THE SUN, EARTH, MOON SYSTEMS
EARTH
Earth is the third of the planets in our solar system. Its surface is mostly water (about 70%), and it has
a moderately dense nitrogen and oxygen atmosphere that supports life. Rich in iron and nickel, Earth
is a dense, molten oblate (slightly flattened) sphere with a solid core and a thin outer crust. Earth
rotates on its axis as it revolves around the Sun. Earth has one natural satellite, the Moon. A
complete revolution of Earth around the Sun takes about one year (365.25 days), while a rotation on
its axis takes one day (24 hours).
Earth orbits the Sun at a distance of about 93,000,000 mi (150,000,000 km). Earth is small by
planetary standards, only one-tenth the size of Jupiter. The circumference of the Earth at the Equator
is 40,075.16 kilometers (24,901.55 miles). Earth's mass is estimated at approximately 6.0×1024 kg
(1.32×1025 lbs). Unlike the outer planets, which are composed mainly of light gases, Earth is made of
heavy elements such as iron and nickel, and is, therefore, much more dense. These characteristics—
small and dense—are typical of the inner four terrestrial planets.
EARTH’S SEASONS
The Earth's revolution and the tilt of the Earth on its axis of about 23.5 degrees is what causes
seasonal changes. The Earth’s orbit is not round, but elliptical, with the sun a little off center of the
ellipse. Earth is closest to the Sun around January 3rd and is farthest from the Sun around July 4th.
This makes our winter’s slightly warmer in the Northern hemisphere than winter in the Southern
hemisphere.
Solstices
A solstice occurs on the two days a year when the sun is directly overhead at 23.5° latitude. The
summer solstice for the Northern Hemisphere is on June 21st. The sun is directly overhead along the
Tropic of Cancer (23.5°N latitude). The Northern Hemisphere is facing toward the sun at this time so
it receives more sun then the Southern Hemisphere. Days are longer in the Northern Hemisphere and
shorter in the Southern Hemisphere.
On December 21 the winter solstice occurs in the Northern Hemisphere. The sun is directly overhead
along the Tropic of Capricorn (23.5° S longitude). At this time the Northern Hemisphere is tilted away
from the sun.
Equinoxes
Halfway between the solstices are 2 days called equinoxes. The equinox is the beginning of spring
and autumn. In an equinox neither hemisphere is facing toward the sun. The length of day and night
are approximately the same. The autumnal equinox is on September 22. The vernal equinox happens
on March 21. The sunlight reaches the Northern and Southern Hemispheres in equal portions.
THE EARTH’S MAGNETIC FIELD
Magnetic fields are generated in the area around magnetic materials and can cause forces to act on
other magnetic materials. These materials can be magnets or electric currents. This field can be
‘seen’ by spreading iron filings around a bar magnet. The filings will align themselves with the
magnetic field.
The field generated by a bar magnet is known a dipole field, as is has two poles. The Earth’s
magnetic field is also a dipole field, resembling that of the bar magnet and is generated by complex
movements of the liquid iron in the Earth’s outer core. The magnetic field is tilted from the rotational
axis of the Earth by about 11° placing the North magnetic pole over northern Canada. The Earth's
magnetic field extends far into space and can trap charged particles, similar to how the iron filings
were trapped by the field around the bar magnet.
The Earth's magnetic field however is influenced by other magnetic fields in space (just as one
magnet can cause another to move on a table, for example). The sun also has a magnetic field of it's
own, which extends far into space, altering the shape of the magnetosphere. On the side pointing at
the sun, the field is compressed and on the other (the night side) the field is stretched such that the
whole magnetosphere resembles a teardrop.
THE MOON
According to the "giant impact" theory, the young Earth had no moon. At some point in Earth's early
history, a rogue planet, larger than Mars, struck the Earth in a great, glancing blow. Instantly, most of
the rogue body and a sizable chunk of Earth were vaporized. The cloud rose to above 13,700 miles
(22,000 kilometers) altitude, where it condensed into many solid particles that orbited the Earth.
Eventually these particles combined to form the moon.
By measuring the ages of lunar rocks, we know that the moon is about 4.6 billion years old, or about
the same age as Earth.
The distance between the Earth and its moon averages about 238,900 miles (384,000 kilometers).
The diameter of the moon is 2,160 miles (3,476 kilometers). The moon's mass—the amount of
material that makes up the moon—is about 1/80 of the Earth's mass.
