Transcript The Sky

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Michael Seeds –
Astronomy: The Solar System and
Beyond, 5th Edition
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The Sky
Astronomy: The Solar System and Beyond
5th 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
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. So, from
your viewpoint, the sky appears to rotate once
around you each day.
– Not only does the sun rise in the East and set
in the West, but so do the stars.
– This apparent motion is caused by the rotation
of our planet.
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
• The ancient astronomers organized
what they saw by naming stars and
groups of stars.
– Some of these names survive to this day.
You may know a few of these groups of stars as
The Sky
• All around the world, ancient
cultures celebrated heroes, gods,
and mythical beasts by naming
groups of stars after them.
– 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 simply celebrate the most
important mythical figures in each culture.
The Sky
• The constellations named within the
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.
The Sky
• To the ancients, a constellation was
a loose grouping of stars.
– Many of the fainter stars were not included
in any constellation. Regions of the
southern sky, not visible to the ancient
astronomers of northern latitudes, were not
identified with constellations.
The Sky
• Constellation boundaries, when they were
defined at all, were only approximate.
– So, a star like
Alpheratz could
be thought of as
part of Pegasus
or part of
• In order to correct these gaps and
ambiguities, astronomers in recent
centuries have added 40 modern
constellations to
fill gaps.
The Sky
The Sky
• In 1928, the International Astronomical
Union established 88 official constellations
with clearly defined boundaries.
– Thus, 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.
The Sky
• In addition to the 88 official constellations,
the sky contains a number of less formally
defined groupings called asterisms.
– The Big Dipper, for example, is a well-known
asterism that is part of the constellation Ursa
Major (Great Bear).
– Another asterism is the Great Square of Pegasus,
which includes three stars from Pegasus and one
(Alpheratz) from Andromeda.
The Sky
• Although constellations and asterisms
are named based on what is visible in
the sky, it is important to remember
that most of these groups
are made up of stars
that are not physically
associated with one
The Sky
• 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
• In addition to
naming groups of
stars, ancient
named the brighter
stars. Modern
still use many of
those names.
The Sky
The Names of the Stars
The Sky
• Whereas the names of the
constellations are in Latin, the
common language of science in
Renaissance Europe, most star
names derive from ancient Arabic.
Although, they have been much
altered by the passing of centuries.
The Names of the Stars
– The name of
Betelgeuse, the bright
red star in Orion, for
example, comes from
the Arabic yad al-jawza,
meaning ‘armpit of
Jawza (Orion).’
– Aldebaran, the bright
red eye of Taurus the
bull, comes from the
Arabic al-dabar-an,
meaning ‘the follower.’
The Sky
The Names of the Stars
The Sky
• Naming individual stars is not very helpful,
because you can see thousands of them and the
names do not help you locate stars in the sky.
The Names of the Stars
• Another way to
identify stars is to
assign Greek
letters to the bright
stars in a
constellation in the
approximate order
of brightness.
– Thus, the brightest
star is usually
designated α
(alpha), the second
brightest β (beta),
and so on.
The Sky
The Names of the Stars
The Sky
• In many constellations, the letters
follow the order of brightness.
– However, in some constellations, by
tradition, mistake, or the personal
preferences of early chartmakers, there
are exceptions.
The Names of the Stars
The Sky
• A star’s Greek-letter designation is the
Greek letter followed by the genitive
(possessive) form of the constellation name.
– For example, 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 identifying a star’s
brightness is only approximate.
– In order to discuss the sky with precision, you
must have an accurate way of referring to the
brightness of stars.
– For that, you must consult
Hipparchus, one of the
first great astronomers.
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, and most astronomers attribute it to the
Greek astronomer Hipparchus (160-127 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, and
successive generations of astronomers
have continued to use the system.
The Brightness of Stars
The Sky
• Modern astronomers can
measure the brightness 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 will discover that
some stars are brighter than 1.0. For example, Vega
(α Lyrae) is so bright that its magnitude, 0.04, is
almost zero.
– A few are so bright that the magnitude scale must
extend into negative numbers. On this scale Sirius,
the brightest
star in the
sky, has a
of -1.47.
