Download The Solar System

Astronomy
 The Universe
The Big Bang Theory is the idea that everything in the universe started out as a superdense mass that
exploded about 15 billion years ago and formed space and galaxies which are still expanding today.
The universe is so huge that distances must be measured in light years. A light year is the distance light
travels in one year, approximately 9.5 trillion kilometers. [To put this in perspective, the closest star to
us outside our solar system is Proxima Centauri which is 40 trillion km away. This means that IF we could
travel the speed of light (and we can’t even come to close to that speed right now), it would still take us 4
years to reach the closest star to us!]
A galaxy is a group of gases, dust, and billions of stars held together by gravity. There are billions of
galaxies in the universe, each with billions of stars. Our galaxy is a spiral-shaped galaxy called the Milky
Way.
There are two “forces” that keep objects moving in the universe:
1. Inertia – the idea that objects in motion stay in motion and objects at rest stay at rest, unless
acted on by a different force
2. Gravity – a force of attraction between objects that have mass
 The greater the mass, the greater gravitational pull
 The greater the distance between two objects, the less the gravitational pull
The weight of an object is the amount of pulling force toward the center of gravity.
Example: Because the Moon has so much less mass than the Earth, there is much less gravitational pull on the Moon than on the
Earth, therefore, you would weigh less on the Moon and have a much easier time jumping into the air. [However, your mass will
remain the same because you didn’t change the amount of matter that is making up your body.]
Before going any further with this topic, it's extremely important that you recognize the difference
between the two basic motions that heavenly bodies have:
rotation: the turning of an object on its axis
revolution: the movement of a body in orbit around an object or point
Most objects in the universe do both of these motions at the same time!
 Stars
- constellation: a group of stars that appears to form a pattern that looks like a familiar object or
character
- as Earth rotates, constellations appear to circle Polaris (the North Star). This creates star trails.
Brightness of Stars
- absolute magnitude: the amount of light actually given off by a star (luminosity/brightness)
- apparent magnitude: the amount of light from a star received on Earth, which depends on brightness
and distance
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Star Classification
- based on temperature and brightness using the H-R Diagram
- the color of a star depends on its surface temperature
hot stars = blue = 30,000 C
cool stars = red = 3,000 C
our Sun = yellow = 5,500 C
Star Energy
- nuclear fusion: four hydrogen atoms fuse together to
form one helium atom, giving off huge amounts of
energy in the process. This is continually happening in
the Sun.
Star Evolution & Origin
(Where do stars come from?)
- Stars begin as a large cloud of gas and dust called a nebula, which contracts due to gravity when it gets
large enough. As temperatures increase, nuclear fusion begins and light is given off. Now it’s a star.
- Most of its life is spent as a main sequence star -- an average size, average temperature star -- using
up its hydrogen fuel. Then the outer layers expand and it becomes a giant (or, if massive enough, a
supergiant)
- If the original mass was low, a hot dense core will be left behind called a white dwarf, then slowly die
out to become a black dwarf.
- If the original mass is large, it will explode into a supernova and then rapidly collapse, forming either a
neutron star or a black hole
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Our Sun is an average star of medium size, temperature, and brightness, about half-way through its life
cycle. In another 5 billion years, it will expand to become a red giant.
 Structure of the Solar System
The Sun is at the center of our solar system. The Sun is classified as a star because it generates energy
through nuclear fusion reactions, thus giving off its own light. [All other objects within the Solar System
give off reflected light.] Although the Sun is an average star, it is a million times greater in size than
the Earth.
As space distances go, the Sun is relatively close to the Earth at about 150 million kilometers. The next
closest stars to us, Proxima and Alpha Centauri, are 40 trillion kilometers away.
In order, from the Sun:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune
[Pluto has been reclassified as a “dwarf planet.”]
Earth is unique among the planets because it is the only one known to support life (and water which
supports life as we know it)
Other “bodies” within our solar system include:
 Asteroids -- solid bodies having no atmosphere with a well-determined orbit. Many are located in a belt
between Mars and Jupiter
 Meteoroids -- chunks of matter that circle the Sun in orbits that cross planetary orbits; could be from
broken up asteroids or comets
- meteor: streak of light produced if the chunk enters Earth’s atmosphere (also called “fireballs”,
“shooting stars”). Sometimes a meteor is produced when a man-made satellite falls to Earth.
- meteorite: any chunk that hits the Earth. These tells us a lot about the composition of Earth and the
solar system
- meteor showers occur whenever Earth crosses the path of a clump of meteoroids
 Comets -- solid chunks of debris and ice with highly elongated elliptical orbits. Many come from beyond
Pluto in the Oort Cloud
 Satellites/Moons -- most of the planets have at least one natural satellite/moon
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 Planetary Motions
Objects in the solar system have two motions:
rotation: the turning of an object on its axis; takes one “day”
revolution: the movement of a body in orbit around an object; takes one “year”
[The closer a planet is to the Sun, the less time it takes to revolve around it.]
