Download DEU 5e Chapter 1 Lecture PPT

Neil F. Comins
Discovering the Essential Universe
Fifth Edition
CHAPTER 1
Discovering the Night Sky
In this chapter you will discover…
• how astronomers organize the night sky to help them locate
objects in it.
• that Earth’s spin on its axis causes day and night.
• how the tilt of Earth’s axis of rotation and Earth’s motion
around the Sun combine to create the seasons.
• that the Moon’s orbit around Earth creates the phases of the
Moon.
• what causes both lunar and solar eclipses.
• the scales of the universe.
WHAT DO YOU THINK?
1. What do astronomers define as constellations?
2. Is the North Star—Polaris—the brightest star in the
night sky?
3. What causes the seasons?
4. How many zodiac constellations are there?
5. Does the Moon have a dark side that we never see
from Earth?
6. Is the Moon ever visible during the daytime?
7. What causes lunar and solar eclipses?
Navigating the Night Sky
• Constellations
• Seasonal Asterisms – Star Guides
In order to more easily locate objects in the sky, we divide
the sky into regions named after familiar patterns of stars
called constellations.
The Night Sky Without and With Light Pollution
Ancient constellations were
imaginary pictures outlined
by familiar patterns of stars.
Modern astronomers divide the sky into
88 official constellations or regions of
space, many of which contain the
ancient star patterns.
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Common Guides to Finding Constellations and Stars
Using the “Big Dipper” as a guide
Common Guides to Finding Constellations and Stars
Common Guides to Finding Constellations and Stars
The “Winter Triangle”
Insight into Science
Compare What You Believe with What Nature and Science
Show You
If you think Polaris is the brightest star in the night sky, how did
you learn that? There is likely to be a lot of information you
learned from a variety of sources that you will have to unlearn
while learning astronomy. While this process is often
uncomfortable, it is essential if you want to understand how
science works and how scientists view the world. Be careful: It is
human nature to change new information to fit our current
beliefs rather than change what we believe. Fight that urge if
science can show you that your beliefs are incorrect!
The “Summer Triangle”
The Celestial Sphere
• Projection of Earth’s equator, poles
• Earth Latitude = Celestial Declination
• Earth Longitude = Celestial Right Ascension
Astronomers describe the universe as an imaginary sphere
surrounding the Earth on which all objects in the sky can be
located, called the CELESTIAL SPHERE.
 As viewed from Earth, the
celestial sphere appears to
rotate around two axis points,
the north and south celestial
poles, which are located
directly above the Earth’s
poles.
 Between these is the celestial
equator, which divides the
celestial sphere into northern
and southern hemispheres.
 We define the position of an
object on the celestial sphere
using two coordinates, right
ascension and declination.
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Angles and Arc Angles
Cyclic motions of the Sun and stars in our sky are
due to motions of Earth.
1. ROTATION = the spin of Earth on its axis. It takes one
day for Earth to complete one rotation.
2. REVOLUTION = the movement of Earth in orbit around
the sun. It takes one year for Earth to complete one
revolution.
3. PRECESSION = the slow conical (top-like) motion of
Earth’s axis of rotation. It takes 26,000 years for Earth to
complete one cycle of precession.
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The Big Dipper
90 – horizon to top of sky (zenith)
5 – angle between Pointer Stars of Big Dipper
1 = 60 arcminutes = 60’
1’ = 60 arcseconds = 60”
The Moon is ½ or 30’ or 1800” in diameter
Star Motions
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•
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Circumpolar
Equatorial
In Between
Rotation of Earth
Revolution of Earth around Sun
The angular distance between the two “pointer stars” at
the front of the Big Dipper is about 5°.
Circumpolar Star Trails
The apparent westward motion of the Sun, Moon,
and stars across our sky each day is caused by
Earth’s rotation.
At middle latitudes, we see the Sun, Moon, and many of the stars
first come into view moving upward, rising at some point along the
eastern horizon. Then, they appear to arc across the sky. Finally,
they disappear beneath the western horizon.