The Brightness of Stars
The Sky
• If you use a telescope to search for very faint
stars, you can find stars much fainter than
the limit for the unaided eye.
– Thus, the magnitude system has also been
extended to numbers larger than sixth magnitude to
fainter stars.
The Brightness of Stars
The Sky
– These numbers are known as apparent visual
magnitudes (mv), and 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. It
is not included in the apparent visual magnitude.
– The subscript ‘v’ reminds you that you are
including only light you can see.
The Brightness of Stars
• Another problem
is distance.
– Very distant stars
look fainter and
nearby stars look
– Apparent visual
magnitude ignores
the effect of distance
and tells only how
bright the star looks,
as seen from Earth.
The Sky
The Brightness of Stars
The Sky
• Sometimes, it is convenient for
astronomers to convert apparent
visual magnitude into 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 Hipparchus.
Building Scientific Arguments
The Sky
• Nonastronomers sometimes
complain that the magnitude scale
is awkward.
– Why would they think it is awkward, and
how did it
get that
Building Scientific Arguments
The Sky
• Two things might make the
magnitude scale seem awkward.
– First, it seems backward: the bigger the
magnitude number, the fainter the star.
– Of course, that is because ancient astronomers
were not measuring the brightness of stars but
rather classifying them, and first-class stars
would be brighter than second-class stars.
Building Scientific Arguments
The Sky
– The second awkward feature of the
magnitude scale is its mathematical relation
to intensity.
– If two stars differ by one magnitude, one is
about 2.5 times brighter than the other. But, if
they differ by two magnitudes, one is 2.5 x
2.5 times brighter.
– This mathematical relationship arises
because of the way human eyes perceive
brightness as ratios of intensity.
Building Scientific Arguments
The Sky
• Now, build your own scientific
argument to analyze the following
– 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
• Ancient astronomers believed the sky was a
great sphere surrounding Earth, with the
stars stuck on the inside like
thumbtacks in a ceiling.
– Modern astronomers know that
the stars are scattered through
space at different distances, but
it is still convenient to think of
the sky as a great starry sphere
enclosing Earth.
The Celestial Sphere
The Sky
• As you study the sky, you will notice
three important points.
– One, the sky appears to rotate westward around
Earth each day, but that is a consequence of the
eastward rotation of Earth. That rotation produces
day and night.
– Two, astronomers measure distances across the
sky as angles and express them as degrees,
minutes, and seconds.
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.
– For example, the star 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 Australia.
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.
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 recorded
nearly two centuries before and realized that the
celestial poles and equator were slowly moving
across the sky.
– Later astronomers understood that this motion is
caused by the top-like motion of Earth.
– 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
– 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.
The Sky
– However, you probably
recall that the axis of even
the most rapidly spinning
top moves as it spins,
describing the surface of
a cone.
– The weight of the top
tends to make it tip, and
this combines with its
rapid rotation to make its
axis sweep around in a
conical motion called
The Sky
The Sky
• Earth spins like a giant top and is tipped
23.5° from vertical.
– Earth’s large mass and rapid rotation keep its axis
of rotation pointed toward
a spot near the star
Polaris, and the axis
would not wander if
Earth were a perfect
The Sky
– However, Earth, because of its rotation, has a
slight bulge around its middle, and the gravity of
the sun and moon pulls on this bulge. That tends to
twist Earth upright in its orbit.
– 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.
The Sky
– Because the celestial poles and equator are
defined by Earth’s rotational axis, precession
moves these reference marks.
– We notice no change at all from night to night
or year to year, but precise measurements
reveal the precessional motion of the celestial
poles and equator.
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 around 12,000 years, the pole will have
moved to within 5° of Vega (α Lyrae).
– The figure shows
the path followed
by the north
celestial pole.
The Sky
The Sky
• As you study astronomy, notice the special
terms used to describe such things as
precession and the celestial sphere.
– You need to know those terms, but science is
about understanding nature, not about
naming its parts.
– Keep in mind that science
is more than just
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
• A circumpolar
constellation is
one that does
not set or rise.
The Sky
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
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
– 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.
The Sky
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 built.
– How would you improve the definition of a
circumpolar constellation to clarify the status of
Ursa Major?
– Would your
help in the
of Orion?