Transcript Sky & Astronomy - Wayne State University Physics and Astronomy

Observing the Sky
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Some Basic Terms & Definitions
Angles
Zenith & nadir
Celestial sphere
Horizon
Ecliptic
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What’s an Angle?
In astronomy as in geometry, one
measures angles in degrees or in radians
A full circle makes 360o or 2p radians
45o
360o
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More about Angles
Examples
Note: p = 3.1415927 ...
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360o
2p
180o
p
90o
p/2
45o
p/4
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Earthly Questions
Do you believe the Earth is round?
If you do (or don’t), what are your
reasons?
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Dome of the Sky
The sky on a clear night appears to the naked eye as
a great hollow dome that is part of a great sphere
which is turning around the observer
This great sphere is called the celestial sphere
The top of the dome is called the zenith
The horizon is
where the dome
meets the Earth
The point on the
celestial sphere
that is directly
opposite the
zenith is called
the nadir
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Celestial Sphere
Today we know that the apparent turning of the celestial
sphere is caused by
the Earth’s spinning
on its axis, which
passes through the
Earth’s North and
South Poles
If this axis is extended
outward, the points
where it meets the
celestial sphere are
defined as the north
celestial pole and the
south celestial pole
The celestial equator
lies halfway between
the celestial poles
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The turning of the
sky looks different
depending on your
latitude on Earth
To an observer at
the Earth’s North
Pole, the stars
circle the zenith
They do not rise
and set
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To an observer at the Earth’s
equator, the celestial poles are
on the horizon
The stars rise straight up and
set straight down
To an observer at an
intermediate latitude, the north
celestial pole is at some position
between overhead and the
horizon
The stars rise and set at an
angle to the horizon
Apparent Motion of the Sun
Because of the Earth’s revolution around the Sun, its
position on our sky
changes day by day
The path that the
Sun appears to take
around the celestial
sphere is called the
ecliptic
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Rising and Setting of the Sun
The Sun gradually changes position on the
celestial sphere, moving each day by one degree
It also rises 4 minutes later each day
Why?
1 day
Sun
Earth
~1o
1o = 24 hours/360 ~ 4 minutes
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Constellations
On a clear night, about 3,000 stars are
visible to the naked eye in sky around us
Ancient Chinese, Egyptians, and Greeks
have grouped them into constellations
Today we use “constellation” to mean
one of the 88 sectors of the sky
Some of the old names are still in use
Constellations are listed in Appendix 14
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Ancient Astronomy (1)
Babylonian, Assyrian, and Egyptian astronomers knew
the approximate length of the year
The Mayans of Central America developed a calendar
based on the planet Venus
In the British Isles, one finds spectacular monuments
(such as Stonehenge) that, one now believes, were
used to track the motion of the Sun and the Moon
Pythagoras already suggested (2500 years ago) that
the Earth should be a sphere
Aristotle (384–322 B.C.) summarized the knowledge
of his day…
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Summary of Aristotle’s Astronomy
The progression of the Moon’s phases results
from our seeing different portions of the
Moon’s sunlit hemisphere during the month
The Sun being farther away from the Earth
than the Moon occasionally causes the Moon
to hide the Sun temporarily from view
We call this a solar eclipse
The Earth must be round for 2 reasons:
The Earth’s shadow on the Moon during Moon
eclipses is always round
The apparent height of the Northern Star (now
called Polaris) decreases as one travels south,
which is inconsistent with a flat Earth, but
explainable with a round Earth
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Ancient Astronomy (2)
Aristarchus (310-230 B.C.) of Samos, Greece,
suggested that the Earth was moving around the
Sun
His ideas were, however, dismissed by Aristotle
and most of the ancient Greek scholars on the
basis that they did not find any observable
parallax of the stars
Parallax is the
apparent shift in
the position of an
object as a result
of the motion of
the observer
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Stellar Parallax
Movie demonstrating the (greatly exaggerated)
parallax motion of a nearby star relative to distant
stars in the background, as seen from the Earth
Note that in the movie none of the stars is moving, and that
the apparent motion of the nearby star is only a reflection of
the Earth’s orbital motion
Note also that the parallax is less if the nearby star is farther
away from the Earth
Astronomers in Aristarchus’ time were not able to
observe stellar parallax because they did not have the
tools that were sufficiently precise
Their inability to detect stellar parallax led them to
accept the geocentric view of the universe
Erastosthenes: a Brilliant Astronomer
Erastosthenes, a Greek living in Alexandria
around 200 B.C., made a measurement of the
size of the Earth based on a very simple, but
powerful observation
He noticed that on the first day of summer at
Syenne, Egypt, sunlight struck the bottom of a
vertical well at noon
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Erastosthenes’ Measurement of the Earth (1)
At the same date and time, in
Alexandria, Egypt, he observed
that the Sun was not directly
overhead (or striking the bottom
of a vertical well)
He observed further that the
light rays from the sun were
instead making an angle of 1/50
of a full circle with the vertical
(approximately 7 degrees)
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Erastosthenes’ Measurement of the Earth (2)
Angle ~ 1/50 of a full circle
Distance ~ 5,000 stadia
Circumference ~ 50 x 5,000 = 250,000 stadia ~ 40,000 km
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Erastosthenes’ measurement of the Earth (3)
He knew the distance between
Syenne and Alexandria to be
5,000 stadia
He was thus able to deduce that
the Earth’s circumference is
50 x 5,000 = 250,000 stadia
Despite the uncertainties with
his definition of stadia, it is
believed that he came within
20% of the actual value of
40,000 km
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Hipparchus
Hipparchus, born in Nicaea (now Turkey), erected a large
observatory on the island of Rhodes around 150 B.C.
