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CHAPTER 2
A USER’S GUIDE TO
THE
SKY
SUMMARY—GUIDEPOST
Excellent brief “Guideposts” questions appear at the
start of each chapter in the text. Hearing
these questions at appropriate points in a lecture or watching for answers to
them while reading will help the student maintain a sense of the unity and
content and not get lost in details. For
Chapter 2 the Guidepost questions (in boldface) with main points of
understanding based on the end-of-chapter summary are:
2-1 THE STARS
How do astronomers
refer to stars and compare their brightness?
Astronomers now divide the sky into 88 constellations.
Although the constellations originated in Greek and Middle Eastern mythology,
the names are Latin. Even the modern constellations, added to fill in the
spaces between the ancient figures, have Latin names. Named groups of stars that are not
constellations are called asterisms.
The names of stars usually come from ancient Arabic, although
modern astronomers often refer to a star by its constellation and a Greek letter
assigned according to its brightness within the constellation.
The magnitude scale is the astronomer’s brightness scale.
First magnitude stars are brighter than second-magnitude stars, which are
brighter than third-magnitude stars, and so on. The magnitude you see when you
look at a star in the sky is its apparent visual magnitude, mV, which includes only types
of light visible to the human eye, and which does not take into account the
star’s distance from Earth.
Flux is a measure of light energy related to intensity. The magnitude of a star is related directly
to the flux of light received on Earth, and so to its intensity.
How do Earth’s
motions affect the appearance of the sky?
The celestial sphere is a scientific model of the sky, to
which the stars appear to be attached. Because Earth rotates eastward, the
celestial sphere appears to rotate westward on its axis.
The north and south celestial poles are the pivots on which
the sky appears to rotate and they define the four directions around the
horizon: the north, south, east and west points. The point directly overhead is the zenith,
and the point on the sky directly underfoot is the nadir.
The celestial equator, an imaginary line around the sky above
Earth’s equator, divides the sky into northern and southern halves.
Astronomers often refer to angular distances “on” the sky as
if the stars, sun, moon, and planets were equivalent to spots painted on a
plaster ceiling. These angular distances are measured in degrees, minutes of
arc, and seconds of arc, and are unrelated to the true distance between the
objects in light-years. The angular
distance across an object is its angular diameter.
What you see of the celestial sphere depends on your
latitude. Much of the southern hemisphere of the sky is not visible from
northern latitudes. To see that part of the sky, you would have to travel
southward over Earth’s surface.
Circumpolar constellations are those close enough to a
celestial pole that they do not rise or set.
The angular distance from the horizon to the north celestial
pole always equals your latitude. This is the basis for celestial navigation.
Precession is caused by the gravitational forces of the moon
and the sun acting on the equatorial bulge of the Earth and causing its axis to
sweep around in a conical motion like the motion of a top’s axis. Earth’s axis of rotation precesses with a
period of 26,000 years, and consequently the celestial poles and celestial
equator move slowly against the background of the stars.
What causes the
seasons?
The rotation of the Earth on its axis produces the cycle of
day and night, and the revolution of the Earth around the sun produces the
cycle of the year.
Because Earth orbits the sun, the sun appears to move
eastward along the ecliptic through the constellations, completing a circuit of
the sky in a year. Because the ecliptic
is tipped 23.5° to the celestial equator, the sun spends half the year in the
northern celestial hemisphere and half in the southern celestial hemisphere.
In each hemisphere’s summer, the sun is above the horizon
longer and shines more directly down on the ground. Both effects cause warmer
weather in that hemisphere and leave Earth’s other hemisphere cooler. In each hemisphere’s winter, the sun is above
the sky fewer hours than in summer and shines less directly, so the winter
hemisphere has colder weather, and the opposite hemisphere has summer. Consequently, the seasons are reversed in
Earth’s southern hemisphere relative to the northern hemisphere.
The beginning of spring, summer, winter, and fall are marked
by the vernal equinox, the summer solstice, the autumnal equinox, and the
winter solstice.
Earth is slightly closer to the sun at perihelion in January
and slightly farther away at aphelion in July.
This has almost no effect on the seasons.
The planets move generally eastward along the ecliptic, and
all but Uranus and Neptune are visible to the unaided eye, looking like
stars. Mercury and Venus never wander
far from the sun and are sometimes visible in the evening sky after sunset or
in the dawn sky before sunrise.
Planets visible in the sky at sunset are traditionally called
evening stars, and planets visible in the dawn sky are called morning stars,
even though they are not actually stars.
The locations of the sun and planets along the zodiac are
diagrammed in a horoscope, which is the basis for the ancient pseudoscience
known as astrology.
2-4
ASTRONOMICAL INFLUENCES ON EARTH’S CLIMATE
How do astronomical
cycles affect Earth’s climate?
According to the Milankovitch hypothesis, changes in the
shape of Earth’s orbit, in its precession, and in its axial tilt can alter the
planet’s heat balance and cause the cycle of ice ages. Evidence found in seafloor samples support
the hypothesis, and it is widely accepted today.
