CHAPTER 16 - THE SOLAR SYSTEM
An Overview
Astronomy is the study of the universe. This includes all
matter, energy, space, and time.
The concept of the year was developed from observations of
the sun's regular cycle of about 365 days.
The concept of the month was developed from observations of
the moon's cycle of about 29.5 days.
The solar system consists of the sun, nine planets and their 70
or so moons, asteroids, comets, meteoroids, interplanetary dust
particles, gasses and a solar wind composed of charged
particles.
Early studies of the solar system incorrectly assumed that the
earth was stationary and everything else revolved around the
earth. This was the Ptolemy version of the solar system and
was called the geocentric (earth-centered) model.
In the early 1500's, Copernicus developed the idea that the sun
was the center of the solar system and that the earth revolved
around it.
Brahe was a Danish astronomer who made the most accurate
measurements at that time of positions of the planets and stars.
He did this without a telescope since it had not been invented
yet.
Kepler was a German mathematician and astronomer who
analyzed the data Brahe collected to develop three laws that
govern the motion of planets in the solar system. They are
collectively called Kepler's laws of planetary motion.
Kepler's first law states that all planets move in elliptical orbits
around the sun with the sun at one focus of the ellipse. An
ellipse is shaped like a circle that has been flattened so that it is
not perfectly round.
The longest distance across an ellipse is called its major axis
and half of this distance is called its semi major axis. The
radius of a planet's orbit is approximately equal to its semi
major axis.
Distances in the solar system are often expressed in terms of
Astronomical Units. 1 AU equals the length of the earth's semi
major axis (average distance from the earth to the sun).
Kepler's second law is the law of equal areas. It states that a
radius vector drawn from any single planet to the sun will
sweep out equal areas in equal times.
This is a result of the conservation of energy. When the planet
is far away from the sun, the radius vector forms a long, skinny
pie slice because it is moving more slowly(less kinetic, more
potential energy). When the planet is close to the sun, the
radius vector forms a short, fat pie slice because it is moving
faster(more kinetic, less potential energy).
Kepler's third law is known as the harmonic law because of the
name of the paper he wrote to describe it (Harmony of the
Worlds). It states that the square of the sidereal period of a
planet is proportional to the cube of its semi major axis. The
equation is:
T2 = kR3
In the equation, T is the sidereal period of the planet(time
measured with respect to a distant star), R is the length of the
semi major axis of the orbit and k is a proportionality constant.
If we express the period in earth years and the radius of the
orbit in AU's, k = 1 year2/AU3
Example
The distance from Mars to the sun averages 1.52 AU. Find the
time in earth years required for Mars to complete one orbit.
Another very important scientist/astronomer was Galileo. He
was the first person to observe the moon and planets through a
telescope. He discovered four of Jupiter’s moons which showed
that the earth was not the only center of motion in the
universe.
Sir Isaac Newton in the late 1600's formulated the laws of
gravitational attraction. He used these laws to explain Kepler's
laws of planetary motion. He invented calculus to help explain
the shape of planetary orbits. His work established the
heliocentric theory of the solar system as the correct model.
Planets are classified by location and/or physical description.
Planets located inside earth's orbit (Mercury and Venus) are
called inferior. Planets located outside earth's orbit are called
superior.
Another method classifies Mercury, Venus, Earth, and Mars as
terrestrial planets since the chemical and physical properties of
the inner planets are relatively similar to those of the earth.
Jupiter, Saturn, Uranus, and Neptune are called Jovian planets
since the outer planets have characteristics similar to those of
Jupiter.
Pluto is an exception since it does not resemble either the Earth
or Jupiter.
If the solar system is viewed from a position far above the
Earth's North Pole, the planets all revolve around the sun in a
counterclockwise direction. This is called prograde motion and
is west to east relative to the sun. All of the planets except
Venus and Uranus also rotate west to east so that the sun rises
in the east. Venus and Uranus rotate east to west which is
called retrograde motion. On these planets the sun rises in the
west and sets in the east.
The Jovian planets are much larger in mass and volume than
the terrestrial planets. They are sometimes called gas giants.
The solar system is shaped like a disk with a very small bump
in the center representing the sun. The planets all orbit the sun
on a relatively thin plane with the exception of Pluto. Pluto's
orbit takes it well above and below the plane of the solar
system.
Time is expressed as either sidereal or synodic. A sidereal
period for a planet would be the time between two
conjunctions involving the planet and a distant star. This type
of time is not significantly affected by the motion of the earth
around the sun.
Synodic time is measured relative to the position in the sky of
the sun. The revolution of the earth around the sun affects
apparent positions relative to the sun.
The sidereal period of the planet Mercury is 88 days. This is
the time required for Mercury to complete its orbit around the
sun. During this 88 day period, the Earth moves in its orbit so
that it takes another 28 days for Mercury to catch back up and
create an inferior conjunction with the sun. This is the time
difference between the sidereal period and synodic period for
Mercury's orbit.
When two objects are in conjunction it means they are on the
same meridian in the sky, usually one above the other.
Planets that lie outside of Earth's orbit can reach opposition.
This means they are near or on a meridian 180 degrees away
from the sun. (Opposite side of the Earth). At opposition a
planet is at its nearest to the Earth and is in the sky during
night hours.