We generalize this motion to make statements such as, “The Sun
rises in the east and sets in the west.”
The stars near the poles of the celestial sphere (shown here)
move in trails that circle the pole and never set. They are
called circumpolar.
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Motion of Stars at the Poles
Because Earth rotates around its poles, stars seen from these locations
appear to move in huge, horizontal circles. This is the same effect you
would get by standing up in a room and spinning around; everything
would appear to move in circles around you. At the North Pole, stars
move left to right, while at the South Pole, they move right to left.
Rising and Setting of Stars at Middle Northern Latitudes
Unlike the motion of the stars at the poles, the stars at all other latitudes
do change angle above the ground throughout the night. This time-lapse
photograph shows stars setting. The latitude determines the angle with
respect to the horizon at which the stars rise and set.
Earth also revolves around the Sun, which changes our view of the
stars.
Rising and Setting of Stars at the Equator
Standing on the equator, you are perpendicular to the axis
around which Earth rotates. As seen from there, the stars
rise straight up on the eastern horizon and set straight down on
the western horizon.
We can see how different stars appear at different times of day by looking at the
position of the Sun against the backdrop of stars. The side of Earth facing the Sun
is experiencing “day,” while the side of Earth turned away from the Sun is
experiencing “night.”
SEPTEMBER
MARCH
Insight into Science
Define Your Terms
From our perspective, the Sun
appears to move through the
stars along a special path
called the ecliptic.
From an outside view, we see
Earth revolve around the Sun.
We define the plane of Earth’s
orbit as the ecliptic plane.
As with interpersonal communication, using the correct
words is very important in science. Usually,
scientific words each have a specific meaning, such as
“rotation” denoting spin and “revolution” meaning one
object orbiting another. Be especially careful to
understand the context in which words with more than
one meaning, such as “ecliptic” and “constellation,” are
used.
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Seasons
• The reason for the seasons is the Earth’s tilt
• In different seasons we receive different
intensities of sunlight and heat
• In the summer, sunlight is more direct and less
spread out
• In the winter, sunlight is less direct and more
spread out
The seasons we experience are due to the Sun’s motion across the celestial sphere.
Seasons are caused because Earth’s axis is tilted with respect to its orbital
plane; as Earth revolves around the Sun, different parts of Earth receive more
direct sunlight (summer), whereas other parts of Earth receive sunlight that is
more spread out (winter).
The Sun’s Daily Path and the Energy It Deposits Here
The point of the Sun’s path
farthest north on the celestial
sphere is called the summer
solstice (JUN 21), whereas the
point of the ecliptic farthest
south is called the winter
solstice (DEC 21).
The two points on the ecliptic
where the Sun crosses the
celestial equator are called
equinoxes. During the vernal
equinox (MAR 21), the Sun is
moving north, while during the
autumnal equinox (SEPT 21),
the Sun is moving south.
Remember that the seasonal
names of the equinoxes and
the solstices refer to seasons in
the NORTHERN hemisphere.
The seasons occurring in the
SOUTHERN hemisphere are
exactly opposite.
Insight into Science
(a) On the winter solstice―first day of winter―the Sun rises farthest south of east, it is lowest in the
noontime sky, stays up the shortest time, and its light and heat are least intense (most spread out) of
any day of the year in the northern hemisphere. (b) On the vernal equinox―first day of spring―the Sun
rises precisely in the east and sets precisely in the west. Its light and heat have been growing more
intense, as shown by the brighter oval of light than in (a). (c) On the summer solstice―first day of
summer― the Sun rises farthest north of east of any day in the year, is highest in the sky at noontime,
stays up the longest time, and its light and heat are most intense of any day in the northern
hemisphere. (d) On the autumnal equinox, the same astronomical conditions exist as on the vernal
equinox.
The Midnight Sun
Expect the Unexpected
The process of science requires that we question the obvious,
that is, what we think we know. Many phenomena in the
universe defy commonsense explanations. The fact that the
changing distance from Earth to the Sun has a minimal effect on
the seasons is an excellent example.