Because the force of gravity at the surface of an object is the result of the object's mass and size, the
surface gravity of the moon is only one-sixth that of the Earth. The force gravity exerts on a person
determines the person's weight. Even though your mass would be the same on Earth and the moon,
if you weigh 132 pounds (60 kilograms) on Earth, you would weigh about 22 pounds (10 kilograms)
on the moon.
The earth rotates about 1000 mph. By comparison, the moon rotates about 10 mph. The rotation of
the moon—the time it takes to spin once around on its own axis—takes the same amount of time as
the moon takes to complete one orbit of the Earth, about 27.3 days. This means the moon's rotation
is synchronized in a way that causes the moon to show the same face to the Earth at all times. One
hemisphere always faces us, while the other always faces away. The lunar far side (aka the dark
side) has been photographed only from spacecraft.
The moon orbits the Earth at an average speed of 2,300 miles an hour (3,700 kilometers an hour).
The moon's gravitational pull on the Earth is the main cause of the rise and fall of ocean tides. The
moon's gravitational pull causes two bulges of water on the Earth's oceans—one where ocean waters
face the moon and the pull is strongest and one where ocean waters face away from the moon and
the pull is weakest. Both bulges cause high tides. As the Earth rotates, the bulges move around it,
one always facing the moon, the other directly opposite. The combined forces of gravity, the Earth's
rotation, and other factors usually cause two high tides and two low tides each day.
The airless lunar surface bakes in the sun at up to 243 degrees Fahrenheit (117 degrees Celsius) for
two weeks at a time (the lunar day lasts about a month). Then, for an equal period, the same spot is
in the dark. The dark side cools to about -272 degrees Fahrenheit (-169 degrees Celsius).
MOON PHASES
The most common incorrect reason given for the cause of the Moon's phases is that we are seeing
the shadow of the Earth on the Moon! This is not correct. The revolution of the Moon around the
Earth makes the Moon appear as if the earth is creating a shadow. The Moon change is caused by
the different angles from which we see the bright part of the Moon's surface.
The shape of the moon appears to change in a repeating cycle of 29.5 days when viewed from the
Earth because the amount of illuminated moon we see varies, depending on the moon's position in
relation to the Earth and the sun. We see the full moon when the sun is directly behind us,
illuminating a full hemisphere of the moon when it is directly in front of us. The new moon, when the
moon is darkened, occurs when the moon is almost directly between Earth and the sun—the sun's
light illuminates only the far side of the moon (the side we can't see from Earth).
Of course, the Moon doesn't generate any light itself; it just reflects the light of the Sun. The Moon
passes through four major shapes during a cycle that repeats itself every 29.5 days. The phases
always follow one another in the same order.
The phases were understood as early as 500 BC by the Greeks.
When the moon rises:
The New Moon always rises at sunrise.
The first quarter at noon.
The Full Moon always rises at sunset.
The last quarter at midnight.
Waxing refers to growing larger.
Waning refers to growing smaller.
ECLIPSES
Lunar Eclipse
Lunar eclipses are actually remarkably simple considering how interesting and dynamic they can be.
Basically, the Earth's shadow blocks most of the sunlight from directly illuminating all -- or a portion -of the moon's surface.
Lunar eclipses only occur during the full moon phase. The Earth plays “monkey in the middle”
between the Sun and the Moon. ).
It's important to note that lunar eclipses don't occur during every full moon because of two factors.
The first has to do with the variations in the orbital planes between the sun, Earth and the moon. The
rotational layout of the sun and Earth forms an ecliptic plane between the two celestial bodies. The
moon, however, doesn't circle the Earth in line with this same plane -- instead, its orbit is about 5
degrees off kilter. Any point where the moon happens to cross the ecliptic plane is called a node, and
the moon must be near a node for an eclipse to occur.
Solar Eclipse
A solar eclipse occurs when the moon passes in a direct line between the Earth and the Sun. The
moon's shadow travels over the Earth's surface and blocks out the sun's light as seen from Earth.
Because the moon orbits the Earth at an angle, approximately 5 degrees relative to the Earth-Sun
plane, the moon crosses the Earth's orbital plane only twice a year. These times are called eclipse
seasons, because they are the only times when eclipses can occur. For an eclipse to take place, the
moon must be in the correct phase during an eclipse season. For a solar eclipse, it must be a new
moon. This condition makes solar eclipses relatively rare.
There are 3 types of solar eclipses:
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Total - The entire central portion of the sun is blocked out.
Partial - Only part of the sun's surface is blocked out.
Annular - Only a small, ring-like sliver of light is seen from the sun's disc.