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					                      The Sky

Astronomy: Horizons
10th edition
  Michael Seeds
                           Chapter 2    The Sky

The Southern Cross I saw every night
abeam. The sun every morning came up
astern; every evening it went down
ahead. I wished for no other compass to
guide me, for these were true.

                 - CAPTAIN JOSHUA SLOCUM
                  Sailing Alone Around the World
                                              The Sky

• The night sky is the rest of the
  universe as seen from our planet.
  – When you look up at the stars, you look out
    through a layer of air only a few hundred
    kilometers deep.
  – Beyond that, space is nearly empty, and the stars
    are scattered light years apart.
                                     The Sky

• Here, you will begin your search for the
  natural laws that govern the universe—
  by trying to understand what the
  universe looks like.
                                              The Sky

• As you read this chapter, keep in mind
  that you live on a planet.
  – Stars are scattered in the void all around you,
    most very distant and some closer.
  – Earth rotates on its axis once a day—and that
    makes the sky appear to revolve around you in a
    daily cycle.
  – Not only does the sun rise in the East and set in
    the West, but so do the stars.
                            The Stars The Sky

• On a dark night, far from city lights,
  you can see a few thousand stars in
  the sky.
                                     The Stars The Sky

• As you begin your study of the sky, the
  first step is to organize what you see—
  by naming groups of stars and
  individual stars and by specifying the
  brightness of individual stars.
  – That will make the sky familiar territory, and you
    will be ready to explore further.
                       Constellations The Sky

• All around the world, ancient cultures
  celebrated heroes, gods, and mythical
  beasts by naming groups of stars after
                               Constellations The Sky

• You should not be surprised that the
  constellations do not look like the creatures
  they represent any more than Columbus,
  Ohio, looks like Christopher Columbus.
  – The constellations
    ‘celebrate’ the most
    important mythical
    figures in each culture.
                                 Constellations The Sky

• The constellations named within Western
  culture originated in Mesopotamia over
  5,000 years ago.
• Other constellations were added by
  Babylonian, Egyptian, and Greek
  astronomers during the classical age.
  – Of these ancient constellations, 48 are still in use.
                               Constellations The Sky

• To the ancients, a constellation was a
  loose grouping of stars.
  – Many of the fainter stars were not included in any
  – Regions of the southern sky not visible to the
    ancient astronomers of northern latitudes were
    not identified with constellations.
                                   Constellations The Sky

• Constellation boundaries, when they
  were defined at all, were only
  – So, a star like Alpheratz could be thought of as part of
    Pegasus or part of Andromeda.
                       Constellations The Sky

• In recent centuries, astronomers have
  added 40 modern constellations to
  fill gaps.
                                  Constellations The Sky

• In 1928, the International Astronomical
  Union established 88 official
  constellations with clearly defined
  – A constellation now represents not a group of stars but
    an area of the sky.
  – Any star within the region belongs to one and only
    one constellation.
  – Alpheratz belongs to Andromeda.
                                Constellations The Sky

• Apart from the 88 official
  constellations, the sky contains a
  number of less formally defined
  groupings called asterisms.
  – The Big Dipper is a well-known asterism that is
    part of the constellation Ursa Major (Great Bear).
                         Constellations The Sky

– Another asterism is the Great Square of
  Pegasus, which includes three stars from
  Pegasus plus Alpheratz from Andromeda.
                                     Constellations The Sky

• Although you can refer to constellations
  and asterisms by name, most are made up
  of stars that are not physically associated
  with one another.
  – Some stars may be many
    times further away than
    others and moving through
    space in different directions.
  – The only thing they have in
    common is that they lie in
    approximately the same
    direction from Earth.
                  The Names of the Stars The Sky

• In addition to naming groups of stars,
  ancient astronomers named the brighter
  stars, and modern astronomers still use
  many of those
                  The Names of the Stars The Sky

• The constellation names come from
  Greek versions translated into Latin, the
  language of science from the fall of
  Rome to the 19th century.
                        The Names of the Stars The Sky

