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Astronomy by eye: Motions in
the Sky
The Celestial Sphere
Intrinsic and Reflex Motion
Reflex Motion from Earth’s
Rotation
Recap
• Astronomy and science
– Science is data driven
– Models, theories and the scientific process
– Science in society:
• Importance of science in society
• judging “quality” of scientic discussion
• pseudoscience
Astrology
• What is astrology?
– Astrology is something which purports that the position
of the planets and the stars at the time of your birth
determines the course of your future life
• Is wondering about this idea in the first place nonscientific?
A. Yes
B. No
•
Given the hypothesis of astrology, what would be
the next step in analyzing it scientifically?
Observations/data relevant to
astrology
• There is no evidence that astrology actually works.
• Note that the predictions of astrology may work sometimes;
almost certainly, some of these predictions will work
sometimes by chance! Certainly, astrology is not a fully
deterministic theory; if it claimed to be so, even a single
example of a failed astrological prediction would invalidate
the theory.
– Other flaw: not all astrologers even predict the same thing!
• Even as a statistical theory, astrology fails to be validated by
experiment.
• A link to some studies on the predictions of astrology:
http://www.astrosociety.org/education/resources/pseudobib.
html#1
Theoretical basis for astrology?
• Is it surprising that astrology doesn’t work? Are there successful scientific
theories that make predictions about astrology?
• Current theoretical understanding, which has been well supported by
observation, says there are four basic forces in nature:
– Gravity
– Electromagnetic force
– Strong force
– Weak force
• When quantitatively calculated, the force arising from planets and stars at
the moment of your birth from any of these sources is much smaller than
the force contribution from many other objects! So in no way would it be
expected that the position of astronomical objects would have an impact
Part II
Astronomy by eye : motions in the sky
Observing the night sky
How many objects were you able to identify in
the sky: Saturn, Venus, North Star, Vega,
Arcturus, others?
A. 0
B. 1
C. 2
D. 3
E. 4 or more
Observing the night sky
Which best describes your observations?
A. I was unable to detect any motion of any objects
across the sky
B. I was able to detect the motion of objects, but was
unable to judge whether they were all moving at the
same rate
C. I was able to detect the motion of objects, and found
they all moved at the same rate
D. I was able to detect the motion of objects, and found
that they moved at different rates
E. I was unable to measure anything or did not complete
the assignment
Observing the
night sky
Which best describes the
observations?
A. The data suggest there is no
motion of any objects across
the sky
B. The data suggest that objects
move, but we are unable to
judge whether they were all
moving at the same rate
C. The data suggest that all
obects move at the same rate
D. The data suggest that objects
moved at different rates
E. We cannot understand these
data
• Data obtained from
measurements taken
two hours apart.
• Note the “wrap” of the
map
Testing the model
Observing that stars move across the sky during the
course of a night
A. Proves that the Earth rotates on its axis
B. Proves that the Earth revolves around the Sun
C. Proves that the Sun revolves around the Earth
D. Rejects the model that the Sun is the only object
that is moving in space
E. Rejects the model that the Earth rotates on its
axis
Class Section II:
Motions in the Sky: Astronomy by Eye
• What astronomical objects do we see with our naked
eyes?
–
–
–
–
Sun
Moon
Planets
Nearby stars
• Many of these are relevant for everyday life: motion
of the Sun, phases of the Moon, etc.
• Want you to come out of astronomy class
understanding “everyday” astronomy!
Positions in the Sky
• How can we describe where astronomical objects are
located in the sky?
– Since we can’t immediately infer distances of astronomical objects by
just looking at them, all we can describe is what direction they are in
– Imagine that stars can be described by their location on an imaginary
sphere centered on the earth: the celestial sphere
– On this sphere, we can describe locations the way we describe
locations on the surface of the Earth, which is also a sphere
• Astronomical longitude (right ascension) and latitude (declination)
– From a given location on Earth, you can’t see the whole sphere, only
the half of it that is above the horizon
• Note that local coordinates that we have been using (azimuth / altitude) depend on
where you are located on earth, and thus aren’t ideal for a coordinate system that
everybody can refer to)
Constellations
• We notice patterns of stars that are seen in different
directions and call these constellations
– Different cultures have come up with different
constellations
• Constellations are nice to know for finding your way
around the sky, but are not really associated
“astronomically”, because stars in a given constellation
may be at very different distances!
– Constellations are not usually clusters! (although
sometimes they are!)
– viewed from some other location in the Galaxy,
constellations would be totally different!
Motions of astronomical objects
• Looking at astronomical objects, one quickly recognizes
that they move across the sky
• Apparent motion of objects can come from
– intrinsic motion of the object: motion that appears because
object itself is moving
– Reflex motion: motion that appears because Earth is moving
• For distant objects (stars), essentially all observed motion
comes from reflex motion
– Although stars definitely have intrinsic motion, they are so far
away that we don’t see much apparent motion over the course
of our lifetimes
– As we will discuss later, we can measure radial motion of
objects, but this doesn’t give apparent sideways motion
Reflex motion from Earth’s rotation
• Earth spins around an imaginary axis, once
per day
• As a result, the celestial sphere appears to
spin around the Earth
– Stars that happen to be located in the
direction of the Earth’s rotation axis
appear to stand still
• There is such a star above the North
pole : Polaris, the North Star
• There is NOT such a star above the
South pole!
– Stars near these directions travel in
small circles around the sky
– Stars far from these directions travel in
large circles around the sky
What’s the best description of Earth’s rotation and
motion of stars?
A. stars appear to move E to W because the
Earth rotates from E to W
B. stars appear to move E to W because the Earth
rotates from W to E
C. stars appear to move W to E because the Earth
rotates from E to W
D. stars appear move W to E because the Earth
rotations from W to E
E. stars don’t appear to move
To do
• Canvas homework on science
• Look up on web: celestial sphere, daily
motion of the stars and Sun, yearly motion of
the Sun