ISP205 Section 2
Lecture#2: Thursday, January 11
Handouts:
Topics:
- Review
- Motion of Sun and Stars - Celestial Sphere
- Motion of Planets
- Astronomy and Astrology - The concept of science
- Review
- Light travels at a fixed speed of 300,000 km/s
- Looking far away is like looking back in time
- Astronomical distances
1 AU = distance sun-earth (average) (1.5x1011 m)
1 light year = distance light travels in a year (9.5x1015 m)
size of the universe: ~14,000,000,000 light years
- Celestial Sphere
- The celestial sphere is an imaginative sphere surrounding the earth
with the
stars attached to it.
- The concept works because the stars are so far away.
Therefore Practically no changes in relative positions of stars from
earth rotation
(there is a very small parallax for stars not too far away
from the rotation
of the earth around the sun - only noticable with large telescopes,
will be discussed in next lecture)
- The Motion of the stars as seen from earth can be described as the
rotation of
the celestial sphere (clockwise if seen from above celestial north
pole).
(Demo: celestial sphere) (figure
1.2)
- Celestial north and south pole: continuation of earth's rotation axis
through celestial sphere
Celestial
equator
: Cut of earth's equatorial plane through celestial sphere
Zenith
: point on celestial sphere directly above observer
- The stars move on circular orbit around celestial north (or south)
pole.
A star directly at the pole is not moving at all. At north pole there is
one: Polaris
Figure 1.4. (Redshift
Demo)
Observer on northern hemisphere at different latitude sees
celestial north pole at different
altitudes in the sky. Altitude = Latitude.
- Ecliptic: Path of the Sun (not fixed to celestial sphere because its
not far away -
we see earths rotation around sun as a movement of the sun on the
celestial sphere.
(Demo: celestial sphere) (see
Figure 1.6)
- Only stars in opposite direction of suns position are visible because
sun is so bright
(we see the stars only at night). Therefore we see different stars at
different times
of the year (winter sky, summer sky)
The sun moves through a sequence of constellations called the zodiac
(old sequence used for
Astrology - discussed later).
(see Figure)
- Points on the path of the sun on the celestial sphere:
Summer solstice : Sun highest in the sky (different times for northern
and southern hemisphere !)
Winter solstice : Sun lowest in the sky (different
times for northern and southern hemisphere !)
Vernal equinox : Sun crosses equator from southern to northern
hemisphere (everywhere same time)
Atumnal equinox: Sun crosses equator from northern to southern
hemisphere (everywhere same time
- even though its called Autumnal equinox it is
always at the same time - in September, which is spring
on the southern hemisphere !)
- Precession: the celestial north and south poles move slowly because of
the precession of earths
rotational axis. One rotation 26000 years.
(see Figure)
(Demo: precession of spinning top)
Polaris will move away from north pole. Then no star pointing
to north available anymore until Vega
will moves in around 14000 AD (see Figure)
Therefore also the Zodiac signs change. Signs used in
Astrology were determined by Ptolemy around
150 AC when he wrote his Tetrabiblios). So in early January the Sun is
in Sagittarius, not in
Capricorn
(Redshift Demo)
- Motion of Planets
- Complicated as motions of planets and earth around the sun add
together.
- Always close to ecliptic as planets rotate in one plane (except Pluto)
- Retrograde motion: sometimes Planets change direction in their
movements on celestial sphere
(Redshift Demo) (Redshift Presentation)
- Astrology, Astronomy, Science
Astronomy is Science (taught in this class)
Astrology is not Science and sometimes even Pseudo Science (not taught in
this class)
- The scientific method:
- Arrive at a Hypothesis
For example by Induction from observations with some creativity.
Different
philosophical schools disagree on how this is done)
- Deduce as many testable predictions as possible
(for complicated hypothesis - or several a model might be needed)
- Compare them with observations (reproducable by many scientists)
- If they agree - go back to 2
If sufficient evidence is accumulated the hypothesis becomes a
theory.
- If they don't agree refine Hypothesis until the do - go then back
to 2 using the
refined hypothesis.
If refinement is not possible - reject Hypothesis and create new one
(go back to 1)
- The main characteristics of the scientific method are:
- Many competing scientists try to carry out the experiments or
observations that
provide the most critical tests for a hypothesis or a theory.
- Scientists are ready to reject any hypothesis or theory that
does not agree with the facts
- Only observed facts are important and all observed facts are taken
into account.
- Characteristics of Pseudo science are for example:
- Pseudo scientists avoid experiments that could disprove their
theory
- Pseudo scientists are never willing to reject their theory
- Pseudo scientists ignore facts that speak against their theory and
emphasize
facts that seem to prove it.
- Astrology is not a Science. If scientific vocabulary is used or
testable predictions are made
it is a Pseudo science.
Examples: Ptolemys Tetrabiblios is still used by Astrologers. Scientists
have used his Almagest
for centuries, but were ready to reject it when evidence against it was
found (earth orbits the sun).