• Most star names come from ancient
  Arabic, though have been altered much
  by the passing centuries.
  – The name of Betelgeuse, the bright red star in Orion,
    comes from the Arabic yad al-jawza, meaning ‘armpit
    of Jawza (Orion).’
  – Names such as Sirius (the Scorched One) and
    Aldebaran (the Follower of the Pleiades) are beautiful
    additions to the mythology of the sky.
                    The Names of the Stars The Sky

• Naming individual stars is not
  very helpful.
  – You can see thousands of them.
  – Names do not help you locate stars in the
    sky or guess their brightness.
                       The Names of the Stars The Sky

• Another way to identify stars is to assign
  Greek letters to the bright stars in a
  constellation in the approximate order of
  – Thus, the brightest
    star is usually
    designated α (alpha),
    the second brightest
    β (beta), and so on.
                 The Names of the Stars The Sky

• For many constellations, the letters
  follow the order of brightness.
• However, some constellations—by
  tradition, mistake, or the personal
  preferences of early chartmakers—are
                         The Names of the Stars The Sky

• To identify a star by its Greek-letter
  designation, you give the Greek letter
  followed by the possessive (genitive) form
  of the constellation name.
  – The brightest star in the constellation Canis Major is α
    Canis Majoris.
  – This identifies both the star and the constellation and
    gives a clue to the relative brightness of the star.
  – Compare this with the ancient name for this star,
    Sirius, which tells you nothing about location or
                       The Names of the Stars The Sky

• This method of estimating a star’s
  brightness is only approximate.
• To discuss the sky with precision, you
  must have an accurate way of referring
  to the brightness of stars.
  – For that, you must consult two of the first great
                        The Brightness of Stars The Sky

• Astronomers measure the brightness of
  stars using the magnitude scale—a system
  that first appeared in the writings of the
  ancient astronomer Claudius Ptolemy
  about 140 AD.
  – The system may have originated earlier than Ptolemy.
  – Most astronomers attribute it to the Greek astronomer
    Hipparchus (190-120 BC).
                        The Brightness of Stars The Sky

• The ancient astronomers divided the
  stars into six classes.
  – The brightest were called first-magnitude stars and
    those that were fainter, second-magnitude.
  – The scale continued downward to sixth-magnitude
    stars—the faintest visible to the human eye.
  – Thus, the larger the magnitude number, the fainter
    the star.
  – This makes sense if you think of the bright stars as
    first-class stars and the faintest stars visible as
    sixth-class stars.
                   The Brightness of Stars The Sky

• Hipparchus is believed to have compiled
  the first star catalog, and he may have
  used the magnitude system in that catalog.
• Almost 300 years later, Ptolemy used the
  magnitude system in his own catalog.
• Successive generations of astronomers
  have continued to use the system.
                        The Brightness of Stars The Sky

• Modern astronomers can measure the
  light of stars to high precision.
  – So, instead of saying that the star known by the
    charming name Chort (Theta Leonis) is third
    magnitude, they can say its magnitude is 3.34.
                         The Brightness of Stars The Sky

• If you measure magnitudes, you would
  discover that some stars are brighter
  than 1.0.
  – Vega (α Lyrae) is so bright that its magnitude, 0.04, is
    almost zero.
                         The Brightness of Stars The Sky

• A few are so bright that the magnitude
  scale must extend into negative
  – Sirius, the brightest star in the sky, has a magnitude
    of -1.47.
                        The Brightness of Stars The Sky

• The faintest stars you can see with your
  unaided eyes are about sixth magnitude.
• If you use a telescope, you will see stars
  much fainter.
  – Thus, the scale has also been extended to include
    numbers larger than sixth magnitude to include fainter
                        The Brightness of Stars The Sky

• These numbers are known as apparent
  visual magnitudes (mv).
• They describe how the stars look to human
  eyes observing from Earth.
  – Although some stars emit large amounts of infrared or
    ultraviolet light, humans can’t see it, and it is not
    included in the apparent visual magnitude.
  – The subscript ‘v’ stands for ‘visual’ and reminds you
    that you are including only light you can see.
                      The Brightness of Stars The Sky

• Another problem is the distance
  to the stars.
  – Very distant stars look fainter and nearby stars
    look brighter.
  – Apparent visual magnitude ignores the effect of
    distance and informs you only how bright the star
    looks as seen from Earth.
                    The Brightness of Stars The Sky

