Download Chapter 2 User`s Guide to the Sky: Patterns and Cycles

Michael Seeds
Dana Backman
Chapter 2
User’s Guide to the Sky:
Patterns and Cycles
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 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 about 100 kilometers deep.
• Beyond that, space is nearly empty—with the planets
of our solar system several AU away and the far more
distant stars scattered many light-years apart.
• You can begin your understanding of
the natural laws that govern the
universe by carefully noting what the
universe looks like and how it behaves.
• Keep in mind that you live on a
planet, a moving platform.
• Earth rotates on its axis once a day.
• So, from our viewpoint, sky objects appear to rotate
around us each day.
• For example, the sun rises in the east and sets in the
west, and so do the stars.
• The sun, the moon, planets, stars, and galaxies all
have an apparent daily motion that is not real but is
caused by a real motion of Earth.
The Stars
• On a dark night, far from city lights,
you can see a few thousand stars.
• Your observations can be summarized by naming
individual stars and groups of stars and by specifying
their relative brightness.
Constellations
• All around the world, ancient cultures
celebrated heroes, gods, and mythical
beasts by naming groups of stars called
constellations.
Constellations
• You should not be surprised that the star
patterns do not look like the creatures they
are named after any more than Columbus,
Ohio, looks like Christopher Columbus.
Constellations
• The constellations named within Western
culture originated in Mesopotamia,
Babylon, Egypt, and Greece beginning as
much as 5,000 years ago.
• Of these ancient constellations, 48 are still in use.
Constellations
• In those former times, a constellation
was simply a loose grouping of bright
stars.
• Many of the fainter stars were not included in any
constellation.
• Regions of the southern sky not visible to the ancient
astronomers living at northern latitudes also were not
identified with constellations.
Constellations
• Constellation boundaries, when they
were defined at all, were only
approximate.
• So, a star like Alpheratz could be thought of as part of
Pegasus and
also part of
Andromeda.
Constellations
• In recent centuries, astronomers have
added 40 modern constellations to
fill gaps.
Constellations
• In 1928, the International Astronomical
Union (IAU) established 88 official
constellations with clearly defined
permanent boundaries that together
cover the entire sky.
• A constellation now represents not a group of stars
but a section of the sky—a viewing direction.
• Any star within the region belongs to only
that one constellation.
Constellations
• In addition to the 88 official
constellations, the sky contains a
number of less formally defined
groupings known as asterisms.
• For example, the Big Dipper is an asterism you
probably recognize that is part of the constellation
Ursa Major (the Great Bear).
Constellations
• Another asterism is the Great Square of Pegasus
that includes three stars from Pegasus and
Alpheratz, now considered to be part of
Andromeda only.
Constellations
• Although constellations and asterisms
are named as if they were real
groupings, most are made up of stars
that are not physically associated with
one another.
• Some stars may be many times farther away than
others in the same constellation and moving through
space in different directions.
Constellations
• 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 astronomers named the
brighter stars.
• Modern astronomers still use
many of those names.
The Names of the Stars
• The names of the constellations are in
Latin or Greek, the languages of science in
Medieval and Renaissance Europe.
The Names of the Stars
• Most individual star names derive
from ancient Arabic, much altered
over centuries.
• The name of Betelgeuse, the bright red star in Orion,
comes from the Arabic phrase ‘yad aljawza,’ meaning
‘armpit of Jawza (Orion).’
• Aldebaran, the bright red eye of Taurus the bull,
comes from the Arabic ‘aldabar an,’ meaning ‘the
follower.’
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 alpha (α),
the second brightest
beta (β), and so on.
The Names of the Stars
• For many constellations, the letters follow
the order of brightness.
• However, some
constellations are
exceptions.
The Names of the Stars
• A Greek-letter star name also
includes the possessive form of the
constellation name.
• For example, the brightest star in the constellation
Canis Major is alpha Canis Majoris.
• This name identifies the star and the constellation
and gives a clue to the relative brightness of the star.
• Compare this with the ancient individual name for that
star, Sirius, which tells you nothing about its location
or brightness.
