Key Ideas — Models of the Earth
The Earth is made up of 4 “spheres”: Atmosphere (air); Hydrosphere(water); Lithosphere (rock); and Biosphere (living).
Evidence that the Earth is round include: photos from spaceflight (best evidence); ships disappearing slowly over the horizon; and Earth’s curved shadow on the moon seen during a lunar eclipse.
The oceans cover almost three-quarters of the surface, but are comparably thinner than a film of water on a basketball.
The Earth’s layers are divided into the crust, mantle, outer core, and inner core.
The core is made of iron and nickel. The outer core is molten (fluid) and the inner core is solid.
Movements within the core create Earth’s magnetic field, which enable compasses to function, among other effects.
Temperature, density, and pressure all increase going from the crust deeper toward the center of the Earth.
A globe is the best model of our planet because both are 3-dimensional spheres.
Measurements show a greater pull of gravity at the poles than at the equator, showing that the poles are slightly closer to center of the Earth.
Because the Earth bulges slightly at the equator and is slightly flattened at the poles it is called an Oblate Spheroid. Remember it is more round than a basketball.
Eratosthenes was able to determine the circumference of the Earth. He used the altitude (angle above the horizon) of the Sun at two positions at noon and the distance between those positions.
Reference lines of latitude and longitude are used to find any position on the earth.
Lines of latitude run east-west, but measure distances north and south of the Equator. They are parallel each other. They go from the Equator (0 degrees) to 90o N (North Pole) and 90o S (South Pole.)
The altitude of Polaris (North Star) is the same as the latitude of the observer.
Longitude is based upon observations of the Sun, comparing the position of the observer with the position at the Prime Meridian, which runs through the Old Royal Observatory at Greenwich, near London, England.
Since Earth spins (rotates) through 360o in one day (24 hours), it turns at a rate of 15o of longitude each hour. This is the basis for standard time zones.
A map can symbolically depict portions of Earth’s surface in more detail than would be possible with any globe Landforms can be represented on a topographic map through the use of contour lines, which connect points of equal elevation.
Other patterns can also be represented by the use of isolines, such as isobars (pressure) and isotherms (temperature) on weather maps.
Isolines can never cross each other. (No point can have two values.)
Hachure marks show depressions (valleys within flat areas).
Where contour lines cross a stream, they always form a “V” pointing upstream.
Contour lines can be used to create a profile, which represents the shape of the landform as seen from a distance. (Imagine looking at a mountain’s shape from the bottom.)
The closer together isolines are, the steeper the gradient (slope or difference in value over a distance.)
Landforms can also be represented by electronic images (especially digital maps that can be manipulated with software to create “fly-throughs” and other effects.)
