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Chapter Six Review: Solar System

Course Code
Brian Wilson

of 13
October 1st, 2010
Intro to Astronomy and Astro Part I
Chapter Six: The Solar System
An Inventory of the Solar System
Discovering our Planetary System
The invention of the telescope, in the 17th century, made it possible for astronomers
to observe the heavens in more detail, leading to the findings of the other planets
that make up our solar system
oAs, they were now able to see objects invisible to the naked eye
The 20th century saw the rise of:
oNon-optical observing aids:
oImprovements in telescopes which led to the observation of continuously
smaller objects (such as asteroids)
o Spacecraft exploration
oAs well as space flight, manned missions to the Moon
As currently explored, our solar system contains:
oThe Sun
oEight planets
o166 moons
oEight asteroids and more than 100 Kuiper Belt objects larger than 300 km in
oTens of thousands of smaller asteroids and Kuiper Belt objects
oMyriad comets a few kilometers in diameter
Comparative Planetology
Comparative planetology: The powerful perspective of comparing and contrasting
the properties of the various worlds we encounter to understand better the
conditions under which the planets form and evolve
oThis is the start of developing a comprehensive theory of the origin and
evolution of our planetary system a theory that explains all, or at least
most, of the solar systems observed properties
Through the discovery of other planets in the universe (mostly in our galaxy)
astronomers have a whole new set of proving grounds in which to compare theory
and reality
Measuring the Planets
The Sun contains about 99.9% of all mass in our solar system
Here is a brief summary of the properties of some of the objects in our solar system
oThe distance from each planet from the Sun is known from Keplers law once
the scale of the solar system was set by radar ranging on Venus
oA planets sidereal orbital period (relative to the stars) can be measured from
repeated observations of its location on the sky, so long as Earths own motion
around the Sun is properly taken into account
oA planets radius is found by measuring the angular size of the planet the
from one side to the other as we see it on the sky
oThe masses of planets with moons may be calculated by applying Newtons
laws on motion and gravity, just by observing the moons orbits around the
oThe masses of Mercury and Venus are a little harder to determine accurately
because these bodies have no natural satellites of their own. Nevertheless, it
is possible to measure their masses by careful observations of their
gravitational influence on other planets or nearby bodies.
oThese techniques for determining mass were available to astronomers well
over a century ago. Today, the masses of most objects have been accurately
measured through their gravitational interactions with artificial satellites
and space probes launched from Earth.
oA planets rotation period may, in principle, be determined simply by
watching surface features alternately appear and disappear as the planet
rotates. With some planets this is too difficult to do (because of atmospheres
or features are just too hard to see)
oThe average density, density is a measure of the compactness of matter can
be computed by dividing an objects mass by its volume.
The Overall Layout of the Solar System
Although the solar system is immense and even though many objects are many AU
apart, the planets all lie very close to the Sun, astronomically speaking
The planets in order from closest to the Sun to furthest:
oUranus, and
With the exception of Mercury all of the planets move on low-eccentricity orbits
oAccordingly, we can think of most planets’ orbits as circles centered on the
The planetary orbits are not evenly spaced, becoming further and further apart as
we move outward from the Sun
All the planets orbit the Sun counter clockwise as seen from above Earths North
Pole and in nearly the same plane as Earth (the plane of the ecliptic), with the
exception of Mercury (7 degrees to the ecliptic)
Terrestrial and Jovian Planets
Planetary Properties