The sky is the ultimate art gallery just above us.
— Ralph Waldo Emerson
The Art of the Universe
We are just an advanced breed of monkeys on a minor planet of a very average star.
— Stephen Hawking
Wait! That's not fair! Earth gets its own page?! Well, yes, because we live here, and therefore we know the most about it. If you think about it, we've been studying the Earthwe came into existence. Important facts you need to know about the Earth are:
Distance from Sun: 149,600,000 km (1 AU)
Orbital period: 1 Earth Year
Axis tilt: 23.45Ëš
Radius: 6378 km
Mass: 5.97x10^24 kg (1 Earth units)
Density: 5.52 g/cm^3
Moons: 1
Atmosphere
Earth has a very unique atmosphere. It is the only atmosphere that we know of that can support life as we know it. Our atmosphere
is made up of 78% nitrogen, 21% oxygen, and 1% various other elements and compounds such as argon and carbon dioxide. There
are five layers of the atmosphere: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
The bottom layer is the troposphere. This is the layer where we live. Most airplanes fly in here and almost all weather occurs in this
layer. It extends from the surface of the Earth to 20 kilometers above. The average temperature is about 290.15K (17ËšC/62.6ËšF) at
the surface and drops to about 222.15K (-51ËšC/-58.9ËšF) as you increase altitude. Density decreases with altitude as well. The size of
the troposphere is different at the poles than it is at the equator. At the poles it's only extends to about 6 kilometers above the Earth,
while at the equator it reaches about 20 kilometers. 80% of the gases in the atmosphere are in the troposphere.
The second layer is the stratosphere. It extends from the end of the stratosphere to around 50 kilometers above the planet's surface.
Weather balloons fly in this layer. 19% of the gases of the atmosphere can be found here. Unlike the troposphere, in the stratosphere
temperature increases as altitude increases; however, density still decreases with an increase in altitude. At the bottom of the
stratosphere, the temperature averages about 213.15K (-60ËšC/-76ËšF) and at the top of the layer the temperature averages around
258.15K (-15ËšC/5ËšF). The increase in temperature is due to the formation of Ozone as oxygen absorbs ultraviolet radiation from the
Sun.
Next is the mesosphere. This is the layer where meteors burn and become visible. It begins where the stratosphere ends and ends
around 85 kilometers above the surface. Density continues to decrease with altitude. Because of the decrease in density, the
temperature decreases as you get higher, as there's less heat from radiation absorption. The temperature spans from 258.15K
(-15ËšC/5ËšF) in the lower regions to around 152.15K (-120ËšC/-184ËšF) in the upper regions.
The thermosphere is the fourth layer of the atmosphere. It is also the largest, expanding from 50 kilometers to about 690 kilometers
above the Earth. Space shuttles and the auroras can be found here. Despite having the lowest density yet, the molecules in this layer
absorb so much high-energy radiation from the Sun that the heat increases as you get higher. At the far reaches of this layer,
temperatures can reach 2273.15K (2000ËšC/3600ËšF) but because the air is so thin it would still feel remarkably cold to us.
The final layer is the exosphere. It extends to around 10,000 kilometers above the surface of the Earth. Most of the molecules and
atoms in this layer escape Earth's gravitational pull and go gallivaning off in space. Satellites orbit in the exosphere.
Interior
There are four layers that make up the Earth: the inner core, the outer core, the mantle, and the
crust. The inner core is the hottest, densest layer. It is a ball of solid iron superheated to
43273.15K (4300ËšC/77432ËšF) that is 5200 kilometers beneath the surface of the planet.
Surrounding the inner core is the outer core, a shell of liquid iron, nickel, and sulfur that is
37273.15K (3700ËšC/66632ËšF). This layer begins 2900 kilometers beneath the surface and
extends to the inner core. Above the inner core is the mantle. It is 2900 kilometers thick and
made of molten iron, magnesium, aluminum, silicon, and oxygen. This layer is around
1273.15K (1000ËšC/1832ËšF). The surface of the planet is made up of the crust. The crust is
much cooler than the interior of the planet, ranging from 217.15K (0ËšC/32ËšF) to nearly
1273.15K (1000ËšC/1832ËšF). It can be anywhere between 0 and 50 kilometers thick.