Scientists routinely test their own ideas by organizing
theory and evidence into a scientific argument.
CHAPTER OUTLINE
2-1 The
Stars
Constellations
The Names of the Stars
Favorite Stars
The Brightness of
Stars
Magnitude and Flux
2-2 The
Sky and Celestial Motion
The Celestial Sphere
Precession
Concept Page: The
Sky Around You
How Do We Know? 2-1: Scientific Models
2-3 The
Cycles of the Sun
The Annual Motion of
the Sun
The Seasons
The Motion of the
Planets
Concept Page: The
Cycle of the Seasons
2-4 Astronomical Influences on Earth's Climate
How Do We Know? 2-2: Pseudoscience
The Hypothesis
The Evidence
How Do We Know 2-3: Evidence as the Foundation of
Science
Scientific Argument: Why was it critical in testing
the Milankovitch hypothesis to determine the ages of ocean sediment?
How Do We Know 2-4: Scientific Arguments
What Are We? Along for the Ride
EDUCATIONAL RESOURCES
Stellarium is an open-source planetarium
program available for download. The
program can be downloaded at: http://www.stellarium.org. Once you have gone to this site, click on the large button that corresponds
to your platform (e.g. Mac OS X, Windows, or Linux). The Stellarium installation package
will download to your computer. There
is also a button to download a User’s Manual (which is rather technical). Many other resources about Stellarium are
available on this site.
For Windows installation: Double click on the stellarium-0.10.2.exe
file to run the installer. Follow the on-screen instructions.
Starting Stellarium in Windows: The Stellarium installer creates an item in
the Start Menu in the Programs section. Select this to run Stellarium. If you cannot find it, type stellarium in the
“Search” box and hit return. You will then get the series of Setup boxes.
Stellarium will then appear as an item in the Start Menu and as a desk-top
icon. Use this icon to start the program
in the future.
Other
internet sites related to this material:
Current observational events: http://www.space.com/nightsky/
Night sky events for the week: http://www.nightskyinfo.com/
ANSWERS TO REVIEW QUESTIONS
1. Our current scientific culture has been greatly influenced by
Western Civilization, and the modern constellations came about as Europeans
began to explore the world during the 15th to the 17th centuries. The southern sky was unknown by the ancient
Greeks and hence uncharted until the age of exploration began. European explorers created constellations in
the southern hemisphere that depicted various devices of their time. Europeans also created a few northern
hemisphere constellations. Because most
of the bright stars were already part of ancient constellations, these newly
created northern hemisphere constellations were developed using faint stars.
2. The brightest stars in each constellation are named by a Greek
letter followed by the possessive form of the Latin constellation name. The constellation name describes the star’s
location in the sky and the Greek letter indicates its brightness relative to
the other stars in the constellation.
The brighter stars have Greek letters nearer the beginning of the Greek
alphabet.
3. a) Ursa Majoris is the brighter of the two because it has a
Greek letter designation that suggests that it is the brightest star in the
constellation of Ursa Majoris.
b) It is difficult to
determine which is brighter; one might guess that α Pegasi should be brighter
than e Scorpii. Both constellations are bright
constellations, and α is the brightest star in Pegasus, while e would be one of the moderately
bright stars in Scorpius. As it turns
out, e Scorpii is slightly
brighter than α Pegasi.
c) α Orionis should be
much brighter than α Telescopii. Orion
is a very bright constellation with several stars brighter than second
magnitude. On the other hand,
Telescopium is a faint constellation with no stars brighter than third magnitude.
4. Astronomers measure the brightness of stars using the magnitude
scale, attributed in its original form to the Greek astronomer Hipparchus
(about 190–120 BC). Ancient astronomers
divided the stars into six classes: the brightest were called first magnitude
and the faintest were called sixth magnitude.
Modern astronomers can measure magnitude to high precision and have
extended the scale to larger numbers for even fainter stars, and to zero and
negative numbers for the brightest stars.
The magnitude scale is confusing because it is an inverse scale, meaning
that bright objects have smaller magnitudes than fainter objects.
5. In the term apparent
visual magnitude, the word apparent
means that the magnitude describes how bright the star appears to us, observing
from Earth.
6. In the term apparent
visual magnitude, the word visual
means that the magnitude includes only light that is visible to the human eye.
7. Although modern astronomers know that the stars are scattered
through space at different distances, it is still convenient to describe the
sky as the celestial sphere: a great sphere enclosing the Earth with the stars
stuck on the inside like thumbtacks in a ceiling This is an example of a simple scientific
model: it describes what we see as the sky appears to turn above us, and
enables us to predict correctly the future positions of the stars at various
times of the night and throughout the year.
8. If the Earth did not turn on its axis with respect to the stars,
then we would not be able to define the celestial poles or equator.
9. If you were on the Earth’s equator, the north celestial pole
would be on your northern horizon and the south celestial pole would be on your
southern horizon.
