Download 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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Dome of the Sky
• The sky on a clear night appears to the naked eye
as a great hollow dome
– The top of the dome is called the zenith
– The horizon is where the dome meets the Earth
– The point of the celestial sphere that is directly opposite
the zenith is called the nadir
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Celestial Sphere
• The view of
an observer
at the north
pole
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Celestial Sphere
• The view of an
observer at
the equator
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Apparent Motion of the Sun
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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 3000 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 Bristish Isles, one finds spectacular
monuments that, one now believes, were used to
track the motion of the Sun and the Moon
• Pythagoras already knew (2500 years ago) the
Earth is round
• Aristotle (384 –322 BC) summarized the
knowledge of his day…
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Summary of Aristotle’s Astronomy
• Progression of the Moon phases
• Sun being farther away from the Earth
than the Moon
• The roundness of the Earth
– Earth’s shadow on the Moon during Moon
eclipses is always round
– The apparent height of the Northern star (now
Polaris) decreases as one travels south –
inconsistent with a flat Earth, but explainable
with a round Earth
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Ancient Astronomy (2)
• Aristarchus (310-230 BC) of Samos,
Greece, suggested long before Copernicus
that the Earth moves around the Sun
• His ideas were, however, dismissed by
Aristotle on the basis that there is no
observable parallax of the stars
• Aristotle made quite reasonable
conclusions based on his observations
which in the end turned out to be wrong!!!
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Parallax
• This is the apparent shift in the position of
an object as a result of the motion of the
observer
q
d
L
L ~ d/ q (radians)
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Erastosthenes: A Brilliant Astronomer
• Erastosthenes, a Greek living in Alexandria
around 200 BC, 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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1/50 of a circle
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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 BC
• 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
• Ptolemy, about the year 140 AD, wrote
Almagest, which is a huge compilation of
astronomical data
• His most important contribution was a
geometrical representation of the observed
motion 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…
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Epicycle
Earth
Equant point
Center of
eccentric
Deferent
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Retrograde Planet Motion:
The Modern View
Planet’s orbit
Earth’s orbit
E
D
E
E
D
B
C
C
C
A
Sun
D
B
A
B
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A
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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
– Copernicus’ model still retained the Aristotelian idea of
uniform circular motions for the planets and used small
epicycles to get the details of the retrograde loops correct
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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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Phases of Venus (1)
• Galileo also made the important discovery
that Venus goes through a complete set of
phases
• The gibbous and full phases of Venus were
impossible in the Ptolemaic model, but
possible in Copernican model
• In the Ptolemaic model, Venus was always
approximately between the Earth and the
Sun and was never found further away
from the Earth than the Sun
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Phases of Venus (2)
• Problem:
– According to the Ptolemaic model, Venus should
always be in a crescent, new, or quarter phase
• Solution:
– Venus has a gibbous or full phase because it
orbits the Sun
– With respect to our viewpoint, Venus could get
on the other side of, or behind, the Sun further
away from us than the Sun
• This is a prime example of the scientific
method and progress…
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Galileo’s Battle with the Church
• Galileo’s astronomical observations confirmed the
Copernican heliocentric model of the universe
– This eventually put him in conflict with the authorities of
the 17th century Church, who still upheld the geocentric
ideas of Aristotle and Ptolemy
– For Galileo himself, there was no contradiction between
the authority of the Church (in matters of religion and
morality) and the authority of nature (as revealed by
experiments, in matters of science)
• Although his observations were consistent with the
heliocentric model, they could also be explained
with a geocentric model like Tycho Brahe's
– But for Galileo, the observations were enough because he
had already been convinced of the heliocentric system
before he used his telescope, and his observations only
confirmed his belief
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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
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A Need for Better Technology!
• 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 views on
how things move were wrong
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Aristotle’s Views on Motion
• 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's observations of moving objects
contradicted the long-unchallenged
physics of Aristotle
• 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
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Galileo’s Studies of Motion (3)
• Galileo's studies on how forces operate also
provided the foundation to prove that the
Earth spins on its axis
• Although the stars and Sun appear to rise
and set every night or day, they are
actually stationary — it is the Earth that
moves
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Beginning of a Revolution
• 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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