The Brightness of Stars
• Astronomers measure the
brightness of stars using the
magnitude scale.
The Brightness of Stars
• The ancient astronomers divided
the stars into six brightness groups.
• The brightest were called first-magnitude stars.
• The scale continued downward to sixth-magnitude stars
—the faintest visible to the human eye.
The Brightness of Stars
• 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 Greek astronomer Hipparchus (190–
120 BC) is believed to have compiled the
first star catalog.
• There is evidence he used the magnitude
system in that catalog.
The Brightness of Stars
• About 300 years later (around AD 140),
the Egyptian-Greek astronomer Claudius
Ptolemy definitely used the magnitude
system in his own catalog.
• Successive generations of astronomers
have continued to use the system.
The Brightness of Stars
• Star brightnesses expressed in this
system are known as apparent visual
magnitudes (mV).
• These describe how the stars look to
human eyes observing from Earth.
The Brightness of Stars
• Brightness is quite subjective.
• It depends on both the physiology of human eyes and
the psychology of perception.
• To be scientifically accurate, you
should refer to flux.
• This is a measure of the light energy from a star that
hits one square meter in one second.
The Brightness of Stars
• With modern scientific instruments, you can
measure the intensity of starlight with high
precision and then use a simple
mathematical relationship that relates light
intensity to apparent visual magnitude.
• So, instead of saying that the star known by the
charming name Chort (Theta Leonis) is about third
magnitude, you can say its magnitude is 3.34.
The Brightness of Stars
• Thus, precise modern measurements
of the brightness of stars are still
connected to observations of apparent
visual magnitude that go back to the
time of Hipparchus.
The Brightness of Stars
• Limitations of the apparent visual
magnitude system have motivated
astronomers to supplement it in
various ways.
The Brightness of Stars
• One, some stars are so bright that
the scale must extend into negative
numbers.
• Sirius, the brightest star in the sky, has a magnitude
of –1.47.
The Brightness of Stars
• Two, with a telescope, you can find stars
much fainter than the limit for your
unaided eyes.
• Thus, the magnitude system has also been extended
to include numbers larger than sixth magnitude to
include fainter stars.
The Brightness of Stars
• Three, although some stars emit
large amounts of infrared or
ultraviolet light, those types of
radiation are invisible to human eyes.
• The subscript ‘V’ in mV is a reminder that you are
counting only light that is visible.
• Other magnitudes systems have been invented to
express the brightness of invisible light arriving at
Earth from the stars.
The Brightness of Stars
• Four, an apparent magnitude informs you
only how bright the star is as seen from
Earth.
• It doesn’t reveal anything about a star’s true
power output—because the star’s distance
is not included.
The Sky and Its Motions
• The sky above you seems to be a
great blue dome in the daytime and a
sparkling ceiling at night.
• Learning to understand the sky
requires that you first recall the
perspectives of people who observed
the sky thousands of years ago.
The Celestial Sphere
• Ancient astronomers believed the sky
was a great sphere surrounding Earth,
with the stars stuck
on the inside—like
thumbtacks in a
ceiling.
The Celestial Sphere
• Modern astronomers know that the stars
are scattered through space at different
distances.
• However, it is still
convenient to think of
the sky as a great sphere
enclosing Earth with stars
all at one distance.
The Celestial Sphere
• The celestial sphere is an example of
a scientific model, a common feature
of scientific thought.
• You can use the celestial sphere as a convenient
model of the sky.
The Celestial Sphere
• As you study the sky, you will
notice three important points.
The Celestial Sphere
• One, sky objects appear to rotate westward
around Earth each day, but that is a
consequence of Earth’s eastward rotation.
• This produces day and night.
The Celestial Sphere
• Two, what you can see of the sky
depends on where you are on Earth.
• For example, Australians see many constellations and
asterisms invisible from North America, but they never
see the Big Dipper.
The Celestial Sphere
• Three, astronomers measure distances
across the sky as angles.
• These are expressed in units of degrees and
subdivisions of degrees called arc minutes and arc
seconds.