10. As the Earth rotates on its axis, all of the stars appear to rotate
westward, in big circles, around the north and south celestial poles. Circumpolar stars are those at a given
latitude that never rise or set. From a
latitude close to a celestial pole, such as Norway
or Tierra del Fuego, you would have many more circumpolar constellations than
at a latitude closer to the equator, such as Hawaii
11. During the winter, light from the sun hits Earth at a more oblique
angle than it does during the summer.
Further, the length of time that the sun is above the horizon is
noticeably shorter during the winter than during the summer.
12. The seasons are reversed.
The season when the sun is highest in the sky at noon in the southern
hemisphere is the season when the sun is lowest in the sky at noon in the
northern hemisphere.
13. Due to the eccentricity of the Earth’s orbit, with perihelion
occurring in early January, the northern hemisphere winters should be slightly
warmer than the southern hemisphere winters.
In the southern hemisphere, winter coincides with the time, in July,
when Earth reaches aphelion, its most distant point from the sun.
14. How Do We Know? – A scientific model can be technically inaccurate
but still be useful in understanding the basic behavior or nature of a
system. Some realistic details of the
system, for example the distance to stars on the celestial sphere, are simply
unimportant when trying to understand the behavior of the system. For the case of the celestial sphere, how
stars move on a daily or yearly basis doesn’t depend on their distance from the
Earth, so those details can be left out.
15. How Do We Know? – Astrology
is a set of theories that purport the belief that the motion of the stars and
planets and other celestial events control the events in our lives. These ideas have no scientific basis and have
repeatedly been tested for accuracy and have repeatedly failed. Such methods or theories with no scientific
basis are therefore classified under pseudoscience.
16. How
Do We Know? – Evidence is reality, and scientists constantly check their ideas
against reality. It is a characteristic of scientific knowledge that it is
supported by evidence. A scientific statement is more than an opinion or a
speculation because it has been tested objectively against reality. Every scientific theory needs to be supported
by evidence in the form of observations and/or experiments.
17. How Do We Know? – Evidence from
experiments and observation is the foundation of science. Evidence is reality, and scientists must
constantly check their ideas against reality.
In order to understand nature, scientists must be objective and not
ignore any known evidence.
ANSWERS TO DISCUSSION QUESTIONS
1.
From earliest times, we humans have tried to make sense of the universe,
seeking to understand nature and our place within it. Especially in ancient cultures without
today’s light pollution, it was a natural impulse to find patterns in the stars
and connect them with history, myths, and legends.
2. Like all stars, Polaris appears to trace a circular path
around the north celestial pole as the Earth rotates on its axis. Polaris is so close to the true pole that
this apparent motion is very small; nonetheless, to find your exact latitude
from the position of Polaris, you would need to know the date and time in order
to figure out Polaris’s location relative to the pole at that moment.
3. The apparent path of the sun across the Earth’s sky is
called the ecliptic. Planets orbiting
other stars would also have such an ecliptic.
If their axes of rotation are tilted, or if they have extremely
eccentric orbits, they will also have seasons.
ANSWERS TO PROBLEMS
1. Star A is brightest. Stars A and B are visible to the unaided
eye. Stars A and B have a flux ratio of
approximately 16.
2. Approximately 2 magnitudes
brighter
3. Approximately 4 magnitudes
brighter
4. Approximately 630
5. Approximately 2800
6. A is brighter than B by a
factor of approximately 170.
7. The sun is 400,000 times
brighter than the full moon.
8. 66.6°; 113.4°
9. If you are at a latitude of
35° north of Earth’s equator,
that angle is also the angular distance from your northern horizon up to the
north celestial pole. From the north
celestial pole, it is an additional 55° up to your zenith and
another 35° (south) to the celestial
equator; your southern horizon is therefore 55° below the celestial equator, and the
south celestial pole (which is 90° from the celestial
equator) is thus 90°– 55° = 35° below your southern horizon.
ANSWERS TO LEARNING TO LOOK
1. As the
caption says, they are well to the right of center. Also look at the July Southern hemisphere
star map upside down with the southern horizon at the bottom. You can see the Southern Cross (Crux) and the
constellation Carina to the right of the South Celestial Pole (the red
dot).
2. The
constellation looks odd in its positioning relative to the horizon because the
stamp depicts the sky as seen from the southern hemisphere. Orion is on the celestial equator. Considering the extreme case, compared to an
observer at the north geographic pole, an observer at the south geographic pole
will see Orion as “upside down” since the two observers are inverted relative
to one another. Try making a drawing to
see this. For northern and southern
hemisphere observers not at the poles, the situation is similar (but less
extreme) and results in differing views of Orion.
3. The photo was likely taken
around 1:00 a.m. to 2:00 a.m. in mid-September
ANSWERS TO SCIENTIFIC ARGUMENTS
1.
The temperatures determined from calcite deposits in Devil’s
Hole may reflect only local climate changes in that specific location. Ocean floor samples on the other hand reflect
global climate conditions.
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