Tectonics
Tectonics is the theory that explains why the Earth's crust is the way it is – how it was shaped and why it moves. Basically, tectonics is the process by which convection in the mantle moves tectonic plates in the crust. There are seven major plates – African, North American, South American, Eurasian, Antarctic, Pacific, and Indo-Australian – and several minor plates. These plates are all moving different directions at a pace of about 2-10 centimeters per year. Where two plates meet is called a boundary, and there are three types of boundaries: convergent, divergent, and transform.
Convergent boundaries are where two tectonic plates are moving towards each other; essentially they're crashing into each other. Where two plates of the same density meet there is often a mountain range. The most famous example of mountains formed by this process today is the Himalayas, where the Indian plate and the Eruasian plate meet and push against each other, raising the crust into
mountains. However, if one plate is made of denser material than the other, for example
seafloor crust (which is young and dense) and continental crust (which is old and less
dense), the more dense plate will subduct, or be pushed under the less dense plate.
A divergent boundary is one where two plates are moving away from each other. The
movement of the plates away from each other creates two parallel faults that tilt away from
each other. This movement is usually on the slower end of plate movement, separating about
2 centimeters each year. Well-known divergent boundaries include the East Africa rift in
Kenya and Ethiopia and the Mid-Atlantic ridge.
The third boundary, a transform boundary, is where two plates slide past each other, moving along the same line but in opposite directions. Unlike the other two boundaries, there aren't any significant features that appear along a boundary of this kind. Instead, earthquakes occur pretty frequenly along these faults. The most famous transform boundary is the San Andreas fault, where part of southwest California is moving north while the rest of the state moves south. Around 10,000 earthquakes occur along this fault each year, although most are too small to notice.
Moon
Distance to Earth: 384,400 km
Orbital period: 27 Earth days
Radius: 1,737.4 km
Mass: 7.35x10^22 kg
Density: 3.3 g/cm^3
The moon is the only natural satellite of Earth. It is tidally locked with the Earth, meaning that we can only ever see one side of it. It is believed to have formed when, early in Earth's history, the planet was hit by an asteroid that flung part of the mantle out into orbit. When it was first formed, there was enough heat to create a molten interior, and thus there was geologic activity, but the moon has since cooled off. It is estimated that the moon has been geologically dead for around 3 billion years, and any geological change today is due to sandblasting by micrometeorites.
The surface of the moon is covered in craters and lunar maria. Maria are relatively smooth fields caused by lava flows. The lunar maria are generally round because they filled craters millions of years after the interior had initially cooled, but had re-heated due to radioactive decay. They are darker than the lunar highlands surrounding them. Most of the craters on the moon were formed during the late heavy bombardment period around 4 billion years ago.
There is no atmosphere on the moon, but there is water ice in the craters on the poles.
Source: solarsystem.nasa.gov
Tidal Forces
Tidal forces are what cause the daily high tides in the oceans. They are result of the gravitational pull increasing on the side of Earth facing the moon. This pull physically stretches the Earth so that it bulges out on the side facing the moon and the side opposite the moon. Although the tidal forces affect both land and water, we only notice the effect on the water because it moves more easily than land
does.
Tidal forces aren't just an interaction between the moon and the Earth, however. They occur
between any two bodies in orbit around each other. The Sun exerts tidal forces on the Earth, and
the Earth exerts tidal forces on the Sun (although they are markedly less noticable due to Earth's
significantly smaller mass). Same with any moon around any other planet.
The moon is tidally locked with the Earth, meaning that the moon has a rotation that is the same length of its orbit. This means that we only ever see one face of the moon. Such an occurrence is common among moons in the Solar System, and there are very few large moons that are not tidally locked with their planets.