This time-lapse photograph was taken on July 19, 1985, at 69°
north latitude in northeastern Alaska. At that latitude,
the Sun is above the horizon continuously from mid-May
until the end of July.
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Precession
This precession causes the
position of the North Celestial
Pole to slowly change over
time. Today, the North Celestial
Pole is near the star Polaris,
which we call the “North Star.”
However, in 3000 BC, Thuban
was close to the North
Celestial Pole and in 14,000
AD, Vega will be in this
location.
Gravitational forces of the Sun and the Moon pulling on Earth as it rotates cause
Earth to undergo a top-like motion called precession. Over a period of 26,000
years, Earth’s rotation axis slowly moves in a circular motion.
Phases of the Moon
• Caused by the changing amount of
illumination seen from the Earth as the Moon
revolves around it.
Another familiar cycle is the lunar cycle. When the Moon orbits Earth, the
amount of the side facing Earth that is lit changes, creating the Moon’s phases.
This phase cycle is called the synodic period and is 29½ days long.
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A synodic month is the time it takes for the Moon to orbit Earth with respect to
the Sun and is 29½ days long.
One common
misconception is that
the Moon is only visible
at night. However, the
time of day in which the
Moon is in our sky
varies depending on its
phase. This picture
clearly displays the
Moon, visible during the
day.
A sidereal month is the time it takes for the Moon to orbit Earth with respect to
the stars and is 27.3 days long.
The two times are
different because
Earth moves in its
orbit around the
Sun as the Moon
moves in its orbit
around Earth.
Eclipses
• Solar Eclipse – the Moon shadows the Earth
During a new or full moon phase, when the Moon, Sun, and Earth are aligned, the
Moon may enter the shadow of Earth, or the shadow of the Moon may reach
Earth, creating eclipses. However, these eclipses do not occur during every full
or new moon because the Moon’s orbit is tilted by 5 with respect to the EarthSun (ecliptic) plane.
– New Moon
• Lunar Eclipse – the Earth shadows the Moon
– Full Moon
PENUMBRAL = the Moon appears
dimmed.
PARTIAL = part of the Moon enters the
umbra of Earth’s shadow and is darkened.
TOTAL = all of the Moon enters Earth’s
shadow and becomes a reddish color, only
lit from light bending around Earth’s
atmosphere.
During a total lunar eclipse, the Moon moves in and out of the
umbra of Earth’s shadow.
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Unlike lunar
eclipses, solar
eclipses occur at
specific places on
Earth, indicated by
the arrow.
Eclipse Paths for Total and Annular Eclipses 2001–2020
A Total Eclipse of the Sun
This map shows the eclipse paths for the 14 total solar and 13 annular
eclipses that occur between 2001 and 2020. In each eclipse, the
Moon’s shadow travels along the eclipse path in a generally eastward
direction across Earth’s surface.
Sometimes eclipses occur
when the Moon is too far away
from Earth to completely cover
the Sun in our sky. When this
occurs, the Moon appears in
the center and a thin ring, or
“annulus,” of light surrounds it.
These are called annular
eclipses.
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What Have Astronomers Discovered in Our Universe?
THE SCALES OF
THE UNIVERSE
The range of objects
we study are from the
extremely small
subatomic particles,
to objects that are
gigantic, such as a
galaxy or the size of
the known universe
itself.
Each division up the
line indicates an
increase in size by
100,000.
Navigating the Night Sky
Summary of Key Ideas
Earthly Cycles
• The stars appear to revolve around Earth
once each night. In fact, it is actually
Earth’s rotation that causes this apparent
motion.
• Earth revolves around the Sun in 365¼
days, defining a year.
• The celestial sphere is an imaginary hollow sphere
centered on Earth. A map of the sky, analogous to a
map of the entire Earth, is impressed on it. The poles
and equator of the celestial sphere are determined by
extending the axis of rotation and the equatorial
plane of Earth out onto the celestial sphere.