Precession
• 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 centuries.
Precession
• More than 2000 years ago, Hipparchus
compared a few of his star positions with
those made by other astronomers nearly two
centuries before him.
• He realized that the celestial poles and
equator were slowly moving relative to the
stars.
Precession
• Later astronomers understood
that this apparent motion is
caused by a special motion of
Earth called precession.
Precession
• If you have ever played with a toy top or
gyroscope, you may recall that the axis of
such a rapidly spinning object sweeps
around relatively slowly in a circle.
• The weight of the top tends to
make it tip.
• This combines with its rapid
rotation to make its axis sweep
around slowly in precession
motion.
Precession
• Earth spins like a giant top, but it does
not spin upright relative to its orbit
around the sun.
• You can say either
that Earth’s axis
is tipped 23.5°
from vertical or
that Earth’s equator
is tipped 23.5°
relative to its orbit.
Precession
• Earth’s large mass and rapid rotation keep
its axis of rotation pointed toward a spot
near Polaris (alpha Ursa Minoris).
• Its axis direction would not move if Earth
were a perfect sphere.
Precession
• However, Earth has a slight bulge around
its middle.
• The gravity of the sun and moon pull on
this bulge, tending to twist Earth’s axis
upright relative to its orbit.
Precession
• The combination of these forces and
Earth’s rotation causes Earth’s axis to
precess in a slow circular sweep—taking
about 26,000 years
for one cycle.
Precession
• As the celestial poles and equator are
defined by Earth’s rotational axis,
precession moves these reference
marks.
• You would notice no change at all from night to night
or year to year.
• Precise measurements, though, reveal their slow
apparent motion.
Precession
• Over centuries, precession has
dramatic effects.
• Egyptian records show
that 4,800 years ago
the north celestial pole
was near Thuban
(alpha Draconis).
• Now, the pole is
approaching Polaris
and will be closest to it
in about 2100.
Precession
• In about 12,000 years, the pole will have moved to
the apparent vicinity of the very bright star Vega
(alpha Lyrae).
• The figure shows
the apparent path
followed by the north
celestial pole over
thousands of years.
The Cycle of the Sun
• Rotation is the turning of a body on its
axis.
• Revolution means the motion of a body
around a point outside the body.
• Earth rotates on its axis—and that produces day and
night.
• Earth also revolves around the sun—and that produces
the yearly cycle.
The Annual Motion of the Sun
• Even in the daytime, the sky is actually
filled with stars.
• However, the glare of sunlight fills Earth’s
atmosphere with scattered light, and you
can only see the brilliant blue sky.
The Annual Motion of the Sun
• If the sun were fainter and you could see
the stars in the daytime, you would notice
that the sun appears to be moving slowly
eastward relative to the background of the
distant stars.
• This apparent motion is caused by the real orbital
motion of Earth around the sun.
The Annual Motion of the Sun
• In January, you would see the sun in front
of the constellation Sagittarius.
• By March, it is in front of Aquarius.
The Annual Motion of the Sun
• Note that your angle of view in the figure
makes the Earth’s orbit seem very elliptical
when it is really almost a perfect circle.
The Annual Motion of the Sun
• Through the year, the sun moves eastward
among the stars following a line called the
ecliptic—the apparent path of the sun
among the stars.
• If the sky were a great screen, the ecliptic would be
the shadow cast by Earth’s orbit.
• In other words, you can call the ecliptic the projection
of Earth’s orbit on the celestial sphere.
The Annual Motion of the Sun
• Earth circles the sun in 365.26 days and,
consequently, the sun appears to go once
around the sky in the same period.
• You don’t notice this motion because you
cannot see the stars in the daytime.
The Annual Motion of the Sun
• However, the motion of the sun
caused by a real motion of Earth has
an important consequence that you
do notice—the seasons.
The Seasons
• The seasons are caused by the
revolution of Earth around the sun
combined with a simple fact you have
already encountered.
• Earth’s equator is tipped 23.5° relative to its orbit.
