Download Which of the following represent the best explanation we currently

Which of the following
represent the best explanation
we currently have?
a) Einstein’s Theory of Relativity
b) Newton’s Law of Gravitation
The concept of theory, as used in science, is
often misunderstood.
! A theory consists of a set of basic principles.
! These principles are often widely accepted.
Early models of the universe attempted to explain the
motion of the five visible planets against the
background of “ fixed”
fixed” stars. The main problem was
that the planets do not move uniformly against the
background of stars, but instead appear to stop, move
backward, then move forward again. This backward
motion is referred to as retrograde motion.
The Path of Mars in 2005-2006 and 2009-2010
Ptolemy explained this motion using a geocentric (Earthcentered) model of the solar system in which the planets
orbited the Earth indirectly, by moving on epicycles
which in turn orbited the Earth.
Geocentric Explanation of Retrograde Motion
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Nicolaus Copernicus developed the
first heliocentric (sun-centered) model
of the solar system. In this model, the
retrograde motion of Mars is seen
when the Earth passes Mars in its orbit
around the Sun.
A Heliocentric
Explanation of
Planetary Motion
We define special positions of the planets in their orbits depending
where they appear in our sky. For example, while at a conjunction, a
planet will appear in the same part of the sky as the Sun, while at
opposition, a planet will appear opposite the Sun in our sky.
However, the cycle of these positions (a synodic
period) is different from the actual orbital period of the
planet around the Sun (a sidereal period) because
both the Earth and the planet orbit around the Sun.
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When a new “star” appeared in the sky during the 16th century, a Danish
astronomer named Tycho Brahe reasoned that the distance of the object
may be determined by measuring the amount of parallax.
The apparent
change in the
location of an
object due to the
difference in
location of the
observer is called
parallax.
Because the parallax of the “star” was too small to measure,
Tycho knew that it had to be among the other stars, thus
disproving the ancient belief that the “heavens” were fixed
and unchangeable.
After Tycho Brahe’
Brahe’s
death, Johannes
Kepler (pictured here
with Tycho in the
background) used
Tycho’
Tycho’s
observations to
deduce the three
laws of planetary
motion.
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KEPLER’S THREE LAWS OF PLANETARY MOTION
LAW #1. The orbit of a planet around the Sun is an ellipse
with the Sun at one focus.
The amount of elongation in a planet’
planet’s orbit is defined as
its orbital eccentricity. An orbital eccentricity of 0 is a
perfect circle while an eccentricity close to 1.0 is nearly a
straight line.
In an elliptical orbit, the distance from a planet to the
Sun varies. The point in a planet’
planet’s orbit closest to the
Sun is called perihelion, and the point farthest from the
Sun is called aphelion.
KEPLER’S THREE LAWS OF PLANETARY MOTION
LAW #2: A line joining the planet and the Sun sweeps
out equal areas in equal intervals of time.
Planet moves
Planet moves
faster in its orbit
when closer to the
Sun.
Kepler's First and
Second Laws
slower in its orbit
when farther away
from the Sun.
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KEPLER’S THREE LAWS OF PLANETARY MOTION
LAW #3: The square of a planet’
planet’s sidereal period around the Sun
is directly proportional to the cube of its semi-major axis.
This law relates the amount of time for the planet to complete one orbit around the
Sun to the planet’s average distance from the Sun.
If we measure the orbital periods (P) in years and distances (a) in astronomical
units, then the law mathematically can be written as P2 = a3 .
Planetary Orbits
Galileo was the first to use a telescope to
examine celestial objects. His
discoveries supported a heliocentric
model of the solar system.
Galileo discovered that Venus, like the
Moon, undergoes a series of phases as
seen from Earth. In the Ptolemaic
(geocentric) model, Venus would be seen
in only new or crescent phases. However,
as Galileo observed, Venus is seen in all
phases, which agrees with the Copernican
model as shown.
Galileo also discovered
moons in orbit around the
planet Jupiter. This was
further evidence that the
Earth was not the center of
the universe.
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