shadow and the Moon
 It is thought that the moons phases are caused by earths shadow falling
on it
 Moons phases are actually caused by the different portions of its day
and night sides at different times as it orbits earth
 A lunar eclipse is the only time earths shadow will fall on the moon
c. Common Misconceptions: moon in the daytime
 The moon is not only out during the night. It is out almost as long during
the day than it is during night.
 We can see the moon only when the light of the sun isn’t drowning it
out

During a first quarter moon we can see the moon in the late afternoon
as it rises through eastern sky
 Third-quarter moon is visible in the morning as it heads toward western
horizon
d. What Causes Eclipses?
 Two basic types of eclipse
 Lunar eclipse: when earth comes directly between the sun and
the moon and earths shadow falls on the moon
 Because the earth is so much bigger than the moon, its
shadow can cover the entire moon
 Can be seen by anyone on the night side of the earth
when it happens
 Solar eclipse: when the moon comes directly between the sun
and earth and the moons shadow falls on earth.
 The moon is a lot smaller than earth so its shadow can
only cover a portion of earth when it happens
 You must be in the narrow pathway that the shadow
moves in order to see it
i this is why we tend to see lunar eclipses more
even though they happen almost as often
ii. Shadow regions
 A shadow consists of two regions:
 Full shadow: (umbra) the dark central region of a shadow.
Sunlight is fully blocked
 Partial shadow: (penumbra) the lighter, outlying regions of a
shadow. Light from only part of the sun is blocked
iii. Lunar Eclipses
 Begins when the moons orbit first carries it into earths partial shadow
 Three types of lunar eclipses that can happen after that:
 Total Lunar Eclipse: a lunar eclipse in which the moon becomes
fully covered by earths full shadow. When the sun, earth, and
moon are nearly perfectly aligned. The moon passes through
earths full shadow
 The most spectacular
 The moon turns dark and red during totality
 Partial Lunar eclipse: a lunar eclipse during which the moon
becomes only partially covered by Earth’s full shadow.
Alignment between the sun, earth, and moon is less perfect,
only part of full moon passes through earths full shadow (with
the rest in partial shadow)
 Penumbral lunar eclipse: when the moon passes only through
earths partial shadow. These are the most common and least
impressive because the full moon only slightly darkens.

Totality: the portion of a total lunar eclipse during which the moon is
fully within the earths full shadow or a total solar eclipse during which
the suns disk is fully blocked by the moon.
 The moon becomes dark and red
 Lasts about an hour with partial phases before and after
 Earth projects shadow with curvature which proves earth is
round during the partial phases
iv. Solar Eclipses
 Three types of solar eclipse
 Total solar eclipse: a solar eclipse during which the sun becomes
fully blocked by the disk of the moon
 When the moon is in part of its orbit and close to earth,
full shadow will cover small part of earth (up to about
270km in diameter)
 The moon slowly creeps in and takes over the moon. It
becomes dark and temperature drops. Animals go to
their homes and crickets will chirp
 Corona: when the moon completely blocks the visible
sun showing a corona
 It takes the sun a couple of ours to come back out but
because we get used to the darkness, it seems a lot
faster than that
 Annular Eclipse: when the moon is its orbit and further from
earth, all of the shadow might not reach earths surface
 A ring of sunlight surrounding the moon, in the small
region of earth directly behind the full shadow
 For either one the region of totality or annularity will be
surrounded by a much larger region (about 7000 km in
diameter) that falls within the moons partial shadow
 Partial solar eclipse: a solar eclipse during which the sun
becomes only partially blocked by the disk of the moon
 Because of earth rotating and moon orbiting, the shadow races
across earth (about 1700km an hours) and totality never lasts
longer than a few minutes in any particular place
 2017 solar eclipse in green river lakes, Wyoming lasted about 2
minutes
v. Conditions for eclipses
 We don’t have as many solar or lunar eclipses as one would think while
looking at the sun, moon, and earths orbit because the moons orbit is
inclined about 5degree to the ecliptic plane
 During the moons orbital inclination the moon spends most time above
or below the ecliptic plane
 Nodes: when the moon crosses through the ecliptic place, once coming
out and once going back in
 Lined about the same way all year, they have almost a straight
line with the sun and earth about 2x a year.
 Eclipse seasons: only time an eclipse can happen, last an average of 5
weeks
 Lasts a few days longer than a cycle of phases
 Happen about 2x a year
 Each season has a lunar eclipse and solar eclipse
 An eclipse can happen when:
 The moon is full (for lunar eclipse) or new (for solar eclipse) and
 The new or full moon happens when the moon is close to a
node, during an eclipse season
 There is always a lunar eclipse at full moon and solar eclipse at new
moon during each eclipse season.
 A second eclipse can happen during an eclipse season
vi. Predicting eclipses
 The nodes slowly move around the moons orbit causing the eclipse
season to happen a little less than 6 months apart (173 days)
 Saros Cycle: the period over which the basic pattern of eclipses repeats,
which is about 18 years, 11 1/3 days.