• Your interpretation of brightness is quite
  subjective—depending on both the
  physiology of human eyes and the
  psychology of perception.
• To be accurate, you should refer to
  intensity—a measure of the light energy
  from a star that hits one square meter in
  one second.
                       The Brightness of Stars The Sky

• A simple relationship connects apparent
  visual magnitudes and the intensity of
  – Thus, modern astronomers
    can measure the brightness
    of stars to high precision,
    while still making
    comparisons to
    observations of apparent
    visual magnitude that go
    back to the time of
                       The Brightness of Stars The Sky

• It’s time for you to review the preceding
  section on constellations, star names, and
• How you review is critical when you study
  a science.
  – Memorizing facts won’t help you much.
  – Organizing your understanding into scientific
    arguments will line the facts up in a meaningful
                      The Brightness of Stars The Sky

• The review tool that follows—Building
  Scientific Arguments—will help you
  review an important concept from the
  – You should use the same process to review each
    concept from the beginning of the section to the
                 Building Scientific Arguments The Sky

• Nonastronomers sometimes
  complain that the magnitude scale is
  – Why would they think it is awkward?
  – How did it get that way?
               Building Scientific Arguments The Sky

• One of the main characteristics of a
  scientific argument is that it is carefully
• In this case, you can identify two things
  that might make the magnitude scale
  seem awkward.
                   Building Scientific Arguments The Sky

• First, it is backward—the bigger the
  magnitude number, the fainter the star.
  – That arose because ancient astronomers were not
    measuring the brightness of stars but rather classifying
  – First-class stars would be brighter than second-class
                  Building Scientific Arguments The Sky

• The second awkward feature of the
  scale is its mathematical relation to
  – If two stars differ by one magnitude, one is about
    2.5 times brighter than the other.
  – However, if they differ by two magnitudes, one is
    2.5 x 2.5 times brighter.
                Building Scientific Arguments The Sky

• Now, extend your scientific
  – If the magnitude scale is so awkward, why do
    you suppose astronomers have used it for over
    two millennia?
                         The Sky and Its Motion The Sky

• The sky seems to be a great blue dome
  in the daytime and a sparkling ceiling at
  – Learning to look at the sky requires that you begin
    of years ago.
                   The Sky and Its Motion The Sky

• Ancient astronomers believed the sky
  was a great sphere surrounding Earth,
  with the stars stuck on the inside like
  thumbtacks in a ceiling.
                          The Sky and Its Motion The Sky

• Modern astronomers know that the stars
  are scattered through space at different
  – However, it is still convenient to think of the sky as a
    great starry sphere enclosing
                           The Celestial Sphere The Sky

• As you study the sky, notice three
  important points.
• One, the sky appears to rotate westward
  around Earth each day.
  – That is a consequence
    of the eastward rotation
    of Earth.
  – That produces day
    and night.
                  The Celestial Sphere The Sky

• Two, astronomers measure distances
  across the sky as angles and express
  them as degrees, minutes, and
                           The Celestial Sphere The Sky

• Three, what you can see of the
  sky depends on where you are
  on Earth.
  – If you lived in Australia, you would see many
    constellations and asterisms invisible from North
    America, but you would never see the Big Dipper.
  – Alpha Centauri is in the southern sky and isn’t visible
    from most of the United States.
  – You could just glimpse it above the southern horizon
    if you were in Miami, but you could see it easily from
                         The Celestial Sphere The Sky

• This is a good time to eliminate a couple
  of common misconceptions.
• Lots of people—without thinking about it
  much—assume the stars are not in the
  sky during the daytime.
  – The stars are there day and night.
  – They are just invisible during the day because the
    sky is lit up by sunlight.
                           The Celestial Sphere The Sky

• Also, many people insist that Polaris is
  the brightest star in the sky.
  – You can see that Polaris is important because of its
    location, not because of its brightness.
                         The Celestial Sphere The Sky

• The celestial sphere is an example of
  a scientific model—a common feature
  of scientific thought.
  – Notice that a scientific model does not have to be
    true to be useful.
                                     Precession The Sky