The Seasons
• There are two important principles
to note about the cycle of seasons.
The Seasons
• One, the seasons are not caused by
variation in the distance between Earth and
the sun.
• Earth’s orbit is nearly circular, so it is always about the
same distance from the sun.
The Seasons
• Two, the seasons are caused by changes
in the amount of solar energy that Earth’s
northern and southern hemispheres
receive at different times of the year—
resulting from the tip of the Earth’s equator
and axis relative to its orbit.
The Seasons
• The seasons are so important as a cycle of
growth and harvest that cultures around
the world have attached great significance
to the ecliptic.
• It marks the center line of the zodiac (‘circle of
animals’).
• Also, the motion of the sun, moon, and the five visible
planets (Mercury, Venus, Mars, Jupiter, and Saturn)
are the basis of the ancient superstition of astrology.
The Seasons
• However, the signs of the
zodiac are no longer important
in astronomy.
The Seasons
• You can look for the planets along
the ecliptic appearing like bright
stars.
• Mars looks quite orange in color.
The Seasons
• As Venus and Mercury orbit inside Earth’s
orbit, they never get far from the sun and are
visible in the west after sunset or in the east
before sunrise.
• Venus can be very bright, but Mercury is difficult to see
near the horizon.
The Seasons
• By tradition, any planet in the
sunset sky is called an evening
star.
• Any planet in the dawn sky is
called a morning star.
The Seasons
• Perhaps the most beautiful is Venus,
which can become as bright as
magnitude -4.7.
• As Venus moves around its orbit, it can dominate the
western sky each evening for many weeks.
• Eventually, its orbit appears to carry it back toward the
sun as seen from Earth, and it is lost in the haze near
the horizon.
• A few weeks later, you can see Venus reappear in the
dawn sky as a brilliant morning star.
• Months later, it will switch back to being an evening star.
The Cycles of the Moon
• The moon orbits eastwards
around Earth once a month.
The Cycles of the Moon
• Starting this evening, look for the
moon in the sky.
• If it is a cloudy night or if the moon is in the wrong
part of its orbit, you may not see it.
• Keep trying on successive evenings.
• Within a week or two, you will see the moon.
• Then, watch for the moon on following evenings.
• You will see it move along its orbit around Earth and
cycling through its phases as it has done for billions
of years.
The Motion of the Moon
• If you watch the moon night after night, you
will notice two things about its motion.
• First, you will see it moving relative to the
background of stars.
• Second, you will notice that the markings on
its face don’t change.
• These two observations will help you understand the
motion of the moon and the origin of the moon’s phases.
The Motion of the Moon
• The moon moves rapidly among the
constellations.
• If you watch the moon for just an hour, you can see it
move eastward against the background of stars by
slightly more than its own apparent diameter.
• Each night when you look at the moon, you will see it is
roughly half the width of a zodiac constellation—about
13 degrees—to the east of its location the night before.
• This movement is the result of the motion of the moon
along its orbit around Earth.
The Cycle of Moon Phases
• The changing shape of the illuminated part
of the moon as it orbits Earth is one of the
most easily observed phenomena in
astronomy.
The Cycle of Moon Phases
• There are three important points to
notice about the phases of the moon.
The Cycle of Moon Phases
• First, the moon always keeps the
same side facing Earth, and you never
see the far side of the moon.
• ‘The man in the moon’
(some cultures see
‘the rabbit in the moon’
instead) is produced by
familiar features on the
moon’s near side.
The Cycle of Moon Phases
• Second, the changing shape of the moon as
it passes through its cycle of phases is
produced by sunlight illuminating different
parts of the side of the moon you can see.
The Cycle of Moon Phases
• You always see the same side of the moon
looking down on you.
• The changing shadows, though, make the
‘man in the moon’ shift moods as the moon
cycles through its phases.
The Cycle of Moon Phases
• Finally, the orbital period of the moon
around Earth is not the same as the
length of a moon phase cycle.
Eclipses
• Eclipses are due to a seemingly
complicated combination of apparent
motions of the sun and moon.
