*The Earth is but one of nine (or more) planets that revolve around the sun.

*The Earth is the fifth largest of the planets in the solar system, ranking below all the gas giants, but above all the rocky planets and the ice planets.  Like all the planets except Mercury and Venus (and the possible tenth planet), Earth has a moon, and ours is larger in proportion to the planet than any other moon (except Pluto’s moon, Charon), and this affects Earth significantly, most notably by causing tides.

*In addition to the other planets, the solar system is full of loose pieces of rock.  Sometimes these fall on the earth, and these are known as meteoroids.  They usually do not hurt us, though, because they burn up in the atmosphere, the envelope of air and other gasses that surround the Earth.  The atmosphere is about 1,000 miles deep, although almost all the mass of the gases in it are within 6 or 7 miles of the surface.

*Earth itself is not quite a perfect sphere; it is 24,900 miles around the equator, but 24818 miles around the poles.  This is caused, in part, by the rotation of the earth on its axis, which tends to spread the mass of the earth towards the centre as it spins.

*The surface of the Earth is made up of the hydrosphere (the water) and the lithosphere (the land).  About 70% is water.  Of that, about 97% is salt water, found in the oceans and seas.

*The Earth has four main oceans:  The Atlantic, Pacific, Indian, and Antarctic. 

*3% of the Earth’s water is fresh water.  About 2% is locked up in glaciers and icebergs so, although it is fresh water, we still cannot drink it.  The 1% left is divided up about equally between water in streams, lakes, and rivers, and that which is underground.

*Water underground is known as groundwater.  This is water in springs, underground caves, and in porous rock that holds water known as aquifers.  This is the water that people tap with wells, and in very dry places, it is very important.  However, some aquifers, including a big one in the American Midwest called the Ogallala Aquifer, are being used up faster than they can be replenished by rain seeping down into them.

*All the water in the world moves from the ocean to the air to the ground and back to the ocean. This movement of water is known as the water cycle. 

*The sun drives the majority of the water cycle.  It does so by heating exposed water (in oceans, lakes, or rivers) so that it turns into a gas.  This is called evaporation.  As water evaporates, the vapour gathers in the air.

*Depending on the condition of the air, the amount of water it can hold varies.  Warm air can hold more molecules of water vapour than cold air.

*Air eventually cools, either by rising too high, by circulating to latitudes farther from the equator, or simply by experiencing the cooling effect of darkness at night.  As the air grows cooler, it approaches the dew point, the point at which a given volume of air becomes too cold to contain all the water vapour it holds. 

*As moist air approaches the dew point, the water molecules begin to turn from vapour into liquid form.  This is known as condensation.  This forms clouds or may become visible as fog (which is why fog is seen most often in cool, damp areas).

*When moist air reaches its dew point, it may deposit some of its moisture as dew, or, if it falls far below the dew point, the water that condenses as clouds will be released as precipitation—the name for water in any form that falls to the ground as a liquid.  Depending on the temperature, this may be rain, snow, or sleet.

*When air that is rich in moisture passes over mountains (due to wind or to the rotation of the earth), it cools off as it rises over them.  As the air gets cooler, clouds form, and eventually rain falls on the mountains.  This means that mountains typically see more rain on the windward side (usually the west side) than the leeward side.  The side that gets less rain is said to be in a rain shadow. 

*In some cases, particularly with very high mountains, a desert forms in the rain shadow area and particularly wet and (usually) fertile areas on the rainy side of the mountains.  In the Appalachian Mountains this is not a big deal, but in other parts of the world it significantly affects local climates.

*Eventually, the water that returns to the Earth through precipitation evaporates again, and the water cycle continues.  The amount of water in the global cycle stays more or less constant, although it can be changed somewhat as water becomes locked up in glaciers and ice caps, or released from them.

*The lithosphere is the solid part of the Earth’s surface.  The exposed land of the lithosphere covers about 30% of the Earth’s surface.

*The highest point on the surface of the Earth is the top of Mount Everest, in Tibet.  Its top is 29,035 feet (5½ miles) above sea level.  The lowest point on land is the shore of the Dead Sea on the border between Israel and Jordan.  It lies 1,349 feet below sea level.  The deepest point known to man is the bottom of the Mariana Trench, a vast underwater chasm, near Guam in the Pacific Ocean.  It is 35,827 feet (6¾ miles) deep.

*The earth is made of four layers.  The inner core is about 4,000 miles below the surface of the Earth, and is very hot.  It is believed to be made of solid nickel and iron.  Although it is hot enough to be liquid, it is under so much pressure that it is not.

*Above the inner core is the outer core, beginning about 1,800 miles below the surface of the Earth.  It is also made of iron and nickel, but it is sort of liquid.

*On top of the outer core is the mantle.  This is mostly solid, but it is solid the way silly putty is solid—it basically holds a shape, but things can slide around in it slowly.

*On top of the mantle is the crust.  This is a very thin layer, between 3 and 30 miles thick.  However, it is where everything on Earth as we know it lives.

*The crust is actually made up of many different plates that slowly float around on the semi-melted surface of the mantle.  They may move as much as 4 inches a year.

*As the plates move around, they sometimes hit one another or slide against one another.  When two plates meet, they either fold, creating mountains, or break. 

*The places where plates break as they move against each other are called fault lines.  These can create earthquakes, and sometimes even volcanoes or tsunamis (caused by earthquakes or big volcanic eruptions under the sea.

*Most of the great features of the world—the Himalayas, the Rockies, the Appalachians—were formed by two plates running into each other and folding up some mountains.  In fact, Mount Everest in the Himalayas is still growing taller—about 1-2 inches taller a year.

*Other features are formed by external forces.  The two main ones are weathering and erosion.

*Weathering happens when rocks break down where they are, either because chemicals in rainwater and the air make them slowly dissolve, or because being exposed to heat and cold (and possible ice forming and melting in their cracks) makes them fall apart.

*Erosion happens when something rubs against a rock repeatedly, wearing it down.  This can be running water (such as in a river, or from the waves of the ocean), wind-blown sand, or even glaciers—huge blocks of ice that once flowed across the earth.  Some still exist in very high mountains.

*The difference between weathering and erosion is that weathering happens to a rock that is sitting in place, on its own, while erosion happens when something from outside comes in contact with a rock, wearing it down.

*Both the processes, but especially erosion, have created many landforms, especially rivers.  As rain falls high in the mountains, it flows downwards.  As it flows, it erodes the rocks it passes over, but it erodes the weakest one first.  It then follows the paths it makes in the weaker rocks, weakening them still further, and deepening the channels it follows.  This created river valleys and canyons, the most impressive of which is the Grand Canyon.

*The Earth has also been eroded by glaciers during the Ice Ages.  The last Ice Age probably ended about 10,000 years ago.  During that time, glaciers made many valleys and lakes in the world.  In the United States, they carved out the Great Lakes, and many other lakes in the North.

*So, the lithosphere is the Earth’s rocks, the hydrosphere the Earth’s water, and the atmosphere the Earth’s air.  As we can see, these things all act together (and against each other).  They also are all tied together because they support the biosphere, all the life on Earth.