He established a pioneering compilation of a very large
number of stars
and stellar objects
He discovered that
the position in the
sky of the north
celestial pole had
altered over the
previous century
and a half
This phenomenon
is called precession
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Ptolemy
Claudius Ptolemaeus (Ptolemy), in
Alexandria about the year 140 A.D.,
wrote Almagest, which is a huge
compilation of astronomical data
His most important contribution was a
geometrical representation of the
observed motions of the planets in the
solar system based on a geocentric
system
The Earth was at the center of things
His complicated geocentric model would
endure more than a thousand years
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Ptolemy’s Cosmological System
• A main circular orbit:
the deferent
• Small circles within:
epicycles
• Explanation of the
retrograde motion of
the planets
• In use until the
Renaissance …
• Movie showing 5-year
geocentric motions of
the Sun, Mars, Mercury,
and Venus
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Retrograde Planet Motion: the Modern View
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Nicolaus Copernicus
Copernicus (1473-1543), a Polish
cleric trained in law and medicine,
but interested in astronomy and
mathematics, initiated an intellectual
revolution that would lead to the
emergence of modern science
He found many deficiencies in the Ptolemaic model
He developed a heliocentric, or Sun-centered, model
of the solar system
He believed that any model of planetary motions must
account for observations
The Ptolemaic model not only failed to do that, but also was
clumsy and not elegant
Movie comparing the Ptolemaic (geocentric) and
Copernican (heliocentric) solar systems
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Galileo (1)
Galileo Galilei (1564-1642 AD)
was the first person known to
use a telescope for astronomical
observations (starting in 1609)
The telescope was originally used as a naval
tool to assess the strength of the opponent's
fleet from a great distance
Galileo found many new things when he
looked through his telescope
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Galileo (2)
His telescope enabled him to see
many, many new fainter stars
that had never been seen before
The superior resolution and
magnification of his telescope
enabled him to
see pits and craters on the Moon
see spots on the Sun :
these objects are not static
they decay, they are not god-like.
discover four moons orbiting Jupiter
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Galilean Satellites
The 4 moons of Jupiter are Io,
Europa, Ganymede, and Callisto
They are called Galilean
satellites in Galileo’s honor
Jupiter and its moons form a mini-model of
the heliocentric system
The moons are not moving around the
Earth, but are centered on Jupiter
He concluded that perhaps other objects,
including the planets, did not move around
the Earth
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Definitive Evidence of Earth’s Motion
In 1729, James Bradley (1693-1762) discovered that
a telescope has to be slightly tilted because of the
Earth's motion
The direction of the telescope must be tilted constantly as the
Earth orbits the Sun
Over a century later, Friedrich W. Bessel (1784-1846)
provided further evidence for the Earth's motion by
measuring the parallax of a nearby star in the late
1830s
The measurements of Bradley and Bessel required
technology and precision beyond that of Galileo's time
The telescope tilt angle is less than half an arc minute
The parallax angle of even the nearest star is less than one
arc second
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Galileo’s Studies of Motion (1)
Galileo also made advances in understanding
how ordinary objects move here on the Earth
He set up experiments to see how things
move under different circumstances
He found that Aristotle's long-unchallenged
views on how things move were wrong
Aristotle’s views:
In order for something to keep moving, at even a
constant speed, a force must be continually applied
A falling object falls at a constant speed
Heavier things always fall more quickly than lighter
things
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Galileo’s Studies of Motion (2)
Galileo discovered that
An object's motion is changed only by
having a force act on it
Objects falling to the ground accelerate as
they fall
All objects, regardless of their size, fall
with the same acceleration in the absence
of air drag
Galileo's studies on how forces operate
also provided the foundation to prove
that the Earth spins on its axis
Beginning of a Revolution
Galileo’s astronomical observations confirmed the
Copernican heliocentric model of the solar system
The new ideas from Copernicus and Galileo began
a revolution in our conception of the cosmos
(universe)
It became clear that the universe is a vast place
and that the Earth’s role in it is relatively
unimportant
There may be other places like Earth in the
universe
As the Earth was demoted from its position at the
center of the universe, so too was humanity
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