Each body in the Solar System has its own period of rotation and revolution.
The orbits of the planets and moons are not perfect circles, but rather ellipses. This means there are
times when the planet is closer to the Sun than others, making the Sun appear larger in the sky. [This
holds true for the Moon’s orbit around the Earth as well.] Comets have the most elliptical shaped orbits.
 Earth Motions
Rotation
- The Earth rotates once (360) in 24 hours (1 “day”)
- The side that faces the Sun experiences daytime, while the side facing away experiences nighttime.
- The Earth rotates on its axis, an imaginary line extending through the North and South Poles
- The North Star, Polaris, is located directly above our axis of rotation; therefore, as we rotate, the
stars seem to move across the sky and circle Polaris.
- Because the Earth rotates from west to east, objects like the Moon and Sun appear to move from east
to west across the sky
Revolution
- The Earth revolves once around the Sun in 365¼ days (1 “year”)
-Because of the extra one quarter of a day, we add a calendar day to the year every four years, on
February 29th. This is called a “Leap Year.”
-Since the Earth’s orbit is elliptical, we aren’t always the same distance from the Sun. We’re closest on
January 3rd (“perihelion”) and farthest away on July 6th (“aphelion”).
 Seasons
Earth’s seasons are caused by the tilt of the Earth’s axis (23½) as it revolves around the Sun, causing
some areas of the Earth to receive a more intense/direct ray of the Sun. The tilt also causes the length
of daylight to vary for locations throughout the year.
When the North Pole is tilted toward
the Sun, the Tropic of Cancer
(Northern Hemisphere) is getting the
Sun’s direct (90°) ray and therefore
has summer. We have more hours of
daylight than darkness on this date.
The North Pole has 24 hours of
daylight on this date…as the Earth
rotates, the Sun never sets below the
horizon. The South Pole experiences
winter and has 24 hours of darkness on
this date.
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When the Northern Hemisphere is tilted toward
the Sun, we are getting a more intense ray of
sunlight and therefore have summer. At the same
time, the Southern Hemisphere is getting a weaker
ray of sunlight and therefore has winter. We are
actually closer to the Sun during our winter season….
distance has nothing to do with our seasons, it’s all
about the angle of the Sun’s rays.
On the Equinoxes, neither Pole is tilted toward the Sun so the 90° ray strikes the Equator. This is our
fall and winter. Every place on the Earth experiences 12 hours of daylight and 12 hours of darkness on
these days.
The Tropic of Cancer and the Tropic
of Capricorn mark the greatest
distance from the Equator that can
receive the direct ray of the Sun (and
therefore have the Sun directly
overhead).
 Motions of the Moon
The Moon appears to rise in the east and set in the west, moving at a rate of 15/hr, due to Earth’s
rotation.
However, the Moon is also orbiting Earth at a rate of 13/day. Since it takes the Earth 50 minutes to
“catch up” with the Moon’s orbit, the Moon rises 50 minutes later each day.
Moon’s period of rotation = 27 days
Moon’s period of revolution = 27 days
Because its period of rotation equals its period of revolution, there is only one side of the Moon always
seen from Earth.
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Moon phases are the changing appearances of the Moon as seen from Earth, caused by the Moon’s
revolution around the Earth.
The period between one cycle of phases = 29½ days, which is longer than the period of revolution because
the Earth has also been revolving around the Sun during that time and the Moon needs to “catch up.”
Phases of the Moon
The photographs show what the moon looks like from Earth. The
drawings show what the moon would look like from space.
 Tides
A tide is the periodic rise and fall of sea level, caused by a giant wave formed by the gravitational
attraction between the Earth, Sun, and Moon. Because the Moon is so much closer, it plays more of a role
than the Sun.
The tidal range is the difference between the level of the ocean at high and low tide.
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 Eclipses
Eclipses occur when the Earth or Moon temporarily blocks the sunlight reaching the other.
Eclipses can occur only when the Sun, Earth, and Moon are perfectly lined up (at New and Full phases) and
along the same plane (which is rare because the Moon’s orbit it tilted 5).
A solar eclipse occurs when the Moon passes in front of
the Sun and blocks it. It takes only a few minutes, occurs
in the middle of the day, and only about 5% of the Earth
can see one when it happens.
A lunar eclipse occurs when the Moon passes into the
Earth’s shadow so we can’t see the Moon. It only occurs at
night and during the Full Moon phase but the whole half of
the Earth that is in nighttime can view it, especially since
the whole process can last several hours.
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