• Yet, they are actually easy to predict
once all the cycles are understood.
Eclipses
• Eclipses are also among the most
spectacular of nature’s sights you
might witness.
Solar Eclipses
• From Earth, you can see a
phenomenon that is not visible from
most planets.
• It happens that the sun is 400 times larger than our
moon and, on the average, 390 times farther away.
• So, the sun and moon have nearly equal angular
apparent diameters.
• Thus, the moon is just about the right size to cover the
bright disk of the sun and cause a solar eclipse.
• In a solar eclipse, it is the sun that is being hidden
(eclipsed) and the moon that is ‘in the way.’
Solar Eclipses
• A shadow consists of two parts.
• The umbra is the region of total
shadow.
• For example, if you were in the umbra of the moon’s
shadow, you would see no portion of the sun.
• The umbra of the
moon’s shadow
usually just barely
reaches Earth’s
surface and covers
a relatively small
circular zone.
Solar Eclipses
• Standing in that umbral zone, you
would be in total shadow—unable to
see any part of the sun’s surface.
• This is called a total eclipse.
Solar Eclipses
• If you moved into the penumbra, you would
be in partial shadow, but could also see
part of the sun peeking around the edge of
the moon.
• This is called a partial eclipse.
Solar Eclipses
• If you are outside the penumbra,
you see no eclipse at all.
Solar Eclipses
• Due to the moon’s orbital motion and
Earth’s rotation, the moon’s shadow
sweeps rapidly across Earth in a long,
narrow path of totality.
• If you want to see a total solar eclipse, you must be in
the path of totality.
Solar Eclipses
• When the umbra of the moon’s shadow
sweeps over you, you see one of the most
dramatic sights in the sky—the totally
eclipsed sun.
Solar Eclipses
• The eclipse begins as the
moon slowly crosses in front of
the sun.
• It takes about an hour for the
moon to cover the solar disk.
Solar Eclipses
• As the last sliver of sun disappears,
dark falls in a few seconds.
• Automatic street lights come on, drivers of cars turn on
their headlights, and birds go to roost.
• The sky usually becomes so dark you can even see
the brighter stars.
Solar Eclipses
• The darkness lasts only a few
minutes.
• This is because the umbra is never more than 270 km
(170 miles) in diameter on Earth’s surface and sweeps
across the landscape at over 1,600 km/hr (1,000 mph).
• The period of totality lasts on average only 2 or 3
minutes and never more than 7.5 minutes.
Solar Eclipses
• During totality you can see subtle features of
the sun’s atmosphere.
• These include red flame-like projections that
are visible only during those moments when
the brilliant disk
of the sun
is completely
covered by
the moon.
Solar Eclipses
• As soon as part of the sun’s disk
reappears, the fainter features vanish
in the glare.
• The period of totality is over.
• The moon moves on in its orbit and, in an hour the
sun, is completely visible again.
Solar Eclipses
• Sometimes, when the moon crosses
in front of the sun, it is too small to
fully cover the sun.
• Then, you would witness an annular
eclipse.
• This is a solar eclipse in which an annulus (‘ring’) of
the sun’s disk is visible around the disk of the moon.
Solar Eclipses
• The eclipse never becomes
total.
• It never quite gets dark.
• You can’t see the faint features of the solar
atmosphere.
Solar Eclipses
• Annular eclipses occur because the
moon follows a slightly elliptical orbit
around Earth.
• If the moon is in the farther part of its orbit during
totality, its apparent diameter will be less than the
apparent diameter of the sun—and you see an
annular eclipse.
Solar Eclipses
• Also, Earth’s orbit is slightly
elliptical.
• So, the Earth-to-sun distance varies slightly.
• So does the apparent diameter of the solar disk.
• These contribute to the effect of the moon’s varying
apparent size.
Solar Eclipses
• If you plan to observe a solar eclipse,
remember that the sun is bright enough
to burn your eyes and cause permanent
damage if you look at it directly.
• This is true whether there is an eclipse or not.