Saros cycle is caused by the changing dates of eclipse season and the 29
1/3 day lunar phases cycle
Astronomers today can predict when an eclipse will happen and what
kind it will be because we know about the orbits of earth and the moon.
Lunar eclipses 2021-2025 in universal time (Greenwich, England)
Date
May 26, 2021
Type
Total
Nov. 19, 2021
May 16,2022
Nov. 8, 2022
May 5, 023
Oct. 28, 2023
Mar. 25, 2024
Sept. 18, 2024
Mar. 14, 2025
Sep. 7, 2025
Partial
Total
Total
Penumbral
Partial
Penumbral
Partial
Total
Total
IV.
Where you can see it
Australia, pacific, western
Americas
Asia, Australia, pacific, Americas
Americas, Europe, Africa
Asia, Australia, pacific, Americas
Europe, Africa, asia, Australia
Europe, Africa, asia, Australia
Asia, Australia, pacific, Americas
Americas, Europe, Africa
Americas, western Africa
Europe, Africa, asia australia
 Shows the path of totality for solar eclipses from 2017-2045
 Next total solar eclipse for the us is April 8, 2024
e. Common Misconceptions: the “dark side” of the moon
 Far side of the sun is not always dark
 New moon the far side faces the sun and is completely lit
 Since moon rotates in about a month, that means points on both near
and far side of the moon alter between 2 weeks of light and 2 weeks of
darkness
 Only time the far side is completely dark is during a full moon since it
faces away from both the sun and earth
The Ancient Mystery of the Planets
 There are five planets that are easy to see with the naked eye
 Mercury: visible infrequently, only right after sunset or before
sunrise. Because it’s close to the sun
 Venus: bright in the early evening toward the west or before
dawn in the east
 Jupiter: when it’s visible it’s the brightest thing in the sky except
for the moon and venus
 Mars: can be seen by its red color. Make sure you’re not just
looking at a red star
 Saturn: other stars are just as bright as Saturn so you have to
know where to look
 Planets do not twinkle as much as stars
a. Why was planetary motion so hard to explain?
 Planet: from a Greek term meaning “wandering star”
 Planet’s don’t simply rise in east and set west like stars do
 Planets vary in speed and brightness
 Planets usually move eastward through constellations but sometimes
they go backward through the zodiac
 Apparent Retrograde Motion: the apparent motion of a planet, as
viewed from earth, during the period of a few weeks or months when it
moves westward relative to the stars in our sky.
 It goes backwards through the zodiacs
 Time depends on the planet
 Planets do not actually change direction, they only look like they
do because earth orbits faster than they do. It looks like they’re
going backwards relative to the stars when earth passes it


People who believed in earth-centered universe had a hard time
explaining apparent retrograde motion and came up with some crazy
explanations
how could the planets sometimes turn around if everything revolves
around the earth?
b. Why did the ancient greeks reject the real explanation for planetary motion?
 A Greek astronomer Aristarchus of Samos suggested that the earth
revolves around the sun in 260 B.C.
 We don’t know how or why he came up with this idea
 It was rejected by his “contemporaries”
 It wasn’t accepted until almost 2000 years later.
 Stellar parallax: the apparent shift in the position of a nearby star
(relative to distant objects) that occurs as we view the star from
different positions in Earth’s orbit of the Sun each year.
 Think of your two eyes as earth on opposite sides of its orbit.
Think of your finger as a nearby star
 We view stars from different places in our orbit at different
times of the year
 Nearby stars should look like they shift back and forth against
the background of more distant stars
 The amount of shift is too small to see with the naked eye
 Can be seen with telescopes
 Proves earth orbits the sun
 Gives the most reliable measurement of distances to nearby
stars.
 Parallax: happens when your eyes view something from opposite sides
of your nose
 Ex: hold your finger out in front of you and look at it while
closing one eye at a time.
 The closer the object is, the worse the parallax is
 The greeks had trouble seeing a sun centered universe because they
couldn’t see stellar parallax. They knew it should happen but couldn’t
detect it. “seeing is believing”
 The greeks believed in a celestial sphere so they though stellar parallax
would look different because stars lie in one spot



They thought that if earth obits the sun they thought that at different
times of year we would be closer to different parts of the celestial
sphere and notices changes in the angular separations of stars
Their conclusions:
 Earth orbits the sun but the stars are too far away to see stellar
parallax. Or
 There is no stellar parallax because earth stays still in the center
of the universe.
They rejected the first (and correct) answer because they couldn’t
imagine the stars being that far away.
c. The Big Picture: Putting Chapter 2 into Perspective
i. It is easy to think we are in the center of a celestial sphere but we are actually a
planet orbiting a star (sun).
 We can understand the things we see in the local sky by looking at the
latitude of the celestial sphere