• In addition to the daily motion of the sky,
  Earth’s rotation adds a second motion to
  the sky that can be detected only over
  – Over 2,000 years ago, Hipparchus compared a few of
    his star positions with those made nearly two centuries
    earlier and realized that the celestial poles and
    equator were slowly moving across the sky.
  – Later astronomers understood that this motion is
    caused by the toplike motion of Earth.
                                         Precession The Sky

• If you have ever played with a gyroscope
  or top, you have seen how the spinning
  mass resists any change in the direction of
  its axis of rotation.
  – The more massive the top and
    the more rapidly it spins, the more
    difficult it is to change the direction
    of its axis of rotation.
                                      Precession The Sky

• However, you probably recall that the axis
  of even the most rapidly spinning top
  sweeps around in a conical motion.
  – That is, the axis of the top pivots, so the axis sweeps
    out the surface of a cone.
  – The weight of the top tends to
    make it tip.
  – This combines with its rapid
    rotation to make its axis sweep
    around in a conical motion called
                                     Precession The Sky

• Earth spins like a giant top, but it does not
  spin upright in its orbit—it is tipped 23.5°
  from vertical.
  – Earth’s large mass and rapid rotation keep its axis of
    rotation pointed toward a spot near Polaris.
  – The axis would not
    wander if Earth
    were a perfect
                            Precession The Sky

• However, Earth, due to its rotation, has
  a slight bulge around its middle.
• The gravity of the sun and of the moon
  pull on this bulge, tending to twist Earth
  upright in its orbit.
                          Precession The Sky

• The combination of these forces and
  Earth’s rotation causes Earth’s axis to
  precess in a conical motion, taking
  about 26,000 years
  for one cycle.
                                   Precession The Sky

• As the celestial poles and equator are
  defined by Earth’s rotational axis,
  precession moves these reference
  – You notice no change at all from night to night or
    year to year.
  – Nevertheless, precise measurements reveal the
    precessional motion of the celestial poles and
                                    Precession The Sky

• Over centuries, precession has
  dramatic effects.
  – Egyptian records show that 4,800 years ago the north
    celestial pole was near the star Thuban (α Draconis).
  – The pole is now approaching Polaris and will be
    closest to it in about 2100.
  – In about 12,000 years, the pole will have moved to
    within 5° of Vega (α Lyrae).
  – Someday, Vega will be a very impressive north star.
                      Precession The Sky

• The figure shows the path followed
  by the north celestial pole.
                                 Precession The Sky

• As you study astronomy, notice the
  special terms used to describe such
  things as precession and the celestial
  – You need to know those terms.
• However, science is about
  understanding nature, not about naming
  its parts.
  – Science is more than just vocabulary.
                Building Scientific Arguments The Sky

• Does everyone see the same
  circumpolar constellations?
 – Here, you must use your imagination and build
   your argument with great care.
 – You can use the celestial sphere as a convenient
   model of the sky.
            Building Scientific Arguments The Sky

• A circumpolar constellation is one
  that does not set or rise.
                   Building Scientific Arguments The Sky

• Which constellations are circumpolar
  depends on your latitude.
  – If you live on Earth’s equator, you see all the
    constellations rising and setting and there are no
    circumpolar constellations at all.
                  Building Scientific Arguments The Sky

– If you live at Earth’s North Pole, all the
  constellations north of the celestial equator never
  set and all the constellations south of the celestial
  equator never rise.
– In that case, every constellation is circumpolar.
                    Building Scientific Arguments The Sky
– At intermediate latitudes,
  the circumpolar regions
  are caps on the sky
  whose angular radius
  equals the latitude of the
– If you live in Iceland, the
  caps are very large and,
  if you live in Egypt, near
  the equator, the caps are
  much smaller.
                 Building Scientific Arguments The Sky
– For people in Canada, Ursa Major is circumpolar,
  but people in Mexico see most of this
  constellation slip below the horizon.
– From much of the United States, some of the
  stars of Ursa Major set and some do not.
– In contrast, Orion rises and sets as seen from
  nearly everywhere on Earth.
– Explorers at Earth’s poles, however, never see
  Orion rise or set.
                  Building Scientific Arguments The Sky

• Now, use the argument you have just
  – How would you improve the definition of a circumpolar
    constellation to clarify the status of Ursa Major?
  – Would your definition help in the case of Orion?

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