Solar Eclipses
• Solar eclipses can be misleading—
tempting you to look at the sun in spite
of its brilliance and thus risking your
eyesight.
Solar Eclipses
• During the few minutes of totality, the
brilliant disk of the sun is hidden, and
it is safe to look at the eclipse.
• However, the partial eclipse phases
and annular eclipses can be
dangerous.
Solar Eclipses
• The figure demonstrates a safe way to
observe the partially eclipsed sun.
Solar Eclipses
• The table will allow you to determine when
some upcoming solar eclipses will be visible
from your location.
Lunar Eclipses
• Occasionally, you can see the moon
darken and turn copper-red in a lunar
eclipse.
Lunar Eclipses
• A lunar eclipse occurs at full moon
when the moon moves through Earth’s
shadow.
• As the moon shines only by reflected sunlight, you see
the moon gradually darken as it enters the shadow.
Lunar Eclipses
• If you were on the moon and in the umbra
of Earth’s shadow, you would see no
portion of the sun.
Lunar Eclipses
• If you moved into the penumbra, you would
be in partial shadow and would see part of
the sun peeking around the edge of Earth—
so the sunlight would be dimmed but not
extinguished.
Lunar Eclipses
• In a lunar eclipse, it is the moon that is
being hidden in the Earth’s shadow and
Earth that is ‘in the way’ of the sunlight.
Lunar Eclipses
• If the orbit of the moon carries it through
the umbra of Earth’s shadow, you see a
total lunar eclipse.
Lunar Eclipses
• As you watch the moon, it first moves
into the penumbra and dims slightly.
• The deeper it moves into the penumbra, the more it
dims.
Lunar Eclipses
• In about an hour, the moon reaches the
umbra, and you see the umbral shadow
darken part of the moon.
• It takes about an hour for the moon to enter the umbra
completely and become totally eclipsed.
Lunar Eclipses
• The period of total eclipse may last as long
as 1 hour 45 minutes.
• However, the timing of the eclipse depends
on where the moon crosses the shadow.
Lunar Eclipses
• When the moon is totally eclipsed, it
does not disappear completely.
• Although it receives no direct sunlight, the moon in the
umbra does receive some sunlight that is refracted
(bent) through Earth’s atmosphere.
Lunar Eclipses
• If you were on the moon during totality, you
would not see any part of the sun—as it
would be entirely hidden behind Earth.
• However, you would be able to see Earth’s atmosphere
illuminated from behind by the sun.
Lunar Eclipses
• The red glow from this ring consisting of all
the Earth’s simultaneous sunsets and
sunrises illuminates the moon during totality
and makes it glow coppery red.
Lunar Eclipses
• If the moon passes a bit too far north or
south of Earth’s shadow, it may only partially
enter the umbra.
• Then, you see a partial lunar eclipse.
Lunar Eclipses
• The part of the moon that remains outside
the umbra in the penumbra receives some
direct sunlight.
• The glare is usually great enough to prevent your seeing
the faint coppery glow of the part of the moon in the
umbra.
Lunar Eclipses
• Lunar eclipses always occur at full
moon but not at every full moon.
• The moon’s orbit is tipped about 5 degrees to the
ecliptic.
• So, most full moons cross the sky north or south of
Earth’s shadow and there is no lunar eclipse that
month.
Lunar Eclipses
• For the same reason, solar eclipses
always occur at new moon but not at
every new moon.
Lunar Eclipses
• The orientation of the moon’s orbit in
space varies slowly.
• As a result, solar and lunar eclipses
repeat in a pattern called the Saros
cycle lasting 18 years 11 1/3 days.
• Ancient peoples who understood the Saros cycle
could predict eclipses without understanding what
the sun and moon really were.
Lunar Eclipses
• Although there are usually no more
than one or two lunar eclipses each
year, it is not difficult to see one.
• You need only be on the dark side of Earth when the
moon passes through Earth’s shadow.
• That is, the eclipse must occur between sunset and
sunrise at your location to be visible.
Lunar Eclipses
• The table will allow you to
determine when some
upcoming lunar eclipses
will be visible from your
location.