The Planet Earth
The planet Earth has some properties that are unique. No
other planet in the solar system has a large amount of surface
water (especially liquid), an atmosphere that contains a large
percent oxygen, a temperate climate, and living organisms.
This water and atmosphere contribute to the Earth's large
albedo. Albedo is the fraction of incident light that is reflected.
The Earth's albedo is .33 and is much larger than the Moon's
which is only .07.
The planet Venus has an albedo of .76 which, along with its
proximity, makes it the third brightest object in the sky.
It is important to differentiate between rotation and
revolution. The Earth revolves around the Sun. Revolution
means to move around another object or point.
The Earth rotates about its own axis. Rotation means to spin
about some internal axis.
The revolution of the Earth is measured in years. The rotation
of the Earth is measured in days.
Because the Earth revolves around the Sun, the Sun follows a
path in the sky over a year's time called the ecliptic. If we plot
the Sun's position on the celestial sphere at the same time every
day we will trace out a circle over a year's time.
This is called the ecliptic plane since eclipses occur when the
moon is on or near the ecliptic plane.
Evidence that the Earth rotates is provided by a device called a
Foucault pendulum. It consists of a very long low mass wire
with a very heavy weight on the end. Once set in motion, it
oscillates in the same plane. Over a period of 24 hours the
plane of oscillation seems to go through a 360 degree change.
This change is due to the rotation of the Earth.
Evidence that the Earth revolves around the Sun comes in the
form of stellar parallax and aberration of starlight.
Stars that are nearby seem to change position slightly over a 6
month period. Since the Earth revolves around the Sun, we are
about 186,000,000 miles from our initial position and looking
at the stars from a slightly different angle. Half of this angle is
called the angle of parallax.
Aberration of starlight involves a slight shift in apparent
position due to the motion of the Earth. The analogy used here
is aberration of raindrops. If you are sitting still in your car in
the rain, the raindrops could be coming straight down. As you
begin to move forward, they seem to be coming at an angle
relative to the vertical. This apparent change in direction due
to movement of the observer is called aberration of raindrops
or starlight.
The Terrestrial Planets
The terrestrial planets have certain characteristics in common.
They are relatively small in size and mass.
They are composed of rocky material and metals.
They are relatively dense.
They have solid surfaces and weak magnetic fields.
Their orbits are close together and close to the Sun.
None has a ring system and only the Earth and Mars have
moons(1 and 2).
They rotate slowly from 24 hours to 243 earth days.
Mercury is the closest planet to the Sun. It has a higher orbital
speed than any other planet.
Venus is the closest planet to the Earth. It has an atmosphere
that is 96% Carbon Dioxide and has clouds composed of
sulfuric acid droplets.
Earth has liquid water on its surface, oxygen in its atmosphere
and life.
Mars is the planet that most closely resembles the Earth. It has
polar caps composed of frozen water and carbon dioxide. It
has the largest known volcano in the solar system, Olympus
Mons.
The Jovian Planets and Pluto
The characteristics of the Jovian planets are:
They are all much larger than the terrestrial planets.
They are mostly gasseous and have no solid surface.
They have relatively low density.
They have solid cores composed of iron, silicates and ice
formed from methane, ammonia, and water
They are farther from the sun than the terrestrial planets.
They experience very low temperatures.
The distances between them are very large.
They have relatively strong magnetic fields.
They have rings and many moons.
They rotate very rapidly.
Jupiter is the largest planet in the solar system. It is nearly
large enough to be a star. The great red spot is a characteristic
of Jupiter. It is thought to be a like a huge hurricane lasting
hundreds of years.
Saturn has easily seen rings. They are composed of ice crystals
and ice coated rocks. Saturn's density is so low that it would
float in water if we could find an appropriate container and
enough water. Saturn's largest moon, Titan, is the only moon
known to have an atmosphere.
Uranus has a blue-green color since the methane in its
atmosphere absorbs red light. Uranus has an axis of rotation
tilted over 80 degrees fron the normal to the ecliptic. Its
rotation is retrograde.
Neptune was discovered through mathematical predictions of
its location. Johann G. Galle first observed Neptune using
predictions derived from variations in Uranus's orbit. It is a
twin of Uranus. It does have a moon, Triton, which orbits
Neptune retrograde.
Pluto is the farthest planet from the sun. Some astronomers
argue that Pluto is not a planet but that it wandered in from
space and was captured by the sun's gravitational field.
The Origin of the Solar System
Several theories have been proposed to explain how the solar
system was formed. The currently accepted theory is the
condensation theory.
According to this theory, the solar system began as a large,
swirling volume of dust and gasses - a rotating primordial
nebula.
Gravity caused the matter to be attracted towards the center of
the cloud. As this happened, the cloud rotated faster due to
conservation of angular momentum.
This rotation caused the cloud to flatten out into a disk shape
with most of the matter located near the center.
As the Sun formed in the center, it eventually reached a
temperature at which fusion began and it became a star.
The material orbiting the sun coalesced into rings and
eventually planets with the exception of the asteroid belt.
P 465 Key Terms, Matching, Multiple Choice, Fill in the Blank
P 466 Questions 1, 4, 5, 9, 10, 11, 12, 18
P 468 Exercises 1, 3, 5