• The surface of the celestial sphere is divided into 88
unequal areas called constellations.
The Seasons
• Earth’s axis of rotation is tilted at an angle of 23½
from a line perpendicular to the plane of Earth’s orbit
around the Sun (the plane of the ecliptic). This tilt,
combined with Earth’s revolution around the Sun,
causes the seasons.
• Equinoxes and solstices are significant points along
Earth’s orbit that are determined by the relationship
between the Sun’s path on the celestial sphere (the
ecliptic) and the celestial equator.
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The Seasons
The Phases of the Moon
• The length of the day is based upon Earth’s rotation
rate and the average motion of Earth around the
Sun. These effects combine to produce the 24-h day
upon which our clocks are based.
• Earth’s axis of rotation slowly changes direction
relative to the stars over thousands of years, a
phenomenon called precession.
• The phases of the Moon are caused by the
changing relative positions of Earth, the
Moon, and the Sun. The Moon completes one
cycle of phases in a synodic month, which
averages 29½ days.
• The Moon completes one orbit around Earth
with respect to the stars in a sidereal month,
which averages 27.3 days.
Eclipses
Eclipses
• The shadow of an object has two parts: the umbra,
where direct light from the source is completely
blocked; and the penumbra, where the light source is
only partially obscured.
• A lunar eclipse occurs when the Moon moves
through Earth’s shadow. During a lunar eclipse, the
Sun, Earth, and the Moon are in alignment, with
Earth between the Sun and the Moon, and the Moon
in the plane of the ecliptic.
• A solar eclipse occurs when a strip of Earth passes
through the Moon’s shadow. During a solar eclipse,
the Sun, Earth, and the Moon are in alignment, with
the Moon between Earth and the Sun, and the Moon
in the plane of the ecliptic.
• Depending on the relative positions of the Sun, the
Moon, and Earth, lunar eclipses may be penumbral,
partial, or total; solar eclipses may be annular,
partial, or total.
Scales of the Universe
WHAT DID YOU THINK?
• Astronomy examines objects that range in size
from the parts of an atom (~10–15 m) to the
size of the observable universe (~1026 m).
• Scientific notation is a convenient shorthand
for writing very large and very small numbers.
• What do astronomers define as constellations?
Astronomers sometimes use the common definition of a
constellation as a pattern of stars. Formally, however, a
constellation is an entire area of the celestial sphere and all
the stars and other objects in it. Viewed from Earth, the
entire sky is covered by 88 different-sized constellations. If
there is any room for confusion, astronomers refer to the
patterns as asterisms.
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WHAT DID YOU THINK?
WHAT DID YOU THINK?
• Is the North Star—Polaris—the brightest star
in the night sky?
No. Polaris is a star of medium brightness compared
other stars visible to the naked eye.
with
WHAT DID YOU THINK?
• How many zodiac constellations are there?
There are 13 zodiac constellations, the least-known one being
Ophiuchus.
WHAT DID YOU THINK?
• Is the Moon ever visible during the daytime?
The Moon is visible at some time during daylight hours
almost every day of the year. Different phases are visible
during different times of the day.
• What causes the seasons?
The tilt of Earth’s rotation axis with respect to the ecliptic
causes the seasons. They are not caused by the changing
distance from Earth to the Sun that results from the shape of
Earth’s orbit.
WHAT DID YOU THINK?
• Does the Moon have a dark side that we never
see from Earth?
Half of the Moon is always dark. Whenever we see less than a
full Moon, we are seeing part of the Moon’s dark side. So, the
dark side of the Moon is not the same as the far side of the
Moon, which we never see from Earth.
WHAT DID YOU THINK?
• What causes lunar and solar eclipses?
When the Moon is crossing the ecliptic in the full or new
phase, the shadows of Earth or the Moon, respectively, then
fall on the Moon or Earth. These shadows on the respective
surfaces are eclipses.
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